| 0/0 |
2026/08/04 00:40 |
flow |
moderation |
10m
Results: map[Actionable:true Explanation:The KASAN use-after-free bug report is **fully actionable and self-consistent**.
### 1. Analysis of the Stack Traces
The report contains all 3 required stack traces, which are highly consistent and
point to the same object type and subsystem:
* **Bad memory access stack**: Occurs in `rb_page_size` (inlined in
`rb_advance_iter`) when advancing a ring buffer iterator
(`ring_buffer_iter_advance`) during a read of the trace file (`seq_read`).
* **Allocation stack**: Occurs in `__rb_allocate_pages` (called from
`ring_buffer_subbuf_order_set`) when allocating new `struct buffer_page` objects
to change the sub-buffer size/order.
* **Free stack**: Occurs in `free_buffer_page` (called from
`ring_buffer_subbuf_order_set`) when freeing the old `struct buffer_page`
objects after the sub-buffer size/order update.
The buggy address is at offset 56 within a 64-byte `kmalloc-64` slab object.
This matches the layout of `struct buffer_page` perfectly, where the `page`
field is located at offset 56 (448 bits):
```c
struct buffer_page {
struct list_head list; // 16 bytes (offset 0)
...
unsigned int order; // 4 bytes (offset 48)
unsigned int id:30; // (offset 52)
unsigned int range:1; // (offset 55)
void *page; // 8 bytes (offset 56)
};
```
---
### 2. Root Cause
1. When a user opens a trace file (e.g., `/sys/kernel/tracing/trace`), a
`trace_iterator` is created, and `ring_buffer_read_start()` is called.
2. `ring_buffer_read_start()` increments `cpu_buffer->resize_disabled` to
prevent the ring buffer from being resized or having its pages freed while the
iterator is active.
3. However, when a user writes to `buffer_subbuf_size` (which calls
`ring_buffer_subbuf_order_set()`), the kernel changes the sub-buffer order/size
and reallocates the ring buffer pages.
4. Unlike `ring_buffer_resize()`, **`ring_buffer_subbuf_order_set()` fails to
check if `cpu_buffer->resize_disabled` is set**.
5. As a result, `ring_buffer_subbuf_order_set()` proceeds to replace the ring
buffer pages and frees the old `struct buffer_page` objects.
6. When the active reader subsequently advances the iterator via
`ring_buffer_iter_advance()`, it accesses `iter->head_page` (which still points
to one of the freed `struct buffer_page` objects), resulting in a
use-after-free.
---
### 3. Proposed Fix
In `ring_buffer_subbuf_order_set()`, check if `cpu_buffer->resize_disabled` is
set for any of the CPU buffers, and return `-EBUSY` if so (similar to how
`ring_buffer_resize()` handles active readers):
```c
for_each_buffer_cpu(buffer, cpu) {
if (!cpumask_test_cpu(cpu, buffer->cpumask))
continue;
cpu_buffer = buffer->buffers[cpu];
+ if (atomic_read(&cpu_buffer->resize_disabled)) {
+ err = -EBUSY;
+ goto error;
+ }
+
if (cpu_buffer->mapped) {
err = -EBUSY;
goto error;
}
```]
|
| 1/1 |
2026/08/04 00:40 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/cde1b9ef9bc0fd8ebc2b773db8e023ba15b86bfc]
|
| 2/1 |
2026/08/04 00:40 |
action |
kernel-builder |
0m
Results: map[KernelObj:/app/workdir/cache/build/387161899d4d98525e72fbd25159a49cf5355674]
|
| 3/1 |
2026/08/04 00:40 |
action |
codesearch-prepare |
0m
Results: map[Index:codesearch-index]
|
| 4/1 |
2026/08/04 00:41 |
agent |
expert |
10m
Model:
Results:
Instruction: You are an experienced Linux kernel developer tasked with determining if the given kernel bug
report is actionable or not. Actionable means that it contains enough info to root cause
the underlying bug, and that the report is self-consistent and makes sense, rather than
e.g. a one-off nonsensical crash induced by a previous memory corruption.
The bug report is about a use-after-free bug generated by KASAN tool.
It should contain 3 stack traces: the bad memory access stack, the heap block allocation stack,
and the heap block free stack. If the report does not contain 3 stacks, it's not actionable.
All 3 stack traces should be related to the same object type,
and usually be in the same kernel subsystem (at least leaf stack frames).
An example of an actionable and consistent report would be: first access stack relates
to an access to a field of struct Foo, allocation/free stacks relate to allocation/free
of the struct Foo.
In inconsistent/nonsensical reports an access may be to a struct Foo, but allocation
stack allocates a different structure in a different subsystem.
Look for other suspicious signals/inconsistencies that can make this report hard to
debug/understand.
In the final reply explain why you think the report is self-consistent and actionable,
or why it's inconsistent and/or not actionable.
Use the provided tools to confirm any assumptions, variables/fields being accessed, etc.
Don't make assumptions about the kernel source code (it may be different from what you assume it is).
Extensively use the provided code access tools (codesearch-*, git-*, grepper, etc)
to examine the actual source code, and confirm any assumptions.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The bug report is:
==================================================================
BUG: KASAN: slab-use-after-free in rb_page_size kernel/trace/ring_buffer.c:391 [inline]
BUG: KASAN: slab-use-after-free in rb_advance_iter+0x40e/0x460 kernel/trace/ring_buffer.c:6024
Read of size 8 at addr ffff88802d79b8b8 by task syz.4.389/6869
CPU: 0 UID: 0 PID: 6869 Comm: syz.4.389 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0x13d/0x4b0 mm/kasan/report.c:482
kasan_report+0xdf/0x1c0 mm/kasan/report.c:595
rb_page_size kernel/trace/ring_buffer.c:391 [inline]
rb_advance_iter+0x40e/0x460 kernel/trace/ring_buffer.c:6024
ring_buffer_iter_advance+0x69/0x90 kernel/trace/ring_buffer.c:6445
trace_find_next_entry_inc kernel/trace/trace.c:2679 [inline]
s_next+0x270/0x410 kernel/trace/trace.c:2711
seq_read_iter+0xac5/0x1270 fs/seq_file.c:263
seq_read+0x344/0x4d0 fs/seq_file.c:163
vfs_read+0x1e4/0xb40 fs/read_write.c:572
ksys_read+0x12a/0x250 fs/read_write.c:716
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f734c99df99
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f734d820028 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00007f734cc25fa0 RCX: 00007f734c99df99
RDX: 0000000000001000 RSI: 0000200000000340 RDI: 0000000000000005
RBP: 00007f734ca34ec4 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f734cc26038 R14: 00007f734cc25fa0 R15: 00007ffcf813c748
</TASK>
Allocated by task 6869:
kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
__kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415
kasan_kmalloc include/linux/kasan.h:263 [inline]
__do_kmalloc_node mm/slub.c:5334 [inline]
__kmalloc_node_noprof+0x339/0x830 mm/slub.c:5346
_kmalloc_node_noprof include/linux/slab.h:1197 [inline]
__rb_allocate_pages+0x399/0x10a0 kernel/trace/ring_buffer.c:2402
ring_buffer_subbuf_order_set+0x3ef/0x18a0 kernel/trace/ring_buffer.c:7379
buffer_subbuf_size_write+0x182/0x280 kernel/trace/trace.c:8221
vfs_write+0x2aa/0x1050 fs/read_write.c:685
ksys_write+0x12a/0x250 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task 6871:
kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
kasan_save_free_info+0x3b/0x70 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:253 [inline]
__kasan_slab_free+0x5f/0x80 mm/kasan/common.c:285
kasan_slab_free include/linux/kasan.h:235 [inline]
slab_free_hook mm/slub.c:2677 [inline]
slab_free mm/slub.c:6377 [inline]
kfree+0x22b/0x6c0 mm/slub.c:6692
free_buffer_page kernel/trace/ring_buffer.c:399 [inline]
ring_buffer_subbuf_order_set+0x116b/0x18a0 kernel/trace/ring_buffer.c:7439
buffer_subbuf_size_write+0x182/0x280 kernel/trace/trace.c:8221
vfs_write+0x2aa/0x1050 fs/read_write.c:685
ksys_write+0x12a/0x250 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
The buggy address belongs to the object at ffff88802d79b880
which belongs to the cache kmalloc-64 of size 64
The buggy address is located 56 bytes inside of
freed 64-byte region [ffff88802d79b880, ffff88802d79b8c0)
The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x2d79b
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000000 ffff88813fe238c0 dead000000000100 dead000000000122
raw: 0000000000000000 0000000800200020 00000000f5000000 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0xd2cc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 1, tgid 1 (swapper/0), ts 13416095140, free_ts 0
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0xfd/0x120 mm/page_alloc.c:1859
prep_new_page mm/page_alloc.c:1867 [inline]
get_page_from_freelist+0xf48/0x3530 mm/page_alloc.c:3946
__alloc_frozen_pages_noprof+0x299/0x2dc0 mm/page_alloc.c:5304
alloc_slab_page mm/slub.c:3266 [inline]
allocate_slab mm/slub.c:3380 [inline]
new_slab+0xa2/0x640 mm/slub.c:3426
refill_objects+0xe3/0x410 mm/slub.c:7310
refill_sheaf mm/slub.c:2804 [inline]
__pcs_replace_empty_main+0x376/0x680 mm/slub.c:4675
alloc_from_pcs mm/slub.c:4773 [inline]
slab_alloc_node mm/slub.c:4905 [inline]
__do_kmalloc_node mm/slub.c:5333 [inline]
__kmalloc_noprof+0x66d/0x820 mm/slub.c:5359
_kmalloc_noprof include/linux/slab.h:992 [inline]
_kzalloc_noprof include/linux/slab.h:1309 [inline]
handler_new_ref+0x1ab/0xc60 drivers/media/v4l2-core/v4l2-ctrls-core.c:1895
v4l2_ctrl_add_handler drivers/media/v4l2-core/v4l2-ctrls-core.c:2456 [inline]
v4l2_ctrl_add_handler+0x223/0x300 drivers/media/v4l2-core/v4l2-ctrls-core.c:2430
vivid_create_controls+0x33ce/0x3e80 drivers/media/test-drivers/vivid/vivid-ctrls.c:2072
vivid_create_instance drivers/media/test-drivers/vivid/vivid-core.c:1933 [inline]
vivid_probe.cold+0x4f1e/0x921e drivers/media/test-drivers/vivid/vivid-core.c:2095
platform_probe+0x106/0x1d0 drivers/base/platform.c:1439
call_driver_probe drivers/base/dd.c:628 [inline]
really_probe+0x241/0xa60 drivers/base/dd.c:706
__driver_probe_device+0x20e/0x450 drivers/base/dd.c:868
driver_probe_device+0x4a/0x140 drivers/base/dd.c:898
__driver_attach+0x21f/0x5b0 drivers/base/dd.c:1292
page_owner free stack trace missing
Memory state around the buggy address:
ffff88802d79b780: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
ffff88802d79b800: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
>ffff88802d79b880: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
^
ffff88802d79b900: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
ffff88802d79b980: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
==================================================================
Reply: The KASAN use-after-free bug report is **fully actionable and self-consistent**.
### 1. Analysis of the Stack Traces
The report contains all 3 required stack traces, which are highly consistent and point to the same object type and subsystem:
* **Bad memory access stack**: Occurs in `rb_page_size` (inlined in `rb_advance_iter`) when advancing a ring buffer iterator (`ring_buffer_iter_advance`) during a read of the trace file (`seq_read`).
* **Allocation stack**: Occurs in `__rb_allocate_pages` (called from `ring_buffer_subbuf_order_set`) when allocating new `struct buffer_page` objects to change the sub-buffer size/order.
* **Free stack**: Occurs in `free_buffer_page` (called from `ring_buffer_subbuf_order_set`) when freeing the old `struct buffer_page` objects after the sub-buffer size/order update.
The buggy address is at offset 56 within a 64-byte `kmalloc-64` slab object. This matches the layout of `struct buffer_page` perfectly, where the `page` field is located at offset 56 (448 bits):
```c
struct buffer_page {
struct list_head list; // 16 bytes (offset 0)
...
unsigned int order; // 4 bytes (offset 48)
unsigned int id:30; // (offset 52)
unsigned int range:1; // (offset 55)
void *page; // 8 bytes (offset 56)
};
```
---
### 2. Root Cause
1. When a user opens a trace file (e.g., `/sys/kernel/tracing/trace`), a `trace_iterator` is created, and `ring_buffer_read_start()` is called.
2. `ring_buffer_read_start()` increments `cpu_buffer->resize_disabled` to prevent the ring buffer from being resized or having its pages freed while the iterator is active.
3. However, when a user writes to `buffer_subbuf_size` (which calls `ring_buffer_subbuf_order_set()`), the kernel changes the sub-buffer order/size and reallocates the ring buffer pages.
4. Unlike `ring_buffer_resize()`, **`ring_buffer_subbuf_order_set()` fails to check if `cpu_buffer->resize_disabled` is set**.
5. As a result, `ring_buffer_subbuf_order_set()` proceeds to replace the ring buffer pages and frees the old `struct buffer_page` objects.
6. When the active reader subsequently advances the iterator via `ring_buffer_iter_advance()`, it accesses `iter->head_page` (which still points to one of the freed `struct buffer_page` objects), resulting in a use-after-free.
---
### 3. Proposed Fix
In `ring_buffer_subbuf_order_set()`, check if `cpu_buffer->resize_disabled` is set for any of the CPU buffers, and return `-EBUSY` if so (similar to how `ring_buffer_resize()` handles active readers):
```c
for_each_buffer_cpu(buffer, cpu) {
if (!cpumask_test_cpu(cpu, buffer->cpumask))
continue;
cpu_buffer = buffer->buffers[cpu];
+ if (atomic_read(&cpu_buffer->resize_disabled)) {
+ err = -EBUSY;
+ goto error;
+ }
+
if (cpu_buffer->mapped) {
err = -EBUSY;
goto error;
}
```
|
| 5/2 |
2026/08/04 00:41 |
llm |
expert |
0m
Model:
Tokens:
input: 7689
output: 38
thoughts: 1554
Thoughts: **Analyzing the KASAN Use-After-Free Report**
Alright, let's dive into this KASAN report and see if we can make sense of it. My initial thought is to assess its actionability β is this something we can actually fix, and does the report itself seem coherent?
First, I'll examine the stack traces. We've got three: one for the bad memory access (the crash), one for the allocation, and one for the free. My immediate focus is to see if these three stacks point to operations on the *same* object or, at least, objects of the same type. This is crucial for understanding a UAF.
The 'bad memory access' stack indicates the crash occurs within `rb_page_size` and `rb_advance_iter` functions of `kernel/trace/ring_buffer.c`. I'll want to remember those locations.
The allocation stack clearly shows memory being allocated via `__rb_allocate_pages`, also within `kernel/trace/ring_buffer.c`, specifically at line 2402. I'm noting the allocated function.
The 'free' stack shows a free operation happens via `free_buffer_page` and, ultimately, from `ring_buffer_subbuf_order_set` within the same file (`kernel/trace/ring_buffer.c`). The line number is 399 within `free_buffer_page` and 7439 within `ring_buffer_subbuf_order_set`.
So, it *appears* that all three stacks are operating within the `kernel/trace/ring_buffer.c` module. That's a strong indication they're related. The allocation and free stacks seem to be manipulating the same kind of objects, or at least the allocation is used within the free function. Now, to confirm this, I need to check the source. I'm going to look at the code around these specific lines in `kernel/trace/ring_buffer.c` to see how the allocation, use, and free operations are connected. Let's see what's actually going on in that code.
|
| 6/2 |
2026/08/04 00:41 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:rb_page_size]
Results: map[SourceCode: 389: static __always_inline unsigned int rb_page_size(struct buffer_page *bpage)
390: {
391: return rb_data_page_size(bpage->page);
392: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 7/2 |
2026/08/04 00:41 |
llm |
expert |
0m
Model:
Tokens:
input: 9834
output: 36
thoughts: 194
Thoughts: **Analyzing a Kernel Memory Access Error**
Okay, so `rb_page_size` is the function in question. It takes a `struct buffer_page *bpage` as input and calls `rb_data_page_size` on `bpage->page`. My immediate thought is, "Right, standard structure access. Nothing inherently wrong there... or is there?" Then I see the error report, a read of size 8, a double-word, at address `ffff88802d79b8b8` by task `syz.4.389/6869`. Now things get interesting.
That faulty address, `ffff88802d79b8b8`, is inside the object located at `ffff88802d79b880`. This memory region is allocated from the `kmalloc-64` cache, meaning the allocation was for 64 bytes. The error message explicitly states the read happens 56 bytes into a *freed* 64-byte region, starting at `ffff88802d79b880`. This screams "use-after-free" or "buffer overflow".
My mind immediately starts forming questions: What's the layout of `struct buffer_page`? How does this function access memory in the `struct`? I need to understand the relationship between the `bpage` pointer, the `page` member, and the buggy address to pinpoint the root cause of this read. This is a classic debugging scenario. Now, to investigate...
|
| 8/2 |
2026/08/04 00:41 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:buffer_page]
Results: map[Fields:[map[Name:list OffsetBits:0 SizeBits:128] map[Name:write OffsetBits:128 SizeBits:64] map[Name:read OffsetBits:192 SizeBits:32] map[Name:entries OffsetBits:256 SizeBits:64] map[Name:real_end OffsetBits:320 SizeBits:64] map[Name:order OffsetBits:384 SizeBits:32] map[Name:id OffsetBits:416 SizeBits:30] map[Name:range OffsetBits:446 SizeBits:1] map[Name:page OffsetBits:448 SizeBits:64]]]
|
| 9/2 |
2026/08/04 00:41 |
llm |
expert |
0m
Model:
Tokens:
input: 10587
output: 39
thoughts: 318
Thoughts: **Analyzing the Buffer Page Structure**
Okay, let's break this down. First, the `struct buffer_page` layout needs to be clarified in my mind. Right, so we have:
* `list` at offset 0, taking up 16 bytes.
* `write` at 16, 8 bytes.
* `read` at 24, 4 bytes.
* `entries` at 32, 8 bytes.
* `real_end` at 40, another 8 bytes.
* Then `order` at 48, 4 bytes.
* `id` is at 52, which is 30 bits.
* `range` is at 55, a single bit.
* And finally, crucially, `page` is at offset 56, and that's an 8-byte pointer. So, summing these, thatβs 448 bits, or 56 bytes in total for the structure.
Therefore, the `page` pointer *is* indeed at offset 56 (0x38), which is exactly where I expect it to be. The bug, at `ffff88802d79b8b8`, looks very familiar. That address is *precisely* 56 bytes within the 64-byte aligned region starting at `ffff88802d79b880`. This confirms that the access is `bpage->page` within `rb_page_size`. This is all very consistent with my expectations.
Now, to understand the source of the problem, I'll need to look at what `__rb_allocate_pages` is actually allocating.
|
| 10/2 |
2026/08/04 00:41 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:__rb_allocate_pages]
Results: map[SourceCode:2358: static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
2359: long nr_pages, struct list_head *pages)
2360: {
2361: struct trace_buffer *buffer = cpu_buffer->buffer;
2362: struct ring_buffer_cpu_meta *meta = NULL;
2363: struct buffer_page *bpage, *tmp;
2364: bool user_thread = current->mm != NULL;
2365: struct ring_buffer_desc *desc = NULL;
2366: long i;
2367:
2368: /*
2369: * Check if the available memory is there first.
2370: * Note, si_mem_available() only gives us a rough estimate of available
2371: * memory. It may not be accurate. But we don't care, we just want
2372: * to prevent doing any allocation when it is obvious that it is
2373: * not going to succeed.
2374: */
2375: i = si_mem_available();
2376: if (i < nr_pages)
2377: return -ENOMEM;
2378:
2379: /*
2380: * If a user thread allocates too much, and si_mem_available()
2381: * reports there's enough memory, even though there is not.
2382: * Make sure the OOM killer kills this thread. This can happen
2383: * even with RETRY_MAYFAIL because another task may be doing
2384: * an allocation after this task has taken all memory.
2385: * This is the task the OOM killer needs to take out during this
2386: * loop, even if it was triggered by an allocation somewhere else.
2387: */
2388: if (user_thread)
2389: set_current_oom_origin();
2390:
2391: if (buffer->range_addr_start)
2392: meta = rb_range_meta(buffer, nr_pages, cpu_buffer->cpu);
2393:
2394: if (buffer->remote) {
2395: desc = ring_buffer_desc(buffer->remote->desc, cpu_buffer->cpu);
2396: if (!desc || WARN_ON(desc->nr_page_va != (nr_pages + 1)))
2397: return -EINVAL;
2398: }
2399:
2400: for (i = 0; i < nr_pages; i++) {
2401:
2402: bpage = alloc_cpu_page(cpu_buffer->cpu);
2403: if (!bpage)
2404: goto free_pages;
2405:
2406: rb_check_bpage(cpu_buffer, bpage);
2407:
2408: /*
2409: * Append the pages as for mapped buffers we want to keep
2410: * the order
2411: */
2412: list_add_tail(&bpage->list, pages);
2413:
2414: if (meta) {
2415: /* A range was given. Use that for the buffer page */
2416: bpage->page = rb_range_buffer(cpu_buffer, i + 1);
2417: if (!bpage->page)
2418: goto free_pages;
2419: /* If this is valid from a previous boot */
2420: if (meta->head_buffer)
2421: rb_meta_buffer_update(cpu_buffer, bpage);
2422: bpage->range = 1;
2423: bpage->id = i + 1;
2424: } else if (desc) {
2425: void *p = ring_buffer_desc_page(desc, i + 1);
2426:
2427: if (WARN_ON(!p))
2428: goto free_pages;
2429:
2430: bpage->page = p;
2431: bpage->range = 1; /* bpage->page can't be freed */
2432: bpage->id = i + 1;
2433: cpu_buffer->subbuf_ids[i + 1] = bpage;
2434: } else {
2435: int order = cpu_buffer->buffer->subbuf_order;
2436: bpage->page = alloc_cpu_data(cpu_buffer->cpu, order);
2437: if (!bpage->page)
2438: goto free_pages;
2439: }
2440: bpage->order = cpu_buffer->buffer->subbuf_order;
2441:
2442: if (user_thread && fatal_signal_pending(current))
2443: goto free_pages;
2444: }
2445: if (user_thread)
2446: clear_current_oom_origin();
2447:
2448: return 0;
2449:
2450: free_pages:
2451: list_for_each_entry_safe(bpage, tmp, pages, list) {
2452: list_del_init(&bpage->list);
2453: free_buffer_page(bpage);
2454: }
2455: if (user_thread)
2456: clear_current_oom_origin();
2457:
2458: return -ENOMEM;
2459: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 11/2 |
2026/08/04 00:41 |
llm |
expert |
0m
Model:
Tokens:
input: 12907
output: 38
thoughts: 49
Thoughts: Okay, here's my interpretation of that thought process, from an expert's perspective:
**Diving into the `bpage` Allocation**
Alright, line 2402 is where things get interesting. `alloc_cpu_page(cpu_buffer->cpu)` is clearly the function call that's responsible for allocating that `bpage` we're working with. I should probably double-check the definition of `alloc_cpu_page`. Make sure that it is doing what I expect it to do, and I should see if there's anything else in this context that I should take into account.
|
| 12/2 |
2026/08/04 00:41 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:alloc_cpu_page]
Results: map[SourceCode: SourceFile:]
|
| 13/2 |
2026/08/04 00:41 |
llm |
expert |
0m
Model:
Tokens:
input: 13146
output: 34
thoughts: 24
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Investigating `alloc_cpu_page`**
Okay, so I see `alloc_cpu_page`... smells like either a macro or an inline function, doesn't it? Given how performance-critical this often is, I'd bet on that. Time to use `grep` to quickly determine its definition. Need to understand its scope and, more importantly, *how* it's implemented. I'm looking for the canonical definition, the source file it lives in, and if it's potentially architecture-dependent. The goal here is to get a handle on what this abstraction is actually *doing* under the hood before I dive any deeper into code that uses it.
|
| 14/2 |
2026/08/04 00:41 |
tool |
grepper |
0m
Args: map[Expression:alloc_cpu_page PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:kernel/trace/ring_buffer.c=394=static void free_buffer_page(struct buffer_page *bpage)
--
kernel/trace/ring_buffer.c-409-
kernel/trace/ring_buffer.c:410:#define alloc_cpu_page(cpu) (struct buffer_page *) \
kernel/trace/ring_buffer.c-411- kzalloc_node(ALIGN(sizeof(struct buffer_page), \
--
kernel/trace/ring_buffer.c=2358=static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
--
kernel/trace/ring_buffer.c-2401-
kernel/trace/ring_buffer.c:2402: bpage = alloc_cpu_page(cpu_buffer->cpu);
kernel/trace/ring_buffer.c-2403- if (!bpage)
--
kernel/trace/ring_buffer.c=2487=rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
--
kernel/trace/ring_buffer.c-2509-
kernel/trace/ring_buffer.c:2510: bpage = alloc_cpu_page(cpu);
kernel/trace/ring_buffer.c-2511- if (!bpage)
]
|
| 15/2 |
2026/08/04 00:41 |
llm |
expert |
0m
Model:
Tokens:
input: 13711
output: 37
thoughts: 91
Thoughts: Here's my summary, as the expert:
**Diving into `alloc_cpu_page`**
Okay, so I see `alloc_cpu_page(cpu)` defined as a macro. Initially, the definition looks pretty straightforward: it's casting the result of `kzalloc_node` to a `struct buffer_page *`. It seems to be allocating memory, aligned to the size of `struct buffer_page`. Wait, before I jump to any conclusions, I should refresh my memory and verify the full details surrounding the macro. Let me quickly check the lines 410-415 of `kernel/trace/ring_buffer.c` to be completely sure. I need to be exact here.
|
| 16/2 |
2026/08/04 00:41 |
tool |
read-file |
0m
Args: map[File:kernel/trace/ring_buffer.c FirstLine:405 LineCount:15]
Results: map[Contents: 405: */
406: #define alloc_cpu_buffer(cpu) (struct ring_buffer_per_cpu *) \
407: kzalloc_node(ALIGN(sizeof(struct ring_buffer_per_cpu), \
408: cache_line_size()), GFP_KERNEL, cpu_to_node(cpu));
409:
410: #define alloc_cpu_page(cpu) (struct buffer_page *) \
411: kzalloc_node(ALIGN(sizeof(struct buffer_page), \
412: cache_line_size()), GFP_KERNEL, cpu_to_node(cpu));
413:
414: static struct buffer_data_page *alloc_cpu_data(int cpu, int order)
415: {
416: struct buffer_data_page *dpage;
417: struct page *page;
418: gfp_t mflags;
419:
]
|
| 17/2 |
2026/08/04 00:41 |
llm |
expert |
0m
Model:
Tokens:
input: 14268
output: 37
thoughts: 377
Thoughts: **Analyzing a Memory Corruption Bug**
Okay, so `sizeof(struct buffer_page)` is confirmed at 64 bytes. We established that from the structure's bit layout (448 bits + 64 bits = 512 bits = 64 bytes). The `ALIGN` macro then, unsurprisingly, aligns it to 64 bytes since the cache line size is also 64 bytes. Therefore, the memory allocator, `kzalloc_node`, is allocating a 64-byte object.
This all lines up perfectly with the error message: "The buggy address belongs to the object at ffff88802d79b880 which belongs to the cache kmalloc-64 of size 64" and that it's "located 56 bytes inside of freed 64-byte region".
Now, let's examine the free stack. We see the following call trace: `kfree -> free_buffer_page -> ring_buffer_subbuf_order_set -> buffer_subbuf_size_write`. The critical functions seem to be `free_buffer_page` and `ring_buffer_subbuf_order_set` at the vicinity of line 7439, given that these functions were involved in the call chain. Itβs now necessary to read the code in and around those two functions.
|
| 18/2 |
2026/08/04 00:41 |
tool |
read-file |
0m
Args: map[File:kernel/trace/ring_buffer.c FirstLine:394 LineCount:12]
Results: map[Contents: 394: static void free_buffer_page(struct buffer_page *bpage)
395: {
396: /* Range pages are not to be freed */
397: if (!bpage->range)
398: free_pages((unsigned long)bpage->page, bpage->order);
399: kfree(bpage);
400: }
401:
402: /*
403: * For best performance, allocate cpu buffer data cache line sized
404: * and per CPU.
405: */
]
|
| 19/2 |
2026/08/04 00:41 |
llm |
expert |
0m
Model:
Tokens:
input: 15148
output: 43
thoughts: 64
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience:
**Analyzing Kernel Memory Management and Ring Buffer Implementation**
Alright, so `free_buffer_page` is definitely calling `kfree(bpage)`. That makes perfect sense, standard memory deallocation. No surprises there. Now, let's dive into this `ring_buffer_subbuf_order_set` function. I need to understand its role in this system. Time to locate it β probably best to just search for the definition directly. Let me find line 7439, or just search for the function name and see where it shows up. Understanding the sub-buffer order setting is key to comprehending the overall ring buffer implementation strategy. I'm guessing it has something to do with the granularity of memory allocation within the ring buffer itself. Let's dig in.
|
| 20/2 |
2026/08/04 00:42 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:ring_buffer_subbuf_order_set]
Results: map[SourceCode:7298: /**
7299: * ring_buffer_subbuf_order_set - set the size of ring buffer sub page.
7300: * @buffer: The ring_buffer to set the new page size.
7301: * @order: Order of the system pages in one sub buffer page
7302: *
7303: * By default, one ring buffer pages equals to one system page. This API can be
7304: * used to set new size of the ring buffer page. The size must be order of
7305: * system page size, that's why the input parameter @order is the order of
7306: * system pages that are allocated for one ring buffer page:
7307: * 0 - 1 system page
7308: * 1 - 2 system pages
7309: * 3 - 4 system pages
7310: * ...
7311: *
7312: * Returns 0 on success or < 0 in case of an error.
7313: */
7314: int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
7315: {
7316: struct ring_buffer_per_cpu *cpu_buffer;
7317: struct buffer_page *bpage, *tmp;
7318: int old_order, old_size;
7319: int nr_pages;
7320: int psize;
7321: int err;
7322: int cpu;
7323:
7324: if (!buffer || order < 0)
7325: return -EINVAL;
7326:
7327: if (buffer->subbuf_order == order)
7328: return 0;
7329:
7330: psize = (1 << order) * PAGE_SIZE;
7331: if (psize <= BUF_PAGE_HDR_SIZE)
7332: return -EINVAL;
7333:
7334: /* Size of a subbuf cannot be greater than the write counter */
7335: if (psize > RB_WRITE_MASK + 1)
7336: return -EINVAL;
7337:
7338: old_order = buffer->subbuf_order;
7339: old_size = buffer->subbuf_size;
7340:
7341: /* prevent another thread from changing buffer sizes */
7342: guard(mutex)(&buffer->mutex);
7343: atomic_inc(&buffer->record_disabled);
7344:
7345: /* Make sure all commits have finished */
7346: synchronize_rcu();
7347:
7348: buffer->subbuf_order = order;
7349: buffer->subbuf_size = psize - BUF_PAGE_HDR_SIZE;
7350:
7351: /* Make sure all new buffers are allocated, before deleting the old ones */
7352: for_each_buffer_cpu(buffer, cpu) {
7353:
7354: if (!cpumask_test_cpu(cpu, buffer->cpumask))
7355: continue;
7356:
7357: cpu_buffer = buffer->buffers[cpu];
7358:
7359: if (cpu_buffer->mapped) {
7360: err = -EBUSY;
7361: goto error;
7362: }
7363:
7364: /* Update the number of pages to match the new size */
7365: nr_pages = old_size * buffer->buffers[cpu]->nr_pages;
7366: nr_pages = DIV_ROUND_UP(nr_pages, buffer->subbuf_size);
7367:
7368: /* we need a minimum of two pages */
7369: if (nr_pages < 2)
7370: nr_pages = 2;
7371:
7372: cpu_buffer->nr_pages_to_update = nr_pages;
7373:
7374: /* Include the reader page */
7375: nr_pages++;
7376:
7377: /* Allocate the new size buffer */
7378: INIT_LIST_HEAD(&cpu_buffer->new_pages);
7379: if (__rb_allocate_pages(cpu_buffer, nr_pages,
7380: &cpu_buffer->new_pages)) {
7381: /* not enough memory for new pages */
7382: err = -ENOMEM;
7383: goto error;
7384: }
7385: }
7386:
7387: for_each_buffer_cpu(buffer, cpu) {
7388: struct buffer_data_page *old_free_data_page;
7389: struct list_head old_pages;
7390: unsigned long flags;
7391:
7392: if (!cpumask_test_cpu(cpu, buffer->cpumask))
7393: continue;
7394:
7395: cpu_buffer = buffer->buffers[cpu];
7396:
7397: raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
7398:
7399: /* Clear the head bit to make the link list normal to read */
7400: rb_head_page_deactivate(cpu_buffer);
7401:
7402: /*
7403: * Collect buffers from the cpu_buffer pages list and the
7404: * reader_page on old_pages, so they can be freed later when not
7405: * under a spinlock. The pages list is a linked list with no
7406: * head, adding old_pages turns it into a regular list with
7407: * old_pages being the head.
7408: */
7409: list_add(&old_pages, cpu_buffer->pages);
7410: list_add(&cpu_buffer->reader_page->list, &old_pages);
7411:
7412: /* One page was allocated for the reader page */
7413: cpu_buffer->reader_page = list_entry(cpu_buffer->new_pages.next,
7414: struct buffer_page, list);
7415: list_del_init(&cpu_buffer->reader_page->list);
7416:
7417: /* Install the new pages, remove the head from the list */
7418: cpu_buffer->pages = cpu_buffer->new_pages.next;
7419: list_del_init(&cpu_buffer->new_pages);
7420: cpu_buffer->cnt++;
7421:
7422: cpu_buffer->head_page
7423: = list_entry(cpu_buffer->pages, struct buffer_page, list);
7424: cpu_buffer->tail_page = cpu_buffer->commit_page = cpu_buffer->head_page;
7425:
7426: cpu_buffer->nr_pages = cpu_buffer->nr_pages_to_update;
7427: cpu_buffer->nr_pages_to_update = 0;
7428:
7429: old_free_data_page = cpu_buffer->free_page;
7430: cpu_buffer->free_page = NULL;
7431:
7432: rb_head_page_activate(cpu_buffer);
7433:
7434: raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
7435:
7436: /* Free old sub buffers */
7437: list_for_each_entry_safe(bpage, tmp, &old_pages, list) {
7438: list_del_init(&bpage->list);
7439: free_buffer_page(bpage);
7440: }
7441: free_pages((unsigned long)old_free_data_page, old_order);
7442:
7443: rb_check_pages(cpu_buffer);
7444: }
7445:
7446: atomic_dec(&buffer->record_disabled);
7447:
7448: return 0;
7449:
7450: error:
7451: buffer->subbuf_order = old_order;
7452: buffer->subbuf_size = old_size;
7453:
7454: atomic_dec(&buffer->record_disabled);
7455:
7456: for_each_buffer_cpu(buffer, cpu) {
7457: cpu_buffer = buffer->buffers[cpu];
7458:
7459: if (!cpu_buffer->nr_pages_to_update)
7460: continue;
7461:
7462: list_for_each_entry_safe(bpage, tmp, &cpu_buffer->new_pages, list) {
7463: list_del_init(&bpage->list);
7464: free_buffer_page(bpage);
7465: }
7466: }
7467:
7468: return err;
7469: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 21/2 |
2026/08/04 00:42 |
llm |
expert |
0m
Model:
Tokens:
input: 18025
output: 38
thoughts: 504
Thoughts: **Analyzing `ring_buffer_subbuf_order_set` and Potential Iterator Issues**
Okay, let's break this down. My focus is immediately drawn to `ring_buffer_subbuf_order_set`. This is where the subbuffer order is altered, and with that change, the sizes of the subbuffers also change. Now, how does it do this?
First, I see that recording is disabled using `atomic_inc(&buffer->record_disabled);`. Then, it calls `synchronize_rcu();` which is good to see as it suggests some care is being taken to handle concurrent access. After this, it gets new pages using `__rb_allocate_pages(cpu_buffer, nr_pages, &cpu_buffer->new_pages)`.
Next, for each CPU, it locks the `cpu_buffer->reader_lock`. Then, crucially, it swaps the old pages with these brand new ones. The old pages are collected into a local list, `old_pages`. Once all this is done, the `cpu_buffer->reader_lock` is unlocked. Finally, those old pages are freed using a `list_for_each_entry_safe` loop and `free_buffer_page(bpage)`.
However, my expert instincts kick in immediately. This process looks a bit risky from a concurrency standpoint. My alarm bells are ringing about active iterators! If an iterator is currently reading from the ring buffer, are there any locks or references held that need to be considered? It seems like this operation is potentially dangerous if there's an iterator active during this page swap.
And that's the precise issue I'm seeing! The bad memory access stack trace I have in front of me:
```
rb_page_size kernel/trace/ring_buffer.c:391 [inline]
rb_advance_iter+0x40e/0x460 kernel/trace/ring_buffer.c:6024
ring_buffer_iter_advance+0x69/0x90 kernel/trace/ring_buffer.c:6445
trace_find_next_entry_inc kernel/trace/trace.c:2679 [inline]
s_next+0x270/0x410 kernel/trace/trace.c:2711
seq_read_iter+0xac5/0x1270 fs/seq_file.c:263
seq_read+0x344/0x4d0 fs/seq_file.c:163
```
This looks like `rb_advance_iter` and potentially `ring_buffer_iter_advance` could be the source of the trouble. I need to dig into these functions to understand how they interact with the ring buffer pages and what locks or references they might hold. This strongly suggests that there's a race condition where an iterator might be accessing the "old" pages after they've been freed or the new pages before they are fully initialized. I will now examine `rb_advance_iter` and `ring_buffer_iter_advance` with close scrutiny.
|
| 22/2 |
2026/08/04 00:42 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:rb_advance_iter]
Results: map[SourceCode:6006: static void rb_advance_iter(struct ring_buffer_iter *iter)
6007: {
6008: struct ring_buffer_per_cpu *cpu_buffer;
6009:
6010: cpu_buffer = iter->cpu_buffer;
6011:
6012: /* If head == next_event then we need to jump to the next event */
6013: if (iter->head == iter->next_event) {
6014: /* If the event gets overwritten again, there's nothing to do */
6015: if (rb_iter_head_event(iter) == NULL)
6016: return;
6017: }
6018:
6019: iter->head = iter->next_event;
6020:
6021: /*
6022: * Check if we are at the end of the buffer.
6023: */
6024: if (iter->next_event >= rb_page_size(iter->head_page)) {
6025: /* discarded commits can make the page empty */
6026: if (iter->head_page == cpu_buffer->commit_page)
6027: return;
6028: rb_inc_iter(iter);
6029: return;
6030: }
6031:
6032: rb_update_iter_read_stamp(iter, iter->event);
6033: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 23/2 |
2026/08/04 00:42 |
llm |
expert |
0m
Model:
Tokens:
input: 19684
output: 40
thoughts: 159
Thoughts: **Debugging a Potential Data Race**
Okay, so I'm looking at this code and something's not right. I see that `iter->head_page` is supposed to be a `struct buffer_page *`. Then it accesses this via `rb_page_size(iter->head_page)`. But, hang on... didn't `ring_buffer_subbuf_order_set` *free* that `iter->head_page`? That's a huge red flag! It screams potential use-after-free, a classic memory corruption issue.
The immediate question is: how does `ring_buffer_subbuf_order_set` coordinate with these iterators? Is there any sort of synchronization involved here? Does it check for active iterators before freeing the memory? Or are the iterators protected somehow, maybe by a lock, mutex, or RCU mechanism?
I need to investigate further. Time to dive into the `ring_buffer_iter_advance` function to see how those iterators are managed. I'm hoping to find some kind of protection or coordination that I've missed, but based on what I see here I'm getting a bad feeling. I need to be sure this isn't a race condition waiting to happen.
|
| 24/2 |
2026/08/04 00:42 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:ring_buffer_iter_advance]
Results: map[SourceCode:6431: /**
6432: * ring_buffer_iter_advance - advance the iterator to the next location
6433: * @iter: The ring buffer iterator
6434: *
6435: * Move the location of the iterator such that the next read will
6436: * be the next location of the iterator.
6437: */
6438: void ring_buffer_iter_advance(struct ring_buffer_iter *iter)
6439: {
6440: struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
6441: unsigned long flags;
6442:
6443: raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
6444: iter->missed_events = 0;
6445: rb_advance_iter(iter);
6446:
6447: raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
6448: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 25/2 |
2026/08/04 00:42 |
llm |
expert |
0m
Model:
Tokens:
input: 20460
output: 35
thoughts: 613
Thoughts: **My Analysis of a Potential Race Condition in the Ring Buffer Implementation**
Okay, hold on a second! I've spotted something potentially nasty in the ring buffer code. `ring_buffer_iter_advance` grabs `cpu_buffer->reader_lock`. But... and this is the kicker... `ring_buffer_subbuf_order_set` *also* acquires `cpu_buffer->reader_lock`! That's already raising a red flag. Let me re-examine `ring_buffer_subbuf_order_set` closely.
Specifically, it looks like it iterates through the CPUs:
```c
for_each_buffer_cpu(buffer, cpu) {
struct buffer_data_page *old_free_data_page;
struct list_head old_pages;
unsigned long flags;
if (!cpumask_test_cpu(cpu, buffer->cpumask))
continue;
cpu_buffer = buffer->buffers[cpu];
raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
...
/* Free old sub buffers */
list_for_each_entry_safe(bpage, tmp, &old_pages, list) {
list_del_init(&bpage->list);
free_buffer_page(bpage);
}
```
Wait a minute... I see it now. The code *unlocks* `cpu_buffer->reader_lock` *before* calling `free_buffer_page(bpage)`!
```c
raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
/* Free old sub buffers */
list_for_each_entry_safe(bpage, tmp, &old_pages, list) {
list_del_init(&bpage->list);
free_buffer_page(bpage);
}
```
Okay, so it releases the lock, *then* deallocates the old pages. That's a classic pattern, but it opens a window. If an iterator, say `iter->head_page`, is currently pointing to one of these soon-to-be-freed pages, then we have a problem. When `ring_buffer_iter_advance` gets called, it grabs the `cpu_buffer->reader_lock` (which is now free, because the other function has released it). It would then access `iter->head_page` which has just been freed by `free_buffer_page()`, right after the spinlock was released! I think that's a use-after-free situation, right?
But wait... am I missing something? How are iterators handled in the face of buffer page changes? Are they invalidated, or updated somehow? I'm going to check to see if `ring_buffer_subbuf_order_set` has anything to do with invalidating or updating the iterators.
Let me search for "ring_buffer_iter" or just "iter" within the `ring_buffer_subbuf_order_set` function, and also anywhere in `ring_buffer.c` relating to resizing or reordering.
Now, let's grep for `struct ring_buffer_iter` or `iter` in `ring_buffer.c`. I need to figure out if there's any mechanism for handling iterators when the underlying pages change.
|
| 26/2 |
2026/08/04 00:42 |
tool |
grepper |
0m
Args: map[Expression:struct ring_buffer_iter PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:kernel/trace/ring_buffer.c=562=struct trace_buffer {
--
kernel/trace/ring_buffer.c-593-
kernel/trace/ring_buffer.c:594:struct ring_buffer_iter {
kernel/trace/ring_buffer.c-595- struct ring_buffer_per_cpu *cpu_buffer;
--
kernel/trace/ring_buffer.c=3442=static struct ring_buffer_event *
kernel/trace/ring_buffer.c:3443:rb_iter_head_event(struct ring_buffer_iter *iter)
kernel/trace/ring_buffer.c-3444-{
--
kernel/trace/ring_buffer.c=3508=rb_event_index(struct ring_buffer_per_cpu *cpu_buffer, struct ring_buffer_event *event)
--
kernel/trace/ring_buffer.c-3516-
kernel/trace/ring_buffer.c:3517:static void rb_inc_iter(struct ring_buffer_iter *iter)
kernel/trace/ring_buffer.c-3518-{
--
kernel/trace/ring_buffer.c=5540=static void rb_update_remote_head(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-5575-
kernel/trace/ring_buffer.c:5576:static void rb_iter_reset(struct ring_buffer_iter *iter)
kernel/trace/ring_buffer.c-5577-{
--
kernel/trace/ring_buffer.c-5610- */
kernel/trace/ring_buffer.c:5611:void ring_buffer_iter_reset(struct ring_buffer_iter *iter)
kernel/trace/ring_buffer.c-5612-{
--
kernel/trace/ring_buffer.c=5625=EXPORT_SYMBOL_GPL(ring_buffer_iter_reset);
--
kernel/trace/ring_buffer.c-5630- */
kernel/trace/ring_buffer.c:5631:int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
kernel/trace/ring_buffer.c-5632-{
--
kernel/trace/ring_buffer.c=5705=static void
kernel/trace/ring_buffer.c:5706:rb_update_iter_read_stamp(struct ring_buffer_iter *iter,
kernel/trace/ring_buffer.c-5707- struct ring_buffer_event *event)
--
kernel/trace/ring_buffer.c=5982=static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-6005-
kernel/trace/ring_buffer.c:6006:static void rb_advance_iter(struct ring_buffer_iter *iter)
kernel/trace/ring_buffer.c-6007-{
--
kernel/trace/ring_buffer.c=6114=static struct ring_buffer_event *
kernel/trace/ring_buffer.c:6115:rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
kernel/trace/ring_buffer.c-6116-{
--
kernel/trace/ring_buffer.c=6249=ring_buffer_peek(struct trace_buffer *buffer, int cpu, u64 *ts,
--
kernel/trace/ring_buffer.c-6279- */
kernel/trace/ring_buffer.c:6280:bool ring_buffer_iter_dropped(struct ring_buffer_iter *iter)
kernel/trace/ring_buffer.c-6281-{
--
kernel/trace/ring_buffer.c=6294=struct ring_buffer_event *
kernel/trace/ring_buffer.c:6295:ring_buffer_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
kernel/trace/ring_buffer.c-6296-{
--
kernel/trace/ring_buffer.c=6360=EXPORT_SYMBOL_GPL(ring_buffer_consume);
--
kernel/trace/ring_buffer.c-6374- */
kernel/trace/ring_buffer.c:6375:struct ring_buffer_iter *
kernel/trace/ring_buffer.c-6376-ring_buffer_read_start(struct trace_buffer *buffer, int cpu, gfp_t flags)
--
kernel/trace/ring_buffer.c-6378- struct ring_buffer_per_cpu *cpu_buffer;
kernel/trace/ring_buffer.c:6379: struct ring_buffer_iter *iter;
kernel/trace/ring_buffer.c-6380-
--
kernel/trace/ring_buffer.c=6417=void
kernel/trace/ring_buffer.c:6418:ring_buffer_read_finish(struct ring_buffer_iter *iter)
kernel/trace/ring_buffer.c-6419-{
--
kernel/trace/ring_buffer.c=6429=EXPORT_SYMBOL_GPL(ring_buffer_read_finish);
--
kernel/trace/ring_buffer.c-6437- */
kernel/trace/ring_buffer.c:6438:void ring_buffer_iter_advance(struct ring_buffer_iter *iter)
kernel/trace/ring_buffer.c-6439-{
]
|
| 27/2 |
2026/08/04 00:42 |
llm |
expert |
0m
Model:
Tokens:
input: 23244
output: 40
thoughts: 23
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Iterator Allocation Conundrum**
Okay, so I'm digging into this `ring_buffer` code and I've hit a conceptual snag. Specifically, the whole iterator thing. My immediate thought is, "Right, how exactly are these iterators being allocated and, crucially, *managed*?" I'm thinking about the lifecycle β where are they born, how are they copied (if at all), and when are they finally relinquished? Garbage collection isn't my main worry; I'm assuming we're dealing with explicit memory management here. What's the thread-safety story, too?
To get to the bottom of this, the first thing I need to do is trace the initialization and subsequent manipulation of these iterators. I need to understand the "contract," if you will. The likely culprit is the `ring_buffer_read_start` function. Let's see what that reveals. That seems like the logical jumping-off point. It's probably responsible for the instantiation or first logical assignment of the reader's iterator. Let's see how that lays out.
|
| 28/2 |
2026/08/04 00:42 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:ring_buffer_read_start]
Results: map[SourceCode:6362: /**
6363: * ring_buffer_read_start - start a non consuming read of the buffer
6364: * @buffer: The ring buffer to read from
6365: * @cpu: The cpu buffer to iterate over
6366: * @flags: gfp flags to use for memory allocation
6367: *
6368: * This creates an iterator to allow non-consuming iteration through
6369: * the buffer. If the buffer is disabled for writing, it will produce
6370: * the same information each time, but if the buffer is still writing
6371: * then the first hit of a write will cause the iteration to stop.
6372: *
6373: * Must be paired with ring_buffer_read_finish.
6374: */
6375: struct ring_buffer_iter *
6376: ring_buffer_read_start(struct trace_buffer *buffer, int cpu, gfp_t flags)
6377: {
6378: struct ring_buffer_per_cpu *cpu_buffer;
6379: struct ring_buffer_iter *iter;
6380:
6381: if (!cpumask_test_cpu(cpu, buffer->cpumask))
6382: return NULL;
6383:
6384: iter = kzalloc_obj(*iter, flags);
6385: if (!iter)
6386: return NULL;
6387:
6388: /* Holds the entire event: data and meta data */
6389: iter->event_size = buffer->subbuf_size;
6390: iter->event = kmalloc(iter->event_size, flags);
6391: if (!iter->event) {
6392: kfree(iter);
6393: return NULL;
6394: }
6395:
6396: cpu_buffer = buffer->buffers[cpu];
6397:
6398: iter->cpu_buffer = cpu_buffer;
6399:
6400: atomic_inc(&cpu_buffer->resize_disabled);
6401:
6402: guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
6403: arch_spin_lock(&cpu_buffer->lock);
6404: rb_iter_reset(iter);
6405: arch_spin_unlock(&cpu_buffer->lock);
6406:
6407: return iter;
6408: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 29/2 |
2026/08/04 00:42 |
llm |
expert |
0m
Model:
Tokens:
input: 24271
output: 32
thoughts: 121
Thoughts: **Analyzing Ring Buffer Resize Disabling**
Okay, hold on a second. I need to focus on this `ring_buffer_read_start` function. Let me check the code... Aha! It's incrementing `cpu_buffer->resize_disabled`. That makes sense in a high-level way, preventing resizes while reading. But, does the setting of sub-buffer order, managed by `ring_buffer_subbuf_order_set`, actually respect this? Does it check `resize_disabled` at all? I need to go back and double-check that function, because that is where the potential race condition would occur.
Wait, I need to confirm: does `ring_buffer_subbuf_order_set` check `resize_disabled`? Let me search the `ring_buffer.c` file for every instance of `resize_disabled`. I need to ensure that the logic correctly prevents modification during a read.
|
| 30/2 |
2026/08/04 00:42 |
tool |
grepper |
0m
Args: map[Expression:resize_disabled PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:kernel/trace/ring_buffer.c=502=struct ring_buffer_per_cpu {
--
kernel/trace/ring_buffer.c-504- atomic_t record_disabled;
kernel/trace/ring_buffer.c:505: atomic_t resize_disabled;
kernel/trace/ring_buffer.c-506- struct trace_buffer *buffer;
--
kernel/trace/ring_buffer.c=2487=rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
--
kernel/trace/ring_buffer.c-2546- atomic_inc(&cpu_buffer->record_disabled);
kernel/trace/ring_buffer.c:2547: atomic_inc(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-2548-
--
kernel/trace/ring_buffer.c=3206=int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
--
kernel/trace/ring_buffer.c-3247- cpu_buffer = buffer->buffers[cpu];
kernel/trace/ring_buffer.c:3248: if (atomic_read(&cpu_buffer->resize_disabled)) {
kernel/trace/ring_buffer.c-3249- err = -EBUSY;
--
kernel/trace/ring_buffer.c-3329- */
kernel/trace/ring_buffer.c:3330: if (atomic_read(&cpu_buffer->resize_disabled)) {
kernel/trace/ring_buffer.c-3331- err = -EBUSY;
--
kernel/trace/ring_buffer.c=6376=ring_buffer_read_start(struct trace_buffer *buffer, int cpu, gfp_t flags)
--
kernel/trace/ring_buffer.c-6399-
kernel/trace/ring_buffer.c:6400: atomic_inc(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6401-
--
kernel/trace/ring_buffer.c=6418=ring_buffer_read_finish(struct ring_buffer_iter *iter)
--
kernel/trace/ring_buffer.c-6424-
kernel/trace/ring_buffer.c:6425: atomic_dec(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6426- kfree(iter->event);
--
kernel/trace/ring_buffer.c=6637=void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu)
--
kernel/trace/ring_buffer.c-6646-
kernel/trace/ring_buffer.c:6647: atomic_inc(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6648- atomic_inc(&cpu_buffer->record_disabled);
--
kernel/trace/ring_buffer.c-6655- atomic_dec(&cpu_buffer->record_disabled);
kernel/trace/ring_buffer.c:6656: atomic_dec(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6657-
--
kernel/trace/ring_buffer.c=6669=void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
--
kernel/trace/ring_buffer.c-6679-
kernel/trace/ring_buffer.c:6680: atomic_add(RESET_BIT, &cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6681- atomic_inc(&cpu_buffer->record_disabled);
--
kernel/trace/ring_buffer.c-6693- */
kernel/trace/ring_buffer.c:6694: if (!(atomic_read(&cpu_buffer->resize_disabled) & RESET_BIT))
kernel/trace/ring_buffer.c-6695- continue;
--
kernel/trace/ring_buffer.c-6699- atomic_dec(&cpu_buffer->record_disabled);
kernel/trace/ring_buffer.c:6700: atomic_sub(RESET_BIT, &cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6701- }
--
kernel/trace/ring_buffer.c=6710=void ring_buffer_reset(struct trace_buffer *buffer)
--
kernel/trace/ring_buffer.c-6720-
kernel/trace/ring_buffer.c:6721: atomic_inc(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6722- atomic_inc(&cpu_buffer->record_disabled);
--
kernel/trace/ring_buffer.c-6733- atomic_dec(&cpu_buffer->record_disabled);
kernel/trace/ring_buffer.c:6734: atomic_dec(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6735- }
--
kernel/trace/ring_buffer.c=7712=int ring_buffer_map(struct trace_buffer *buffer, int cpu,
--
kernel/trace/ring_buffer.c-7747-
kernel/trace/ring_buffer.c:7748: atomic_inc(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-7749-
--
kernel/trace/ring_buffer.c-7769- rb_free_meta_page(cpu_buffer);
kernel/trace/ring_buffer.c:7770: atomic_dec(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-7771- }
--
kernel/trace/ring_buffer.c=7797=int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
--
kernel/trace/ring_buffer.c-7828- rb_free_meta_page(cpu_buffer);
kernel/trace/ring_buffer.c:7829: atomic_dec(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-7830-
]
|
| 31/2 |
2026/08/04 00:42 |
llm |
expert |
0m
Model:
Tokens:
input: 26197
output: 39
thoughts: 624
Thoughts: **A Critical Oversight in Ring Buffer Resizing**
Okay, so I've been looking closely at the `ring_buffer_subbuf_order_set` function and something's really standing out. I just realized that it *doesn't* check the `resize_disabled` flag! That's a huge issue, isn't it? Let me go over the code again. Right, it takes a mutex, increments the `record_disabled` counter, synchronizes, sets the `subbuf_order` and `subbuf_size`, and then iterates through the CPUs. It *doesn't* validate the `resize_disabled` flag.
Wait, isn't changing the subbuf order effectively resizing/reallocating the buffer pages? That seems like a glaring omission. I was expecting to see a check against `resize_disabled` somewhere, but there isn't one. Let me be absolutely sure; okay, I've checked and rechecked, and there's *no* check for `resize_disabled` in `ring_buffer_subbuf_order_set`. The function *does* check `cpu_buffer->mapped`, but that's not the same thing. It is missing the crucial `atomic_read(&cpu_buffer->resize_disabled)` check.
Now, let's compare that to `ring_buffer_resize`. Okay, `ring_buffer_resize` *does* check `resize_disabled` for each CPU buffer *before* attempting the resize. It uses the `atomic_read(&cpu_buffer->resize_disabled)` calls as expected. And, within `ring_buffer_resize`, it makes sure that `cpu_buffer->resize_disabled` is not set *before* allocating the buffer pages.
So, `ring_buffer_resize` respects the `resize_disabled` flag, but `ring_buffer_subbuf_order_set` completely bypasses it. That's a pretty serious inconsistency. I need to figure out how long this has been going on, or if there is any reason that this is happening. I need to run a `git log` to check the history of this function.
|
| 32/2 |
2026/08/04 00:42 |
tool |
git-log |
7m
Args: map[CodeRegexp:ring_buffer_subbuf_order_set Count:10 Since:3 years]
Results: map[Output:bade44fe5462 tracing: Move snapshot code out of trace.c and into trace_snapshot.c
694b3f6fe0b6 tracing: Rename trace_array field max_buffer to snapshot_buffer
2f84b39f4847 tracing: Update subbuffer with kilobytes not page order
aa067682adf1 tracing: Update snapshot order along with main buffer order
2808e31ec12e ring-buffer: Add interface for configuring trace sub buffer size
]
|
| 33/2 |
2026/08/04 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 27452
output: 26
thoughts: 33
Thoughts: Okay, here's my take on those thoughts, framed as my own:
**Analyzing Commit 2808e31ec12e: Introducing `ring_buffer_subbuf_order_set`**
Alright, let's pull up that specific commit, `2808e31ec12e`. The name, `ring_buffer_subbuf_order_set`, is immediately intriguing. Given my understanding of ring buffers and sub-buffers, my initial hypothesis is that this function likely deals with configuring the *ordering* or *allocation policy* of sub-buffers within a ring buffer. I'm guessing this might involve setting some sort of order parameter that influences how the sub-buffers are managed. Perhaps it relates to allocation sizes, memory layout, or perhaps even a specific priority scheme for access or re-use. I'm keen to dive in and see how they implemented this. Is it a bitwise flag-based approach, a lookup table, or something more sophisticated? What other supporting functions were added or modified in the same commit, and how did they all combine to achieve whatever this new functionality provided? Let's check the code and see whatβs going on.
|
| 34/2 |
2026/08/04 00:50 |
tool |
git-show |
0m
Args:
Results: map[Output:commit 2808e31ec12e5fbe2ae25acc027fcdc67b1fb7f0
Author: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Date: Tue Dec 19 13:54:17 2023 -0500
ring-buffer: Add interface for configuring trace sub buffer size
The trace ring buffer sub page size can be configured, per trace
instance. A new ftrace file "buffer_subbuf_order" is added to get and
set the size of the ring buffer sub page for current trace instance.
The size must be an order of system page size, that's why the new
interface works with system page order, instead of absolute page size:
0 means the ring buffer sub page is equal to 1 system page and so
forth:
0 - 1 system page
1 - 2 system pages
2 - 4 system pages
...
The ring buffer sub page size is limited between 1 and 128 system
pages. The default value is 1 system page.
New ring buffer APIs are introduced:
ring_buffer_subbuf_order_set()
ring_buffer_subbuf_order_get()
ring_buffer_subbuf_size_get()
Link: https://lore.kernel.org/linux-trace-devel/20211213094825.61876-4-tz.stoyanov@gmail.com
Link: https://lore.kernel.org/linux-trace-kernel/20231219185628.298324722@goodmis.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vincent Donnefort <vdonnefort@google.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index ce46218ce46d..12573306b889 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -202,6 +202,10 @@ struct trace_seq;
int ring_buffer_print_entry_header(struct trace_seq *s);
int ring_buffer_print_page_header(struct trace_buffer *buffer, struct trace_seq *s);
+int ring_buffer_subbuf_order_get(struct trace_buffer *buffer);
+int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order);
+int ring_buffer_subbuf_size_get(struct trace_buffer *buffer);
+
enum ring_buffer_flags {
RB_FL_OVERWRITE = 1 << 0,
};
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index d9f656502400..20fc0121735d 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -507,6 +507,7 @@ struct trace_buffer {
bool time_stamp_abs;
unsigned int subbuf_size;
+ unsigned int subbuf_order;
unsigned int max_data_size;
};
@@ -5761,6 +5762,78 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
}
EXPORT_SYMBOL_GPL(ring_buffer_read_page);
+/**
+ * ring_buffer_subbuf_size_get - get size of the sub buffer.
+ * @buffer: the buffer to get the sub buffer size from
+ *
+ * Returns size of the sub buffer, in bytes.
+ */
+int ring_buffer_subbuf_size_get(struct trace_buffer *buffer)
+{
+ return buffer->subbuf_size + BUF_PAGE_HDR_SIZE;
+}
+EXPORT_SYMBOL_GPL(ring_buffer_subbuf_size_get);
+
+/**
+ * ring_buffer_subbuf_order_get - get order of system sub pages in one buffer page.
+ * @buffer: The ring_buffer to get the system sub page order from
+ *
+ * By default, one ring buffer sub page equals to one system page. This parameter
+ * is configurable, per ring buffer. The size of the ring buffer sub page can be
+ * extended, but must be an order of system page size.
+ *
+ * Returns the order of buffer sub page size, in system pages:
+ * 0 means the sub buffer size is 1 system page and so forth.
+ * In case of an error < 0 is returned.
+ */
+int ring_buffer_subbuf_order_get(struct trace_buffer *buffer)
+{
+ if (!buffer)
+ return -EINVAL;
+
+ return buffer->subbuf_order;
+}
+EXPORT_SYMBOL_GPL(ring_buffer_subbuf_order_get);
+
+/**
+ * ring_buffer_subbuf_order_set - set the size of ring buffer sub page.
+ * @buffer: The ring_buffer to set the new page size.
+ * @order: Order of the system pages in one sub buffer page
+ *
+ * By default, one ring buffer pages equals to one system page. This API can be
+ * used to set new size of the ring buffer page. The size must be order of
+ * system page size, that's why the input parameter @order is the order of
+ * system pages that are allocated for one ring buffer page:
+ * 0 - 1 system page
+ * 1 - 2 system pages
+ * 3 - 4 system pages
+ * ...
+ *
+ * Returns 0 on success or < 0 in case of an error.
+ */
+int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
+{
+ int psize;
+
+ if (!buffer || order < 0)
+ return -EINVAL;
+
+ if (buffer->subbuf_order == order)
+ return 0;
+
+ psize = (1 << order) * PAGE_SIZE;
+ if (psize <= BUF_PAGE_HDR_SIZE)
+ return -EINVAL;
+
+ buffer->subbuf_order = order;
+ buffer->subbuf_size = psize - BUF_PAGE_HDR_SIZE;
+
+ /* Todo: reset the buffer with the new page size */
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(ring_buffer_subbuf_order_set);
+
/*
* We only allocate new buffers, never free them if the CPU goes down.
* If we were to free the buffer, then the user would lose any trace that was in
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 76dd0a4c8cb5..a010aba4c4a4 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9358,6 +9358,51 @@ static const struct file_operations buffer_percent_fops = {
.llseek = default_llseek,
};
+static ssize_t
+buffer_order_read(struct file *filp, char __user *ubuf, size_t cnt, loff_t *ppos)
+{
+ struct trace_array *tr = filp->private_data;
+ char buf[64];
+ int r;
+
+ r = sprintf(buf, "%d\n", ring_buffer_subbuf_order_get(tr->array_buffer.buffer));
+
+ return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
+}
+
+static ssize_t
+buffer_order_write(struct file *filp, const char __user *ubuf,
+ size_t cnt, loff_t *ppos)
+{
+ struct trace_array *tr = filp->private_data;
+ unsigned long val;
+ int ret;
+
+ ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
+ if (ret)
+ return ret;
+
+ /* limit between 1 and 128 system pages */
+ if (val < 0 || val > 7)
+ return -EINVAL;
+
+ ret = ring_buffer_subbuf_order_set(tr->array_buffer.buffer, val);
+ if (ret)
+ return ret;
+
+ (*ppos)++;
+
+ return cnt;
+}
+
+static const struct file_operations buffer_order_fops = {
+ .open = tracing_open_generic_tr,
+ .read = buffer_order_read,
+ .write = buffer_order_write,
+ .release = tracing_release_generic_tr,
+ .llseek = default_llseek,
+};
+
static struct dentry *trace_instance_dir;
static void
@@ -9824,6 +9869,9 @@ init_tracer_tracefs(struct trace_array *tr, struct dentry *d_tracer)
trace_create_file("buffer_percent", TRACE_MODE_WRITE, d_tracer,
tr, &buffer_percent_fops);
+ trace_create_file("buffer_subbuf_order", TRACE_MODE_WRITE, d_tracer,
+ tr, &buffer_order_fops);
+
create_trace_options_dir(tr);
#ifdef CONFIG_TRACER_MAX_TRACE
]
|
| 35/2 |
2026/08/04 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 30351
output: 54
thoughts: 50
Thoughts: Okay, let's dive into this. Here's how I'm thinking about it:
**Analyzing `ring_buffer_subbuf_order_set`**
Alright, I need to get a clear picture of what's going on with `ring_buffer_subbuf_order_set`. First things first, I'm going to pull up the current implementation in `kernel/trace/ring_buffer.c`. I want to refresh my memory on the exact code, the data structures it operates on, and the logic it employs. I need a solid understanding of the *current* state before I start making assumptions or looking for issues.
Once I've got that fresh in my mind, the next thing I'll do is check the commit history. I want to see if there have been any recent modifications to this function. Understanding what changes have been made will give me critical context. Were there bug fixes? Performance optimizations? New features added that might have inadvertently introduced a problem or changed the intended behavior? Knowing the recent history will help me pinpoint the potential root cause of any problems faster, especially if there was a recent modification. I also want to evaluate the design rationale behind any such changes and their potential implications.
|
| 36/2 |
2026/08/04 00:50 |
tool |
git-log |
0m
Args: map[CodeRegexp:ring_buffer_subbuf_order_set Count:10 PathPrefix:kernel/trace/ring_buffer.c Since:3 years]
Results: map[Output:2808e31ec12e ring-buffer: Add interface for configuring trace sub buffer size
]
|
| 37/2 |
2026/08/04 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 30747
output: 35
thoughts: 163
Thoughts: **Investigating Ring Buffer Allocation Issues**
Okay, let's take a closer look at what's happening with the ring buffer. I need to examine the history of `kernel/trace/ring_buffer.c`, specifically around the functions `ring_buffer_subbuf_order_set` or `buffer_subbuf_size_write`. I suspect there might be something going on there with sub-buffer allocation.
Hmm, the allocation and free stacks are showing calls to `ring_buffer_subbuf_order_set` at different offsets within `kernel/trace/ring_buffer.c`. I'm seeing `+0x3ef` at line 7379 and `+0x116b` at line 7439. That could be a crucial clue. It suggests we're getting into this function from different places, maybe with different parameters.
Now, let's broaden the search a bit. I'll search for `buffer_subbuf_size_write` in `kernel/trace/trace.c` to see if it's related. Hopefully, the surrounding code will shed some light on the ring buffer memory allocation scheme.
|
| 38/2 |
2026/08/04 00:50 |
tool |
grepper |
0m
Args: map[Expression:buffer_subbuf_size_write PathPrefix:kernel/trace/trace.c]
Results: map[Output:kernel/trace/trace.c=8190=static ssize_t
kernel/trace/trace.c:8191:buffer_subbuf_size_write(struct file *filp, const char __user *ubuf,
kernel/trace/trace.c-8192- size_t cnt, loff_t *ppos)
--
kernel/trace/trace.c=8262=static const struct file_operations buffer_subbuf_size_fops = {
--
kernel/trace/trace.c-8264- .read = buffer_subbuf_size_read,
kernel/trace/trace.c:8265: .write = buffer_subbuf_size_write,
kernel/trace/trace.c-8266- .release = tracing_release_generic_tr,
]
|
| 39/2 |
2026/08/04 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 31409
output: 39
thoughts: 24
Thoughts: Here's my analysis of the provided context, presented as a personal thought process:
**Diving into `buffer_subbuf_size_write`**
Okay, time to get my hands dirty and jump into the `kernel/trace/trace.c` file. Specifically, I need to focus on the `buffer_subbuf_size_write` function. Based on the name, I'm expecting this function deals with the size of subbuffers related to tracing within the kernel. I'm already anticipating some of the nuances I might find: how is the size determined? Is there dynamic allocation? What are the potential impacts on performance and memory usage when writing trace data? Are there any locking mechanisms involved to protect concurrent access to these subbuffers? I'll be looking for how this function interacts with the overall tracing infrastructure, how it handles potential errors, and if there are any optimizations in place for frequent write operations. This should be an interesting piece of code to dissect.
|
| 40/2 |
2026/08/04 00:50 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/trace.c Name:buffer_subbuf_size_write]
Results: map[SourceCode:8190: static ssize_t
8191: buffer_subbuf_size_write(struct file *filp, const char __user *ubuf,
8192: size_t cnt, loff_t *ppos)
8193: {
8194: struct trace_array *tr = filp->private_data;
8195: unsigned long val;
8196: int old_order;
8197: int order;
8198: int pages;
8199: int ret;
8200:
8201: ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
8202: if (ret)
8203: return ret;
8204:
8205: val *= 1024; /* value passed in is in KB */
8206:
8207: pages = DIV_ROUND_UP(val, PAGE_SIZE);
8208: order = fls(pages - 1);
8209:
8210: /* limit between 1 and 128 system pages */
8211: if (order < 0 || order > 7)
8212: return -EINVAL;
8213:
8214: /* Do not allow tracing while changing the order of the ring buffer */
8215: tracing_stop_tr(tr);
8216:
8217: old_order = ring_buffer_subbuf_order_get(tr->array_buffer.buffer);
8218: if (old_order == order)
8219: goto out;
8220:
8221: ret = ring_buffer_subbuf_order_set(tr->array_buffer.buffer, order);
8222: if (ret)
8223: goto out;
8224:
8225: #ifdef CONFIG_TRACER_SNAPSHOT
8226:
8227: if (!tr->allocated_snapshot)
8228: goto out_max;
8229:
8230: ret = ring_buffer_subbuf_order_set(tr->snapshot_buffer.buffer, order);
8231: if (ret) {
8232: /* Put back the old order */
8233: cnt = ring_buffer_subbuf_order_set(tr->array_buffer.buffer, old_order);
8234: if (WARN_ON_ONCE(cnt)) {
8235: /*
8236: * AARGH! We are left with different orders!
8237: * The max buffer is our "snapshot" buffer.
8238: * When a tracer needs a snapshot (one of the
8239: * latency tracers), it swaps the max buffer
8240: * with the saved snap shot. We succeeded to
8241: * update the order of the main buffer, but failed to
8242: * update the order of the max buffer. But when we tried
8243: * to reset the main buffer to the original size, we
8244: * failed there too. This is very unlikely to
8245: * happen, but if it does, warn and kill all
8246: * tracing.
8247: */
8248: tracing_disabled = 1;
8249: }
8250: goto out;
8251: }
8252: out_max:
8253: #endif
8254: (*ppos)++;
8255: out:
8256: if (ret)
8257: cnt = ret;
8258: tracing_start_tr(tr);
8259: return cnt;
8260: }
SourceFile:kernel/trace/trace.c]
|
| 41/2 |
2026/08/04 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 32716
output: 36
thoughts: 95
Thoughts: **Analyzing the Write Buffer Logic**
Okay, so I'm looking at this `buffer_subbuf_size_write` function. Seems like the flow is: first, it calls `tracing_stop_tr(tr)`. Then, it sets the buffer order with `ring_buffer_subbuf_order_set(tr->array_buffer.buffer, order)`. Finally, it restarts the trace with `tracing_start_tr(tr)`. Interesting⦠but hold on. What exactly *does* `tracing_stop_tr(tr)` do? Is it suspending some sort of iterator, or something similar? This part is crucial for understanding the impact on the ring buffer and the overall tracing process. Gotta dig into that function definition now.
|
| 42/2 |
2026/08/04 00:50 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/trace.c Name:tracing_stop_tr]
Results: map[SourceCode:1657: static void tracing_stop_tr(struct trace_array *tr)
1658: {
1659: struct trace_buffer *buffer;
1660:
1661: guard(raw_spinlock_irqsave)(&tr->start_lock);
1662: if (tr->stop_count++)
1663: return;
1664:
1665: /* Prevent the buffers from switching */
1666: arch_spin_lock(&tr->max_lock);
1667:
1668: buffer = tr->array_buffer.buffer;
1669: if (buffer)
1670: ring_buffer_record_disable(buffer);
1671:
1672: #ifdef CONFIG_TRACER_SNAPSHOT
1673: buffer = tr->snapshot_buffer.buffer;
1674: if (buffer)
1675: ring_buffer_record_disable(buffer);
1676: #endif
1677:
1678: arch_spin_unlock(&tr->max_lock);
1679: }
SourceFile:kernel/trace/trace.c]
|
| 43/2 |
2026/08/04 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 33336
output: 34
thoughts: 223
Thoughts: **Analyzing a Tracing-Related Memory Access Issue**
Okay, so `tracing_stop_tr` just disables recording, it doesn't actually halt iterators in progress. I need to keep that in mind. Right, let's go back and dissect that memory access stack trace, again. The task involved in the problematic read is `syz.4.389/6869`. It's calling `seq_read` on a trace file, like those under `/sys/kernel/tracing/`. Now I see the call chain: `trace_find_next_entry_inc` within `kernel/trace/trace.c`, then `s_next`, also in `kernel/trace/trace.c`, which leads to `seq_read_iter` in `fs/seq_file.c`, and finally `seq_read` in `fs/seq_file.c`. Hmm, I should probably take a closer look at `s_next` in `kernel/trace/trace.c`.
|
| 44/2 |
2026/08/04 00:50 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/trace.c Name:s_next]
Results: map[SourceCode:2691: static void *s_next(struct seq_file *m, void *v, loff_t *pos)
2692: {
2693: struct trace_iterator *iter = m->private;
2694: int i = (int)*pos;
2695: void *ent;
2696:
2697: WARN_ON_ONCE(iter->leftover);
2698:
2699: (*pos)++;
2700:
2701: /* can't go backwards */
2702: if (iter->idx > i)
2703: return NULL;
2704:
2705: if (iter->idx < 0)
2706: ent = trace_find_next_entry_inc(iter);
2707: else
2708: ent = iter;
2709:
2710: while (ent && iter->idx < i)
2711: ent = trace_find_next_entry_inc(iter);
2712:
2713: iter->pos = *pos;
2714:
2715: return ent;
2716: }
SourceFile:kernel/trace/trace.c]
|
| 45/2 |
2026/08/04 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 34168
output: 40
thoughts: 17
Thoughts: Okay, let's dive into this. Here's my take on those thoughts, as if they're my own:
**Analyzing `trace_find_next_entry_inc`**
Alright, so we're looking at `trace_find_next_entry_inc`. My initial thought is to immediately consider its context within the tracing framework. I want to understand its role in iterating through the trace entries. Specifically, what's its relationship to the overall data structure that holds the trace information? Is it a key function in a linked list implementation, an array, or something more complex? I need to think about efficiency here. How does this function handle potential gaps or non-sequential entries within the trace? Are there any potential performance bottlenecks associated with its incrementing behavior? I also immediately start to think about the locking mechanisms used to ensure thread safety when accessing trace data. I'll need to know whether its handling of concurrency is proper. Also, are we incrementing to the *next* entry, or the next available one? Is there a subtle difference there that impacts how this is used? I'm going to want to confirm its behavior around the end of the trace, and any special considerations for wrap-around or circular buffers. These are the immediate questions that come to mind. I'm already formulating a mental checklist to analyze its logic and performance characteristics.
|
| 46/2 |
2026/08/04 00:50 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/trace.c Name:trace_find_next_entry_inc]
Results: map[SourceCode:2667: /* Find the next real entry, and increment the iterator to the next entry */
2668: void *trace_find_next_entry_inc(struct trace_iterator *iter)
2669: {
2670: struct ring_buffer_iter *buf_iter;
2671:
2672: iter->ent = __find_next_entry(iter, &iter->cpu,
2673: &iter->lost_events, &iter->ts);
2674:
2675: if (iter->ent) {
2676: iter->idx++;
2677: buf_iter = trace_buffer_iter(iter, iter->cpu);
2678: if (buf_iter)
2679: ring_buffer_iter_advance(buf_iter);
2680: }
2681:
2682: return iter->ent ? iter : NULL;
2683: }
SourceFile:kernel/trace/trace.c]
|
| 47/2 |
2026/08/04 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 34792
output: 34
thoughts: 137
Thoughts: **Diving into the Trace Iterator**
Okay, so I'm tracing through this code, and it looks like `trace_find_next_entry_inc` is calling `ring_buffer_iter_advance(buf_iter)`. Makes sense. Now, `buf_iter` is a `struct ring_buffer_iter`, which is what I'd expect. But I'm suddenly curious: how exactly is this `trace_iterator` constructed in the first place? I need to understand the lifecycle here.
Right, I seem to recall that when a user opens that `/sys/kernel/tracing/trace` file, the kernel calls `tracing_open_generic_tr` or something similar, which, if memory serves me right, allocates a `trace_iterator` structure and sets up the `ring_buffer_iter` for each CPU using `ring_buffer_read_start`. That's where I need to look. I'll check `tracing_open_generic_tr` or whatever the open function is in the tracing code now. I need to see how these iterators are initialized.
|
| 48/2 |
2026/08/04 00:51 |
tool |
grepper |
0m
Args: map[Expression:ring_buffer_read_start PathPrefix:kernel/trace/trace.c]
Results: map[Output:kernel/trace/trace.c=3367=__tracing_open(struct inode *inode, struct file *file, bool snapshot)
--
kernel/trace/trace.c-3449- iter->buffer_iter[cpu] =
kernel/trace/trace.c:3450: ring_buffer_read_start(iter->array_buffer->buffer,
kernel/trace/trace.c-3451- cpu, GFP_KERNEL);
--
kernel/trace/trace.c-3456- iter->buffer_iter[cpu] =
kernel/trace/trace.c:3457: ring_buffer_read_start(iter->array_buffer->buffer,
kernel/trace/trace.c-3458- cpu, GFP_KERNEL);
]
|
| 49/2 |
2026/08/04 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 35400
output: 32
thoughts: 1226
Thoughts: **A Deep Dive into a Use-After-Free Bug in Ring Buffer Resizing**
Okay, here's what I've got so far. It starts with a hunch and solidifies into a clear understanding of a critical bug in the ring buffer resizing mechanism. I was looking at `__tracing_open`, which calls `ring_buffer_read_start`. That function increments `cpu_buffer->resize_disabled`. Makes sense, we don't want to resize while someone is reading.
But now, I'm digging into `ring_buffer_subbuf_order_set`. And it *doesn't* check `cpu_buffer->resize_disabled`! Even though the resize is disabled, `ring_buffer_subbuf_order_set` goes ahead and frees the old pages while there might still be an active iterator. That felt wrong, so I had to verify that it *should* check the resize disable. I went back and checked `ring_buffer_resize`, which *does* check `cpu_buffer->resize_disabled` and returns -EBUSY if it's set. So, the resize function honors the disable, but `ring_buffer_subbuf_order_set` doesn't?
I scrutinized the entire `ring_buffer_subbuf_order_set` function again, and there's no check for `resize_disabled`. I also verified there are no other functions that might be involved. Yes, `ring_buffer_resize` honors it. No, `ring_buffer_subbuf_order_set` does not.
Wait. Is this a real bug?
Yes! It's a classic use-after-free scenario. One thread is reading the trace, which sets `resize_disabled` and creates an iterator. Another thread is writing to `buffer_subbuf_size`, triggering `ring_buffer_subbuf_order_set`. Since `ring_buffer_subbuf_order_set` skips the `resize_disabled` check, it cheerfully frees the old buffer pages. Then, the reader thread advances its iterator (`ring_buffer_iter_advance`), accessing `iter->head_page`. Boom! That's a use-after-free because the page has been freed!
Okay, time to double-check my hypothesis. Is there some other lock or synchronization that could prevent this? Let's trace it through. Let's look at locks.
Reader: `seq_read` -> `s_next` -> `trace_find_next_entry_inc` -> `ring_buffer_iter_advance` -> `raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags)`
Writer: `buffer_subbuf_size_write` -> `ring_buffer_subbuf_order_set` -> `guard(mutex)(&buffer->mutex)`
Digging deeper, the writer acquires `cpu_buffer->reader_lock`, replaces the pages, *releases* the `cpu_buffer->reader_lock`, and *then* frees the old pages. The reader acquires the same `cpu_buffer->reader_lock` in `ring_buffer_iter_advance`. The writer is long gone when the reader acquires the same lock. It's too late. The reader's `iter->head_page` still points to one of those now-freed pages! This is a clean, precise, and beautiful root cause, confirmed.
I will need to verify if there's any other place that checks `resize_disabled` or if there's any other way to prevent this. I am going to see if `ring_buffer_subbuf_order_set` should return `-EBUSY` when there is an active reader if it checked `resize_disabled`. Yes! If `resize_disabled` is set, the buffer cannot be resized because someone is iterating. I searched the kernel code for comments about `resize_disabled`, and this is confirmed.
|
| 50/2 |
2026/08/04 00:51 |
tool |
grepper |
0m
Args: map[Expression:resize_disabled PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:kernel/trace/ring_buffer.c=502=struct ring_buffer_per_cpu {
--
kernel/trace/ring_buffer.c-504- atomic_t record_disabled;
kernel/trace/ring_buffer.c:505: atomic_t resize_disabled;
kernel/trace/ring_buffer.c-506- struct trace_buffer *buffer;
--
kernel/trace/ring_buffer.c=2487=rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
--
kernel/trace/ring_buffer.c-2546- atomic_inc(&cpu_buffer->record_disabled);
kernel/trace/ring_buffer.c:2547: atomic_inc(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-2548-
--
kernel/trace/ring_buffer.c=3206=int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
--
kernel/trace/ring_buffer.c-3247- cpu_buffer = buffer->buffers[cpu];
kernel/trace/ring_buffer.c:3248: if (atomic_read(&cpu_buffer->resize_disabled)) {
kernel/trace/ring_buffer.c-3249- err = -EBUSY;
--
kernel/trace/ring_buffer.c-3329- */
kernel/trace/ring_buffer.c:3330: if (atomic_read(&cpu_buffer->resize_disabled)) {
kernel/trace/ring_buffer.c-3331- err = -EBUSY;
--
kernel/trace/ring_buffer.c=6376=ring_buffer_read_start(struct trace_buffer *buffer, int cpu, gfp_t flags)
--
kernel/trace/ring_buffer.c-6399-
kernel/trace/ring_buffer.c:6400: atomic_inc(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6401-
--
kernel/trace/ring_buffer.c=6418=ring_buffer_read_finish(struct ring_buffer_iter *iter)
--
kernel/trace/ring_buffer.c-6424-
kernel/trace/ring_buffer.c:6425: atomic_dec(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6426- kfree(iter->event);
--
kernel/trace/ring_buffer.c=6637=void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu)
--
kernel/trace/ring_buffer.c-6646-
kernel/trace/ring_buffer.c:6647: atomic_inc(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6648- atomic_inc(&cpu_buffer->record_disabled);
--
kernel/trace/ring_buffer.c-6655- atomic_dec(&cpu_buffer->record_disabled);
kernel/trace/ring_buffer.c:6656: atomic_dec(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6657-
--
kernel/trace/ring_buffer.c=6669=void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
--
kernel/trace/ring_buffer.c-6679-
kernel/trace/ring_buffer.c:6680: atomic_add(RESET_BIT, &cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6681- atomic_inc(&cpu_buffer->record_disabled);
--
kernel/trace/ring_buffer.c-6693- */
kernel/trace/ring_buffer.c:6694: if (!(atomic_read(&cpu_buffer->resize_disabled) & RESET_BIT))
kernel/trace/ring_buffer.c-6695- continue;
--
kernel/trace/ring_buffer.c-6699- atomic_dec(&cpu_buffer->record_disabled);
kernel/trace/ring_buffer.c:6700: atomic_sub(RESET_BIT, &cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6701- }
--
kernel/trace/ring_buffer.c=6710=void ring_buffer_reset(struct trace_buffer *buffer)
--
kernel/trace/ring_buffer.c-6720-
kernel/trace/ring_buffer.c:6721: atomic_inc(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6722- atomic_inc(&cpu_buffer->record_disabled);
--
kernel/trace/ring_buffer.c-6733- atomic_dec(&cpu_buffer->record_disabled);
kernel/trace/ring_buffer.c:6734: atomic_dec(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-6735- }
--
kernel/trace/ring_buffer.c=7712=int ring_buffer_map(struct trace_buffer *buffer, int cpu,
--
kernel/trace/ring_buffer.c-7747-
kernel/trace/ring_buffer.c:7748: atomic_inc(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-7749-
--
kernel/trace/ring_buffer.c-7769- rb_free_meta_page(cpu_buffer);
kernel/trace/ring_buffer.c:7770: atomic_dec(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-7771- }
--
kernel/trace/ring_buffer.c=7797=int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
--
kernel/trace/ring_buffer.c-7828- rb_free_meta_page(cpu_buffer);
kernel/trace/ring_buffer.c:7829: atomic_dec(&cpu_buffer->resize_disabled);
kernel/trace/ring_buffer.c-7830-
]
|
| 51/2 |
2026/08/04 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 39076
output: 38
thoughts: 23
Thoughts: Okay, here's my summary, taking the perspective of someone deeply familiar with code and the specific function `ring_buffer_resize`:
**Investigating `resize_disabled` in `ring_buffer_resize`**
Alright, let's dive in. I need to quickly determine the impact of `resize_disabled` within the `ring_buffer_resize` function. My instinct is to check for any existing comments. Comments are my best friend here. They'll hopefully clarify the intended behavior when that flag is set, or if there is any nuance with edge cases or subtle dependencies I might overlook. I'm really looking for any indication of *how* `resize_disabled` modifies the normal resize behavior. Is it a simple skip, or does it trigger error handling or perhaps a fallback mechanism? I'm hoping I don't have to wade through the code itself for too long; a good, concise comment would save me time.
|
| 52/2 |
2026/08/04 00:51 |
tool |
read-file |
0m
Args: map[File:kernel/trace/ring_buffer.c FirstLine:3240 LineCount:20]
Results: map[Contents:3240: if (cpu_id == RING_BUFFER_ALL_CPUS) {
3241: /*
3242: * Don't succeed if resizing is disabled, as a reader might be
3243: * manipulating the ring buffer and is expecting a sane state while
3244: * this is true.
3245: */
3246: for_each_buffer_cpu(buffer, cpu) {
3247: cpu_buffer = buffer->buffers[cpu];
3248: if (atomic_read(&cpu_buffer->resize_disabled)) {
3249: err = -EBUSY;
3250: goto out_err_unlock;
3251: }
3252: }
3253:
3254: /* calculate the pages to update */
3255: for_each_buffer_cpu(buffer, cpu) {
3256: cpu_buffer = buffer->buffers[cpu];
3257:
3258: cpu_buffer->nr_pages_to_update = nr_pages -
3259: cpu_buffer->nr_pages;
]
|
| 53/2 |
2026/08/04 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 39645
output: 15
thoughts: 635
Thoughts: **Analyzing Ring Buffer Resizing and Concurrency**
Okay, so I've zeroed in on a subtle concurrency issue within the ring buffer resizing mechanism. The comment immediately caught my eye: "Don't succeed if resizing is disabled, as a reader might be manipulating the ring buffer and is expecting a sane state while this is true." That's the crux of it! And changing the subbuffer size or order *is* a form of resizing.
My instinct is to scrutinize `ring_buffer_subbuf_order_set` and see if the `resize_disabled` flag is being respected properly on a per-CPU basis. It looks like I was right! The code in `ring_buffer_subbuf_order_set` already attempts to prevent resizing if a CPU buffer has `resize_disabled` set. Good. But let's see where else it's used.
The code checks if `cpu_buffer->mapped` is set, but it looks like `cpu_buffer->mapped` is also protected by `resize_disabled`, since `ring_buffer_map` also increments `resize_disabled`. That means the current check would also cover the mapped case! That's excellent - a little more robust than I thought.
Now, let me just confirm that `resize_disabled` is incremented/decremented in other key functions like `ring_buffer_reset_cpu`, `ring_buffer_reset_online_cpus`, and `ring_buffer_reset`. That further cements my confidence in this approach. Yes, it's used consistently across these functions. This looks like a solid and correct solution.
Okay, I've got a clear picture of the bug and how to fix it. This is definitely actionable. Time to construct a comprehensive explanation for the patch!
|
| 54/2 |
2026/08/04 00:51 |
tool |
set-results |
0m
Args:
Results:
|
| 55/2 |
2026/08/04 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 40686
output: 854
thoughts: 649
Thoughts: **Final Thoughts on the Bug Report Analysis**
Okay, time to put the finishing touches on this response. Let's break down what we've got here. First, the analysis is very solid, and it's crystal clear.
Looking at the summary, the bug report itself is spot on. We're dealing with a KASAN slab-use-after-free, and the report lays out the evidence perfectly. I'm pleased that it outlines all the traces we need: The bad memory access (specifically `rb_page_size` called in the `rb_advance_iter` function during iteration). Then, we have the allocation and deallocation calls with the free operation `free_buffer_page` taking place during buffer configuration. These are all highly consistent. We're dealing with the same subsystem β `kernel/trace/ring_buffer.c` β and the same object type, a `struct buffer_page` of 64 bytes, allocated using `kmalloc-64`. The offset of the bad access, 56 bytes, hits that `page` field precisely.
Next, the root cause is explained. It makes total sense. When we open a trace file, `__tracing_open` gets called, and an iterator is created that calls `ring_buffer_read_start` for each CPU. This increments the `cpu_buffer->resize_disabled` flag, which is supposed to prevent resizing or freeing pages while the iterator is active. The problem is when someone messes with the `buffer_subbuf_size` (calling `ring_buffer_subbuf_order_set`). The code changes the sub-buffer order/size and reallocates the ring buffer pages. The critical flaw here is that, unlike `ring_buffer_resize`, `ring_buffer_subbuf_order_set` doesn't check `cpu_buffer->resize_disabled`. So, it goes ahead and replaces, then frees, the old `struct buffer_page` objects. When the reader then tries to advance the iterator, we get a use-after-free because `iter->head_page` now points to memory that's been freed.
The proposed fix is straightforward and elegant. In `ring_buffer_subbuf_order_set`, we need to add a check for `atomic_read(&cpu_buffer->resize_disabled)`. If any CPU buffer has this set, we return `-EBUSY`. This is exactly what `ring_buffer_resize` already does to prevent resizing while readers are active. It provides a simple and familiar solution, and should fix this bug and prevent future occurrences of this issue.
Given the clarity and actionability of this report, I am satisfied. I will now integrate this clearly into the final response.
|
| 56/1 |
2026/08/04 00:51 |
action |
format-explanation |
0m
Results: map[Explanation:The KASAN use-after-free bug report is **fully actionable and self-consistent**.
### 1. Analysis of the Stack Traces
The report contains all 3 required stack traces, which are highly consistent and
point to the same object type and subsystem:
* **Bad memory access stack**: Occurs in `rb_page_size` (inlined in
`rb_advance_iter`) when advancing a ring buffer iterator
(`ring_buffer_iter_advance`) during a read of the trace file (`seq_read`).
* **Allocation stack**: Occurs in `__rb_allocate_pages` (called from
`ring_buffer_subbuf_order_set`) when allocating new `struct buffer_page` objects
to change the sub-buffer size/order.
* **Free stack**: Occurs in `free_buffer_page` (called from
`ring_buffer_subbuf_order_set`) when freeing the old `struct buffer_page`
objects after the sub-buffer size/order update.
The buggy address is at offset 56 within a 64-byte `kmalloc-64` slab object.
This matches the layout of `struct buffer_page` perfectly, where the `page`
field is located at offset 56 (448 bits):
```c
struct buffer_page {
struct list_head list; // 16 bytes (offset 0)
...
unsigned int order; // 4 bytes (offset 48)
unsigned int id:30; // (offset 52)
unsigned int range:1; // (offset 55)
void *page; // 8 bytes (offset 56)
};
```
---
### 2. Root Cause
1. When a user opens a trace file (e.g., `/sys/kernel/tracing/trace`), a
`trace_iterator` is created, and `ring_buffer_read_start()` is called.
2. `ring_buffer_read_start()` increments `cpu_buffer->resize_disabled` to
prevent the ring buffer from being resized or having its pages freed while the
iterator is active.
3. However, when a user writes to `buffer_subbuf_size` (which calls
`ring_buffer_subbuf_order_set()`), the kernel changes the sub-buffer order/size
and reallocates the ring buffer pages.
4. Unlike `ring_buffer_resize()`, **`ring_buffer_subbuf_order_set()` fails to
check if `cpu_buffer->resize_disabled` is set**.
5. As a result, `ring_buffer_subbuf_order_set()` proceeds to replace the ring
buffer pages and frees the old `struct buffer_page` objects.
6. When the active reader subsequently advances the iterator via
`ring_buffer_iter_advance()`, it accesses `iter->head_page` (which still points
to one of the freed `struct buffer_page` objects), resulting in a
use-after-free.
---
### 3. Proposed Fix
In `ring_buffer_subbuf_order_set()`, check if `cpu_buffer->resize_disabled` is
set for any of the CPU buffers, and return `-EBUSY` if so (similar to how
`ring_buffer_resize()` handles active readers):
```c
for_each_buffer_cpu(buffer, cpu) {
if (!cpumask_test_cpu(cpu, buffer->cpumask))
continue;
cpu_buffer = buffer->buffers[cpu];
+ if (atomic_read(&cpu_buffer->resize_disabled)) {
+ err = -EBUSY;
+ goto error;
+ }
+
if (cpu_buffer->mapped) {
err = -EBUSY;
goto error;
}
```]
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