| 0/0 |
2026/07/02 00:22 |
flow |
assessment-security |
3m
Results: map[DenialOfService:false Explanation:Based on the provided crash report and source code analysis, here is the
assessment of the bug:
**Analysis:**
The bug report indicates a `hung_task_timeout` where the task `syz.0.110` is
blocked for more than 143 seconds trying to acquire the `inode_lock` in
`__blockdev_direct_IO` (called via `fat_direct_IO`).
Looking at the NMI backtrace for the other CPU (CPU 0), we can see that task
`syz-executor/5599` is actively executing `__bfs` inside
`kernel/locking/lockdep.c`:
```
RIP: 0010:__bfs kernel/locking/lockdep.c:1815 [inline]
...
RIP: 0010:check_prevs_add kernel/locking/lockdep.c:3284 [inline]
RIP: 0010:validate_chain kernel/locking/lockdep.c:3908 [inline]
RIP: 0010:__lock_acquire+0x18fa/0x2ce0 kernel/locking/lockdep.c:5237
```
The task on CPU 0 is holding the internal `lockdep` raw spinlock (which disables
interrupts and preemption) and is performing a breadth-first search (`__bfs`) on
the lock dependency graph to validate a newly observed lock dependency.
Syzkaller frequently generates massive lock dependency graphs by creating many
files/directories and acquiring their locks in various complex orders. When the
lockdep graph becomes extremely large, the O(V + E) `__bfs` algorithm can take
an excessively long time to complete. Because this search is performed with
interrupts disabled, it completely stalls the CPU.
This CPU stall prevents other tasksβincluding whichever task is currently
holding the `inode_lock` that `syz.0.110` is waiting forβfrom being scheduled
and making progress. Consequently, the waiting task hangs for over 143 seconds,
triggering the `hung_task_timeout` panic.
**Conclusion:**
This issue is entirely caused by the overhead of the kernel lock validator
(`lockdep`), which is a debugging tool (`CONFIG_PROVE_LOCKING`). In production
kernel builds, `lockdep` is disabled, meaning the `__bfs` graph search code is
not compiled in and this CPU stall cannot occur. Therefore, this is a debugging
artifact and poses no security risk or denial-of-service threat in production
environments. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/07/02 00:22 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/07/02 00:22 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/7be5f73ee7b4d1978813accf8bb54cd892ccd800]
|
| 3/1 |
2026/07/02 00:22 |
action |
kernel-builder |
0m
Results: map[KernelObj:/app/workdir/cache/build/2ba2e838bc5bd125055e7f7b7f28b9592a389426]
|
| 4/1 |
2026/07/02 00:22 |
action |
codesearch-prepare |
0m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/07/02 00:23 |
agent |
expert |
2m
Model:
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Instruction: You are an experienced Linux kernel security engineer. Your task is to analyze given kernel bug report
and determine its security impact based on the following dimensions.
Use the provided tools to examine the source code, check for capability checks (e.g., capable(), ns_capable()),
and understand the nature of the bug. Analyze the given kernel build and configuration.
You can check the kernel config by grepping ".config" file; you can check kernel cmdline by grepping
".config" file for "CONFIG_CMDLINE=". Assume sysctl parameters have default values.
But analyze for the corresponding production build w/o debugging tools enabled (like KASAN, KMSAN, UBSAN).
Try different strategies when analyzing the bug:
- think of ways in which the vulnerable code is unreachable
- or the other way around: try to come up with different ideas of how an unprivileged user can reach the bug
If still unsure err on the side of the bug being non-exploitable/not-accessible.
In the final reply, provide a reasoning for your assessment.
Analysis dimensions:
* Exploitable:
Determine if the bug can result in memory corruption, elevated privileges, or an information leak.
Memory safety issues are almost always exploitable (KASAN or UBSAN reports for use-after-free, out-of-bounds;
refcounting issues, corrupted lists, etc). When kernel is crashing on a completely wild pointer access
(e.g. user-space address, or non-canonical address, but not on NULL or address corresponding to KASAN shadow
for NULL address), including both data accesses and control transfers, that also usually implies possibility
of exploitation. Such reports usually say "unable to handle kernel paging request".
Uses of uninitialized values detected by KMSAN may be exploitable b/c attacker frequently can affect uninit
values with spraying techniques. However, for these exploitability depends on how exactly the uninit value
is used in the code, and what it affects.
Information leaks are exploitable on their own and should be classified as such. A bug that copies kernel
memory contents to userspace (e.g. an out-of-bounds read whose result is returned to the caller, or
uninitialized stack/heap bytes written to a user buffer) is exploitable: it can reveal kernel pointer
values and defeat KASLR, expose sensitive data such as cryptographic keys or other processes' memory, and
serves as a necessary building block in most modern kernel privilege-escalation exploit chains. Do not classify
an information leak as non-exploitable solely because it does not directly cause a memory write or control-flow
hijack; the leak itself is the exploit primitive.
Think of what happens after the bug is triggered. Some bugs cause kernel panic and halt execution,
they are harder to exploit. For example, BUG reports halts the kernel. However, WARNING reports don't halt
execution in production builds. Debug bug detection tools (like KASAN, KMSAN, KCSAN, UBSAN) are also not enabled
in production builds, so attacker can freely exploit these bugs w/o being detected by these tools.
If you see an integer overflow, think how the overflowed value used later (if it's used as allocation size,
or an array index). If you see an out-of-bounds read, think if it's followed by an out-of-bounds write as well.
Some KCSAN data-races may be exploitable by skilled attackers as well. Think what data structures got corrupted
as the result of data races and how. However, note that kernel has lots of "benign" data races that don't lead
to any runtime misbehavior at all.
* Denial Of Service:
Determine if the bug can result in denial-of-service. Most bugs can, since they cause system crash,
hangs, deadlocks, or resource leaks. This is mostly applicable to WARNING bugs that won't cause system crash
in production. For these think what will be consequences of the violation of the kernel assumptions flagged
by the WARNING. In some cases the unexpected condition is also properly handled by the normal control flow
(e.g. with "if (WARN_ON(...))"), these won't cause denial-of-service. If the condition is not handled,
then it may or may not cause denial-of-service.
* Accessible From Unprivileged Processes:
Determine if the bug can be reached from a typical (non-root) user process that does NOT have any special capabilities
(like CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON) or access to device nodes restricted to root.
Assume that unprivileged_bpf_disabled=1, that is eBPF loading is not accessible. However, cBPF (classical BPF)
is still accessible to non-root processes.
Assume that user namespaces are not accessible, that is, the process cannot get the mentioned capabilities even
within a new user namespace (checked by ns_capable() function in the kernel sources).
* Accessible From User Namespaces:
Determine if the bug can be reached within a user-namespace where the process has all capabilities
(including CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON). Such capabilities are checked with ns_capable()
function in the kernel sources.
* VM Guest Trigger:
Determine if the bug can be triggered from the context of a typical KVM guest (e.g., set up by a QEMU VMM).
Consider accesses to standard Linux host paravirtualized features (virtio-blk, virtio-net, etc.),
and handling of VM exits in the KVM code.
* VM Host Trigger in The Confidential Computing Context:
Determine if the bug can be triggered in a confidential computing guest kernel from the context of a KVM host.
Consider access to standard Linux guest paravirtualized features (virtio-blk, virtio-net, etc.).
* Ethernet Network Trigger:
Determine if the bug can be triggered by processing ingress network Ethernet traffic, either directly (network stack)
or via drivers exposed to network data.
* Other Remote Trigger:
Determine if the bug can be triggered by processing remote traffic other than Ethernet (Wifi, Bluetooth, NFC, etc).
* Peripheral Trigger:
Determine if the bug can be triggered via an untrusted peripheral device that can be physically plugged
into a system, such as a USB device or a niche hardware driver handling external hardware inputs.
This is particularly important for mobile and desktop environments where users can plug in unknown devices.
* Malicious Filesystem Trigger:
Determine if the bug can be triggered by the kernel mounting and parsing a malicious filesystem image.
This is highly critical for Desktop and Mobile environments where external media or downloaded images
might be auto-mounted.
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 kernel bug report is:
INFO: tas[ 305.832337][ T38] INFO: task syz.0.110:6435 blocked for more than 143 seconds.
Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.0.110 state:D stack:25560 pid:6435 tgid:6418 ppid:5600 task_flags:0x400040 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5510 [inline]
__schedule+0x16dc/0x5500 kernel/sched/core.c:7234
__schedule_loop kernel/sched/core.c:7311 [inline]
rt_mutex_schedule+0x76/0xf0 kernel/sched/core.c:7607
rt_mutex_slowlock_block kernel/locking/rtmutex.c:1670 [inline]
__rt_mutex_slowlock kernel/locking/rtmutex.c:1747 [inline]
__rt_mutex_slowlock_locked+0x1f84/0x25b0 kernel/locking/rtmutex.c:1787
rt_mutex_slowlock+0xbd/0x170 kernel/locking/rtmutex.c:1827
__rt_mutex_lock kernel/locking/rtmutex.c:1842 [inline]
rwbase_write_lock+0x14c/0x720 kernel/locking/rwbase_rt.c:251
inode_lock include/linux/fs.h:1024 [inline]
__blockdev_direct_IO+0x358/0x33d0 fs/direct-io.c:1136
blockdev_direct_IO include/linux/fs.h:3130 [inline]
fat_direct_IO+0x1be/0x2f0 fs/fat/inode.c:284
generic_file_read_iter+0x346/0x560 mm/filemap.c:2983
new_sync_read fs/read_write.c:493 [inline]
vfs_read+0x59e/0xa90 fs/read_write.c:574
ksys_read+0x156/0x270 fs/read_write.c:716
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f7cd55bce59
RSP: 002b:00007f7cd37ed028 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00007f7cd5836090 RCX: 00007f7cd55bce59
RDX: 0000000000002020 RSI: 0000200000005300 RDI: 0000000000000004
RBP: 00007f7cd5652e6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f7cd5836128 R14: 00007f7cd5836090 R15: 00007ffc32114cb8
</TASK>
Showing all locks held in the system:
5 locks held by kworker/u8:1/14:
1 lock held by khungtaskd/38:
#0: ffffffff8dfc2fc0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#0: ffffffff8dfc2fc0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:840 [inline]
#0: ffffffff8dfc2fc0 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180 kernel/locking/lockdep.c:6775
2 locks held by getty/5356:
#0: ffff888029a350a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x25/0x70 drivers/tty/tty_ldisc.c:243
#1: ffffc90003cc62e0 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x460/0x1360 drivers/tty/n_tty.c:2211
3 locks held by syz-executor/5599:
4 locks held by syz-executor/5603:
5 locks held by udevd/5722:
1 lock held by udevd/5926:
#0: ffff8880229a6608 (&sb->s_type->i_mutex_key#9){++++}-{4:4}, at: inode_lock_shared include/linux/fs.h:1039 [inline]
#0: ffff8880229a6608 (&sb->s_type->i_mutex_key#9){++++}-{4:4}, at: blkdev_read_iter+0x2e6/0x430 block/fops.c:829
3 locks held by syz.0.110/6422:
2 locks held by syz.0.110/6435:
#0: ffff888029d75528 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x252/0x320 fs/file.c:1259
#1: ffff88804035e430 (&sb->s_type->i_mutex_key#33){++++}-{4:4}, at: inode_lock include/linux/fs.h:1024 [inline]
#1: ffff88804035e430 (&sb->s_type->i_mutex_key#33){++++}-{4:4}, at: __blockdev_direct_IO+0x358/0x33d0 fs/direct-io.c:1136
8 locks held by syz.6.384/7884:
2 locks held by syz.6.384/7899:
#0: ffff88803a304500 (sb_writers#27){.+.+}-{0:0}, at: mnt_want_write+0x41/0x90 fs/namespace.c:494
#1: ffff88806071d2f8 (&type->i_mutex_dir_key#19/1){+.+.}-{4:4}, at: inode_lock_nested include/linux/fs.h:1069 [inline]
#1: ffff88806071d2f8 (&type->i_mutex_dir_key#19/1){+.+.}-{4:4}, at: __start_dirop fs/namei.c:2918 [inline]
#1: ffff88806071d2f8 (&type->i_mutex_dir_key#19/1){+.+.}-{4:4}, at: start_dirop fs/namei.c:2942 [inline]
#1: ffff88806071d2f8 (&type->i_mutex_dir_key#19/1){+.+.}-{4:4}, at: filename_create+0x200/0x370 fs/namei.c:4951
4 locks held by syz.2.387/7890:
3 locks held by syz.5.388/7896:
=============================================
NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 38 Comm: khungtaskd Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
nmi_cpu_backtrace+0x274/0x2d0 lib/nmi_backtrace.c:122
nmi_trigger_cpumask_backtrace+0x17a/0x380 lib/nmi_backtrace.c:65
trigger_all_cpu_backtrace include/linux/nmi.h:162 [inline]
__sys_info lib/sys_info.c:157 [inline]
sys_info+0x135/0x170 lib/sys_info.c:165
check_hung_uninterruptible_tasks kernel/hung_task.c:353 [inline]
watchdog+0xfd7/0x1030 kernel/hung_task.c:561
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 5599 Comm: syz-executor Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
RIP: 0010:__bfs kernel/locking/lockdep.c:1815 [inline]
RIP: 0010:__bfs_backwards kernel/locking/lockdep.c:1860 [inline]
RIP: 0010:check_irq_usage kernel/locking/lockdep.c:2796 [inline]
RIP: 0010:check_prev_add kernel/locking/lockdep.c:3169 [inline]
RIP: 0010:check_prevs_add kernel/locking/lockdep.c:3284 [inline]
RIP: 0010:validate_chain kernel/locking/lockdep.c:3908 [inline]
RIP: 0010:__lock_acquire+0x18fa/0x2ce0 kernel/locking/lockdep.c:5237
Code: 13 25 ff 0f 00 00 39 05 b0 d6 d7 11 73 06 89 05 a8 d6 d7 11 48 8b 02 48 89 df 4c 39 e8 0f 84 34 fe ff ff 4c 89 78 30 48 8b 00 <4c> 39 e8 75 f4 48 89 df e9 20 fe ff ff 48 89 fb 48 c7 c7 b0 02 e9
RSP: 0018:ffffc900043ff618 EFLAGS: 00000002
RAX: ffffffff96234578 RBX: 0000000000000101 RCX: 0000000000000002
RDX: ffffffff95fd2078 RSI: ffff888030bc2bd0 RDI: 0000000000000101
RBP: 74e67a4e533e33e7 R08: ffffc900043ff5e0 R09: 0000000000000020
R10: 0000000000000000 R11: ffffffff81a124a0 R12: 0000000000000001
R13: ffffffff931a9348 R14: ffff888030bc1f00 R15: ffffc900043ff6a8
FS: 000055556714d500(0000) GS:ffff888126111000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fd848963000 CR3: 000000005a8fc000 CR4: 00000000003526f0
Call Trace:
<TASK>
lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:132 [inline]
_raw_spin_lock_irqsave+0x40/0x60 kernel/locking/spinlock.c:166
debug_object_deactivate+0x9a/0x250 lib/debugobjects.c:936
debug_hrtimer_deactivate kernel/time/hrtimer.c:490 [inline]
remove_and_enqueue_same_base kernel/time/hrtimer.c:1252 [inline]
__hrtimer_start_range_ns kernel/time/hrtimer.c:1402 [inline]
hrtimer_start_range_ns_common+0x3ee/0xb90 kernel/time/hrtimer.c:1481
hrtimer_start_range_ns+0x111/0x3f0 kernel/time/hrtimer.c:1503
hrtimer_start include/linux/hrtimer.h:223 [inline]
hrtick_cond_restart kernel/sched/core.c:939 [inline]
hrtick_schedule_exit kernel/sched/core.c:1001 [inline]
finish_lock_switch kernel/sched/core.c:5240 [inline]
finish_task_switch+0x3c4/0xc60 kernel/sched/core.c:5359
context_switch kernel/sched/core.c:5513 [inline]
__schedule+0x16e4/0x5500 kernel/sched/core.c:7234
preempt_schedule_common+0x82/0xd0 kernel/sched/core.c:7413
preempt_schedule_thunk+0x16/0x40 arch/x86/entry/thunk.S:12
__raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:179 [inline]
_raw_spin_unlock_irqrestore+0x74/0x80 kernel/locking/spinlock.c:198
rcu_report_unblock_qs_rnp kernel/rcu/tree.c:2454 [inline]
rcu_preempt_deferred_qs_irqrestore+0x911/0xb90 kernel/rcu/tree_plugin.h:571
__rcu_read_unlock+0x83/0xe0 kernel/rcu/tree_plugin.h:438
generic_update_time fs/inode.c:2257 [inline]
touch_atime+0x565/0x670 fs/inode.c:2332
file_accessed include/linux/fs.h:2272 [inline]
iterate_dir+0x37d/0x4e0 fs/readdir.c:113
__do_sys_getdents64 fs/readdir.c:399 [inline]
__se_sys_getdents64+0xf1/0x280 fs/readdir.c:384
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fe0e05d57f3
Code: c7 c0 e8 ff ff ff 64 c7 00 16 00 00 00 31 c0 eb 9e e8 81 9b fd ff 90 b8 ff ff ff 7f 48 39 c2 48 0f 47 d0 b8 d9 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 05 c3 0f 1f 40 00 48 c7 c2 e8 ff ff ff f7 d8
RSP: 002b:00007fff0997e968 EFLAGS: 00000293 ORIG_RAX: 00000000000000d9
RAX: ffffffffffffffda RBX: 0000555567168a30 RCX: 00007fe0e05d57f3
RDX: 0000000000008000 RSI: 0000555567168a60 RDI: 0000000000000005
RBP: 0000555567168a60 R08: 00007fe0e07fde20 R09: 0000000000000001
R10: 0000000000000000 R11: 0000000000000293 R12: 0000555567168a34
R13: ffffffffffffffe8 R14: 0000000000000016 R15: 00007fff09980c10
</TASK>
Reply: Based on the provided crash report and source code analysis, here is the assessment of the bug:
**Analysis:**
The bug report indicates a `hung_task_timeout` where the task `syz.0.110` is blocked for more than 143 seconds trying to acquire the `inode_lock` in `__blockdev_direct_IO` (called via `fat_direct_IO`).
Looking at the NMI backtrace for the other CPU (CPU 0), we can see that task `syz-executor/5599` is actively executing `__bfs` inside `kernel/locking/lockdep.c`:
```
RIP: 0010:__bfs kernel/locking/lockdep.c:1815 [inline]
...
RIP: 0010:check_prevs_add kernel/locking/lockdep.c:3284 [inline]
RIP: 0010:validate_chain kernel/locking/lockdep.c:3908 [inline]
RIP: 0010:__lock_acquire+0x18fa/0x2ce0 kernel/locking/lockdep.c:5237
```
The task on CPU 0 is holding the internal `lockdep` raw spinlock (which disables interrupts and preemption) and is performing a breadth-first search (`__bfs`) on the lock dependency graph to validate a newly observed lock dependency.
Syzkaller frequently generates massive lock dependency graphs by creating many files/directories and acquiring their locks in various complex orders. When the lockdep graph becomes extremely large, the O(V + E) `__bfs` algorithm can take an excessively long time to complete. Because this search is performed with interrupts disabled, it completely stalls the CPU.
This CPU stall prevents other tasksβincluding whichever task is currently holding the `inode_lock` that `syz.0.110` is waiting forβfrom being scheduled and making progress. Consequently, the waiting task hangs for over 143 seconds, triggering the `hung_task_timeout` panic.
**Conclusion:**
This issue is entirely caused by the overhead of the kernel lock validator (`lockdep`), which is a debugging tool (`CONFIG_PROVE_LOCKING`). In production kernel builds, `lockdep` is disabled, meaning the `__bfs` graph search code is not compiled in and this CPU stall cannot occur. Therefore, this is a debugging artifact and poses no security risk or denial-of-service threat in production environments.
|
| 6/2 |
2026/07/02 00:23 |
llm |
expert |
0m
Model:
Tokens:
input: 10718
output: 38
thoughts: 773
Thoughts: **Analyzing a Hung Task: A Deep Dive**
Okay, so we've got a hung task, `syz.0.110:6435`, that's been blocked for a significant amount of time β over 143 seconds. The stack trace is screaming `rt_mutex_slowlock_block`, indicating a real-time mutex contention, which is the immediate issue. I need to figure out why.
The stack trace points toward `inode_lock` within `__blockdev_direct_IO`. That's where it's getting stuck. Hold on... the task seems to be *trying* to acquire `inode_lock` in `__blockdev_direct_IO`, but... isn't it already held by the same task? That's definitely suspicious. It's a classic case of self-deadlock.
Let's verify that. I'll check the locks held by this specific task. Ah, here it is: two locks are held. One is `f->f_pos_lock`, which is fine, but the other one is `inode_lock` and the trace says this lock is held at `__blockdev_direct_IO`. Well, that settles it; it is trying to take the lock that it is already holding, which is causing the deadlock. Let's pinpoint where that's happening with absolute certainty. I'll jump right to the code at `fs/direct-io.c:1136` inside `__blockdev_direct_IO`. Let's see what's happening there to cause this.
|
| 7/2 |
2026/07/02 00:23 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/direct-io.c Name:__blockdev_direct_IO]
Results: map[SourceCode:1076: /*
1077: * This is a library function for use by filesystem drivers.
1078: *
1079: * The locking rules are governed by the flags parameter:
1080: * - if the flags value contains DIO_LOCKING we use a fancy locking
1081: * scheme for dumb filesystems.
1082: * For writes this function is called under i_rwsem and returns with
1083: * i_rwsem held, for reads, i_rwsem is not held on entry, but it is
1084: * taken and dropped again before returning.
1085: * - if the flags value does NOT contain DIO_LOCKING we don't use any
1086: * internal locking but rather rely on the filesystem to synchronize
1087: * direct I/O reads/writes versus each other and truncate.
1088: *
1089: * To help with locking against truncate we incremented the i_dio_count
1090: * counter before starting direct I/O, and decrement it once we are done.
1091: * Truncate can wait for it to reach zero to provide exclusion. It is
1092: * expected that filesystem provide exclusion between new direct I/O
1093: * and truncates. For DIO_LOCKING filesystems this is done by i_rwsem,
1094: * but other filesystems need to take care of this on their own.
1095: *
1096: * NOTE: if you pass "sdio" to anything by pointer make sure that function
1097: * is always inlined. Otherwise gcc is unable to split the structure into
1098: * individual fields and will generate much worse code. This is important
1099: * for the whole file.
1100: */
1101: ssize_t __blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
1102: struct block_device *bdev, struct iov_iter *iter,
1103: get_block_t get_block, dio_iodone_t end_io,
1104: int flags)
1105: {
1106: unsigned i_blkbits = READ_ONCE(inode->i_blkbits);
1107: unsigned blkbits = i_blkbits;
1108: unsigned blocksize_mask = (1 << blkbits) - 1;
1109: ssize_t retval = -EINVAL;
1110: const size_t count = iov_iter_count(iter);
1111: loff_t offset = iocb->ki_pos;
1112: const loff_t end = offset + count;
1113: struct dio *dio;
1114: struct dio_submit sdio = { NULL, };
1115: struct buffer_head map_bh = { 0, };
1116: struct blk_plug plug;
1117: unsigned long align = offset | iov_iter_alignment(iter);
1118:
1119: /* watch out for a 0 len io from a tricksy fs */
1120: if (iov_iter_rw(iter) == READ && !count)
1121: return 0;
1122:
1123: dio = kmem_cache_alloc(dio_cache, GFP_KERNEL);
1124: if (!dio)
1125: return -ENOMEM;
1126: /*
1127: * Believe it or not, zeroing out the page array caused a .5%
1128: * performance regression in a database benchmark. So, we take
1129: * care to only zero out what's needed.
1130: */
1131: memset(dio, 0, offsetof(struct dio, pages));
1132:
1133: dio->flags = flags;
1134: if (dio->flags & DIO_LOCKING && iov_iter_rw(iter) == READ) {
1135: /* will be released by direct_io_worker */
1136: inode_lock(inode);
1137: }
1138: dio->is_pinned = iov_iter_extract_will_pin(iter);
1139:
1140: /* Once we sampled i_size check for reads beyond EOF */
1141: dio->i_size = i_size_read(inode);
1142: if (iov_iter_rw(iter) == READ && offset >= dio->i_size) {
1143: retval = 0;
1144: goto fail_dio;
1145: }
1146:
1147: if (align & blocksize_mask) {
1148: if (bdev)
1149: blkbits = blksize_bits(bdev_logical_block_size(bdev));
1150: blocksize_mask = (1 << blkbits) - 1;
1151: if (align & blocksize_mask)
1152: goto fail_dio;
1153: }
1154:
1155: if (dio->flags & DIO_LOCKING && iov_iter_rw(iter) == READ) {
1156: struct address_space *mapping = iocb->ki_filp->f_mapping;
1157:
1158: retval = filemap_write_and_wait_range(mapping, offset, end - 1);
1159: if (retval)
1160: goto fail_dio;
1161: }
1162:
1163: /*
1164: * For file extending writes updating i_size before data writeouts
1165: * complete can expose uninitialized blocks in dumb filesystems.
1166: * In that case we need to wait for I/O completion even if asked
1167: * for an asynchronous write.
1168: */
1169: if (is_sync_kiocb(iocb))
1170: dio->is_async = false;
1171: else if (iov_iter_rw(iter) == WRITE && end > i_size_read(inode))
1172: dio->is_async = false;
1173: else
1174: dio->is_async = true;
1175:
1176: dio->inode = inode;
1177: if (iov_iter_rw(iter) == WRITE)
1178: dio->opf = REQ_OP_WRITE | REQ_SYNC | REQ_IDLE;
1179: else
1180: dio->opf = REQ_OP_READ;
1181:
1182: /*
1183: * For AIO O_(D)SYNC writes we need to defer completions to a workqueue
1184: * so that we can call ->fsync.
1185: */
1186: if (dio->is_async && iov_iter_rw(iter) == WRITE) {
1187: retval = 0;
1188: if (iocb_is_dsync(iocb))
1189: retval = dio_set_defer_completion(dio);
1190: else if (!dio->inode->i_sb->s_dio_done_wq) {
1191: /*
1192: * In case of AIO write racing with buffered read we
1193: * need to defer completion. We can't decide this now,
1194: * however the workqueue needs to be initialized here.
1195: */
1196: retval = sb_init_dio_done_wq(dio->inode->i_sb);
1197: }
1198: if (retval)
1199: goto fail_dio;
1200: }
1201:
1202: /*
1203: * Will be decremented at I/O completion time.
1204: */
1205: inode_dio_begin(inode);
1206:
1207: sdio.blkbits = blkbits;
1208: sdio.blkfactor = i_blkbits - blkbits;
1209: sdio.block_in_file = offset >> blkbits;
1210:
1211: sdio.get_block = get_block;
1212: dio->end_io = end_io;
1213: sdio.final_block_in_bio = -1;
1214: sdio.next_block_for_io = -1;
1215:
1216: dio->iocb = iocb;
1217:
1218: spin_lock_init(&dio->bio_lock);
1219: dio->refcount = 1;
1220:
1221: dio->should_dirty = user_backed_iter(iter) && iov_iter_rw(iter) == READ;
1222: sdio.iter = iter;
1223: sdio.final_block_in_request = end >> blkbits;
1224:
1225: /*
1226: * In case of non-aligned buffers, we may need 2 more
1227: * pages since we need to zero out first and last block.
1228: */
1229: if (unlikely(sdio.blkfactor))
1230: sdio.pages_in_io = 2;
1231:
1232: sdio.pages_in_io += iov_iter_npages(iter, INT_MAX);
1233:
1234: blk_start_plug(&plug);
1235:
1236: retval = do_direct_IO(dio, &sdio, &map_bh);
1237: if (retval)
1238: dio_cleanup(dio, &sdio);
1239:
1240: if (retval == -ENOTBLK) {
1241: /*
1242: * The remaining part of the request will be
1243: * handled by buffered I/O when we return
1244: */
1245: retval = 0;
1246: }
1247: /*
1248: * There may be some unwritten disk at the end of a part-written
1249: * fs-block-sized block. Go zero that now.
1250: */
1251: dio_zero_block(dio, &sdio, 1, &map_bh);
1252:
1253: if (sdio.cur_page) {
1254: ssize_t ret2;
1255:
1256: ret2 = dio_send_cur_page(dio, &sdio, &map_bh);
1257: if (retval == 0)
1258: retval = ret2;
1259: dio_unpin_page(dio, sdio.cur_page);
1260: sdio.cur_page = NULL;
1261: }
1262: if (sdio.bio)
1263: dio_bio_submit(dio, &sdio);
1264:
1265: blk_finish_plug(&plug);
1266:
1267: /*
1268: * It is possible that, we return short IO due to end of file.
1269: * In that case, we need to release all the pages we got hold on.
1270: */
1271: dio_cleanup(dio, &sdio);
1272:
1273: /*
1274: * All block lookups have been performed. For READ requests
1275: * we can let i_rwsem go now that its achieved its purpose
1276: * of protecting us from looking up uninitialized blocks.
1277: */
1278: if (iov_iter_rw(iter) == READ && (dio->flags & DIO_LOCKING))
1279: inode_unlock(dio->inode);
1280:
1281: /*
1282: * The only time we want to leave bios in flight is when a successful
1283: * partial aio read or full aio write have been setup. In that case
1284: * bio completion will call aio_complete. The only time it's safe to
1285: * call aio_complete is when we return -EIOCBQUEUED, so we key on that.
1286: * This had *better* be the only place that raises -EIOCBQUEUED.
1287: */
1288: BUG_ON(retval == -EIOCBQUEUED);
1289: if (dio->is_async && retval == 0 && dio->result &&
1290: (iov_iter_rw(iter) == READ || dio->result == count))
1291: retval = -EIOCBQUEUED;
1292: else
1293: dio_await_completion(dio);
1294:
1295: if (drop_refcount(dio) == 0) {
1296: retval = dio_complete(dio, retval, DIO_COMPLETE_INVALIDATE);
1297: } else
1298: BUG_ON(retval != -EIOCBQUEUED);
1299:
1300: return retval;
1301:
1302: fail_dio:
1303: if (dio->flags & DIO_LOCKING && iov_iter_rw(iter) == READ)
1304: inode_unlock(inode);
1305:
1306: kmem_cache_free(dio_cache, dio);
1307: return retval;
1308: }
SourceFile:fs/direct-io.c]
|
| 8/2 |
2026/07/02 00:23 |
llm |
expert |
0m
Model:
Tokens:
input: 15645
output: 40
thoughts: 1398
Thoughts: **Analyzing a Potential Direct I/O Deadlock**
Okay, so I'm looking at this stack trace, and it seems like we're getting blocked in `__blockdev_direct_IO` at line 1136, specifically at the `inode_lock(inode)` call. The stack trace confirms this, pointing directly to `rwbase_write_lock` within `inode_lock`.
Initially, lockdep seemed to indicate that the current task *already* held the `inode_lock` that it was trying to acquire, which is strange. I thought, could this be a recursive lock attempt, which would definitely cause a deadlock, but now I know that `rwbase_write_lock` is called from within `inode_lock`!
I considered the call path: `generic_file_read_iter` -> `fat_direct_IO` -> `blockdev_direct_IO` -> `__blockdev_direct_IO`. I checked if `generic_file_read_iter` might be holding the lock, but no, it's not by default. Then I figured that if we're blocked, someone else *must* be holding the lock. Let's look at the other tasks.
I examined the lock information from other tasks, specifically looking at what locks were held. I was focusing on the specific lock class `&sb->s_type->i_mutex_key#33`, which is what lockdep flagged. I looked for any other tasks holding that key. Nope, nothing.
I went back to the lockdep output for the blocking task (`syz.0.110/6435`) and noticed it was trying to acquire the same lock, but now I know it's blocked, it's trying to acquire, not held.
Ah! The critical clue is the `rt_mutex_slowlock_block`. That tells me it is blocked in a real-time environment. In `PREEMPT_RT`, `rwsem` uses `rt_mutex`.
Now I realize that lockdep only shows locks that are *held*, not ones the task is blocked trying to acquire. That explains why it appeared in both places! The task is blocked, not holding, and lockdep is displaying it incorrectly.
So, in summary, it looks like a task is blocked trying to acquire an `inode_lock`, which is a `rt_mutex` in this `PREEMPT_RT` environment, which means it should be a blocking wait.
|
| 9/2 |
2026/07/02 00:23 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/locking/rwbase_rt.c Name:rwbase_write_lock]
Results: map[SourceCode: 244: static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
245: unsigned int state)
246: {
247: struct rt_mutex_base *rtm = &rwb->rtmutex;
248: unsigned long flags;
249:
250: /* Take the rtmutex as a first step */
251: if (rwbase_rtmutex_lock_state(rtm, state))
252: return -EINTR;
253:
254: /* Force readers into slow path */
255: atomic_sub(READER_BIAS, &rwb->readers);
256:
257: rwbase_pre_schedule();
258:
259: raw_spin_lock_irqsave(&rtm->wait_lock, flags);
260: if (__rwbase_write_trylock(rwb))
261: goto out_unlock;
262:
263: rwbase_set_and_save_current_state(state);
264: trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
265: for (;;) {
266: /* Optimized out for rwlocks */
267: if (rwbase_signal_pending_state(state, current)) {
268: rwbase_restore_current_state();
269: __rwbase_write_unlock(rwb, 0, flags);
270: rwbase_post_schedule();
271: trace_contention_end(rwb, -EINTR);
272: return -EINTR;
273: }
274:
275: if (__rwbase_write_trylock(rwb))
276: break;
277:
278: raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
279: rwbase_schedule();
280: raw_spin_lock_irqsave(&rtm->wait_lock, flags);
281:
282: set_current_state(state);
283: }
284: rwbase_restore_current_state();
285: trace_contention_end(rwb, 0);
286:
287: out_unlock:
288: raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
289: rwbase_post_schedule();
290: return 0;
291: }
SourceFile:kernel/locking/rwbase_rt.c]
|
| 10/2 |
2026/07/02 00:23 |
llm |
expert |
0m
Model:
Tokens:
input: 18369
output: 32
thoughts: 1714
Thoughts: **Analyzing a Blocking Issue in the Kernel**
Okay, here's the situation as I see it. I'm looking at a kernel trace where a task (presumably performing direct I/O) is blocked on `inode_lock`. I see that the `rwbase_rtmutex_lock_state` function is involved, which calls `__rt_mutex_lock`. Lockdep shows that `lock_acquire` *should* have been called. Since `lock_acquire` is called before blocking in `rwsem` situations, I would expect the task to be blocked on `inode_lock` at `__blockdev_direct_IO`.
The immediate question is: who holds the lock? If no one holds it, why is it blocked?
I then see another task, `syz-executor/5599`, and the NMI backtrace reveals that it's stuck in `__lock_acquire` within `lockdep`, specifically running `__bfs`. I recall that `__bfs` represents a lockdep graph search, and if the graph is substantial, it can consume a considerable amount of time. Is this task looping in lockdep? If so, is it holding some locks or simply taking too much CPU?
It turns out that task 5599 is running on CPU 0. Task 38 (khungtaskd) is running on CPU 1. Task 6435 is blocked for 143 seconds.
I check what locks `syz-executor/5599` is holding, but the report only says it holds 3 locks, which isn't very helpful. There is a note that the output might be truncated, as it mentions other tasks holding locks but doesn't list them, either.
I then see the process 5599 call stack from `__se_sys_getdents64` down to `__bfs`. It's doing `iterate_dir`, which I know involves the `inode_lock` or `i_rwsem` of the directory.
If `syz.0.110/6435` is blocked on the `inode_lock`, and this isn't showing up as held by the blocked task, then the lock could potentially be held by a task whose locks aren't being printed. I suspect that the reason for the deadlock could be that `syz-executor/5599` is looping in lockdep, potentially holding the `inode_lock`, due to the long lockdep call. The reason could be related to `iterate_dir`. I should check the code for `iterate_dir` in `fs/readdir.c` to see how it uses the `inode_lock`.
|
| 11/2 |
2026/07/02 00:23 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/readdir.c Name:iterate_dir]
Results: map[SourceCode: 87: int iterate_dir(struct file *file, struct dir_context *ctx)
88: {
89: struct inode *inode = file_inode(file);
90: int res = -ENOTDIR;
91:
92: if (!file->f_op->iterate_shared)
93: goto out;
94:
95: res = security_file_permission(file, MAY_READ);
96: if (res)
97: goto out;
98:
99: res = fsnotify_file_perm(file, MAY_READ);
100: if (res)
101: goto out;
102:
103: res = down_read_killable(&inode->i_rwsem);
104: if (res)
105: goto out;
106:
107: res = -ENOENT;
108: if (!IS_DEADDIR(inode)) {
109: ctx->pos = file->f_pos;
110: res = file->f_op->iterate_shared(file, ctx);
111: file->f_pos = ctx->pos;
112: fsnotify_access(file);
113: file_accessed(file);
114: }
115: inode_unlock_shared(inode);
116: out:
117: return res;
118: }
SourceFile:fs/readdir.c]
|
| 12/2 |
2026/07/02 00:23 |
llm |
expert |
0m
Model:
Tokens:
input: 21130
output: 34
thoughts: 989
Thoughts: **Analyzing the Task Hang in a Production Context**
Okay, so I'm looking at this stack trace and trying to understand why this task, `syz-executor/5599`, is essentially hanging. The trace shows it's currently executing `__bfs`, which is a breadth-first search within the lockdep subsystem. Initially, my thought process was following the lock acquisition sequence: `iterate_dir` holds `inode->i_rwsem`, which is `inode_lock_shared`. The stack trace reveals the process is within `file_accessed(file)` before releasing the lock, which means `syz-executor/5599` *does* hold that shared lock. If another process, like `syz.0.110/6435`, tried to acquire the *exclusive* lock on the same directory, there would be a block, which, I assumed, was happening, given the `fat_direct_IO` operation that was in process. However, `fat_direct_IO` implies that the process is trying to read a file instead of a directory. So, I questioned, if the `fat_direct_IO` could have been called on a directory.
But then, the stack trace shifted. Instead of lock contention, I noticed `syz-executor/5599` was preempted! Itβs in `preempt_schedule_thunk` -> `__schedule`, which means it was interrupted. This is critical: `preempt_schedule_thunk` is called from `_raw_spin_unlock_irqrestore`, which enables preemption. If `NEED_RESCHED` is set, the process is preempted. So the scheduler is kicking in.
Now, things get interesting. `__schedule` eventually calls `context_switch`, which goes to `finish_task_switch`. In `finish_task_switch`, we have `finish_lock_switch`, which triggers a chain of events: `hrtick_schedule_exit`, `hrtick_cond_restart`, `hrtimer_start`, `debug_object_deactivate`, which finally leads to a spinlock being acquired, and *that* is where `__bfs` is invoked β and that's when this task seemingly gets stuck.
The critical observation is that `syz-executor/5599` is *not* stuck in a potentially infinite loop within the file system itself; it's in `__bfs` *during the scheduler's lock acquisition process*. Furthermore, the NMI backtrace confirms CPU 0 is hung in `__bfs`.
Now the important detail: This is *lockdep*, a debugging tool. The prompt specifies this needs to be analyzed for a production build *without* debugging tools. Therefore, this issue is a lockdep problem or a lockdep performance issue that will *not* be present in the production build because lockdep is disabled. We can move on.
|
| 13/2 |
2026/07/02 00:24 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/locking/lockdep.c Name:__bfs]
Results: map[SourceCode:1706: /*
1707: * Breadth-First Search to find a strong path in the dependency graph.
1708: *
1709: * @source_entry: the source of the path we are searching for.
1710: * @data: data used for the second parameter of @match function
1711: * @match: match function for the search
1712: * @target_entry: pointer to the target of a matched path
1713: * @offset: the offset to struct lock_class to determine whether it is
1714: * locks_after or locks_before
1715: *
1716: * We may have multiple edges (considering different kinds of dependencies,
1717: * e.g. ER and SN) between two nodes in the dependency graph. But
1718: * only the strong dependency path in the graph is relevant to deadlocks. A
1719: * strong dependency path is a dependency path that doesn't have two adjacent
1720: * dependencies as -(*R)-> -(S*)->, please see:
1721: *
1722: * Documentation/locking/lockdep-design.rst
1723: *
1724: * for more explanation of the definition of strong dependency paths
1725: *
1726: * In __bfs(), we only traverse in the strong dependency path:
1727: *
1728: * In lock_list::only_xr, we record whether the previous dependency only
1729: * has -(*R)-> in the search, and if it does (prev only has -(*R)->), we
1730: * filter out any -(S*)-> in the current dependency and after that, the
1731: * ->only_xr is set according to whether we only have -(*R)-> left.
1732: */
1733: static enum bfs_result __bfs(struct lock_list *source_entry,
1734: void *data,
1735: bool (*match)(struct lock_list *entry, void *data),
1736: bool (*skip)(struct lock_list *entry, void *data),
1737: struct lock_list **target_entry,
1738: int offset)
1739: {
1740: struct circular_queue *cq = &lock_cq;
1741: struct lock_list *lock = NULL;
1742: struct lock_list *entry;
1743: struct list_head *head;
1744: unsigned int cq_depth;
1745: bool first;
1746:
1747: lockdep_assert_locked();
1748:
1749: __cq_init(cq);
1750: __cq_enqueue(cq, source_entry);
1751:
1752: while ((lock = __bfs_next(lock, offset)) || (lock = __cq_dequeue(cq))) {
1753: if (!lock->class)
1754: return BFS_EINVALIDNODE;
1755:
1756: /*
1757: * Step 1: check whether we already finish on this one.
1758: *
1759: * If we have visited all the dependencies from this @lock to
1760: * others (iow, if we have visited all lock_list entries in
1761: * @lock->class->locks_{after,before}) we skip, otherwise go
1762: * and visit all the dependencies in the list and mark this
1763: * list accessed.
1764: */
1765: if (lock_accessed(lock))
1766: continue;
1767: else
1768: mark_lock_accessed(lock);
1769:
1770: /*
1771: * Step 2: check whether prev dependency and this form a strong
1772: * dependency path.
1773: */
1774: if (lock->parent) { /* Parent exists, check prev dependency */
1775: u8 dep = lock->dep;
1776: bool prev_only_xr = lock->parent->only_xr;
1777:
1778: /*
1779: * Mask out all -(S*)-> if we only have *R in previous
1780: * step, because -(*R)-> -(S*)-> don't make up a strong
1781: * dependency.
1782: */
1783: if (prev_only_xr)
1784: dep &= ~(DEP_SR_MASK | DEP_SN_MASK);
1785:
1786: /* If nothing left, we skip */
1787: if (!dep)
1788: continue;
1789:
1790: /* If there are only -(*R)-> left, set that for the next step */
1791: lock->only_xr = !(dep & (DEP_SN_MASK | DEP_EN_MASK));
1792: }
1793:
1794: /*
1795: * Step 3: we haven't visited this and there is a strong
1796: * dependency path to this, so check with @match.
1797: * If @skip is provide and returns true, we skip this
1798: * lock (and any path this lock is in).
1799: */
1800: if (skip && skip(lock, data))
1801: continue;
1802:
1803: if (match(lock, data)) {
1804: *target_entry = lock;
1805: return BFS_RMATCH;
1806: }
1807:
1808: /*
1809: * Step 4: if not match, expand the path by adding the
1810: * forward or backwards dependencies in the search
1811: *
1812: */
1813: first = true;
1814: head = get_dep_list(lock, offset);
1815: list_for_each_entry_rcu(entry, head, entry) {
1816: visit_lock_entry(entry, lock);
1817:
1818: /*
1819: * Note we only enqueue the first of the list into the
1820: * queue, because we can always find a sibling
1821: * dependency from one (see __bfs_next()), as a result
1822: * the space of queue is saved.
1823: */
1824: if (!first)
1825: continue;
1826:
1827: first = false;
1828:
1829: if (__cq_enqueue(cq, entry))
1830: return BFS_EQUEUEFULL;
1831:
1832: cq_depth = __cq_get_elem_count(cq);
1833: if (max_bfs_queue_depth < cq_depth)
1834: max_bfs_queue_depth = cq_depth;
1835: }
1836: }
1837:
1838: return BFS_RNOMATCH;
1839: }
SourceFile:kernel/locking/lockdep.c]
|
| 14/2 |
2026/07/02 00:24 |
llm |
expert |
0m
Model:
Tokens:
input: 24781
output: 36
thoughts: 2216
Thoughts: **Analysis of a Lockdep-Induced Hung Task**
Okay, so `__bfs` is a function related to lockdep, right? If CPU 0 is stuck there, it's a lockdep issue. It's doing a graph search. Now, `syz-executor/5599` is stuck in `__bfs` inside `lockdep`, so it holds the `lockdep` lock, which is a raw spinlock. Crucially, this means preemption is disabled for that task. The backtrace shows `syz-executor/5599` called `_raw_spin_unlock_irqrestore`, which should re-enable interrupts/preemption, but then it was preempted! It got preempted after unlocking the lock, and upon being scheduled back in, it triggered `lockdep` again through `hrtimer_start`. So now, it's stuck in `__bfs` with interrupts disabled, meaning it *is* actively running but not making progress.
`khungtaskd` detected task 6435 is blocked for a long time on `inode_lock`. Task 5599 holds that lock (shared) because it's doing `iterate_dir`. The problem is, task 5599 is the one that's stuck in `__bfs` for a long time! The lockdep graph is huge, or there's an infinite loop. But lockdep is just a debugging tool. If it's disabled, `__bfs` isn't called, task 5599 would quickly release the lock, and task 6435 wouldn't be blocked. So, the hung task is completely due to lockdep.
There's no other reason task 5599 would be stuck, as it's actively executing line 1815 in `lockdep.c`, a loop over dependencies. Syzkaller often causes large lockdep graphs because it creates lots of locks and dependencies. This leads to long lockdep processing and causes hung task timeouts. This is a known issue, and it's resolved by disabling lockdep in production.
Now, I need to check for a potential deadlock. Task 6435 is doing `fat_direct_IO`, and task 5599 is doing `getdents64` on a directory. Are they potentially operating on the same filesystem and causing a deadlock? Task 6435 is blocked on the file's `inode_lock` (exclusive), while `getdents64` on a directory holds the directory's `inode_lock`. They are different inodes, so there's no inherent deadlock.
Checking which tasks have locks held: I see `filename_create` (task 7899) holds the directory's `i_mutex_dir_key` lock. Task 6435 is blocked on a different lock, specifically `&sb->s_type->i_mutex_key#33`, which is the file's `inode_lock`. Who is holding it? It's not listed. If task 5599 *was* holding it, it would show up. However, if task 5599 is stuck in `__bfs`, then it's *not* making progress and won't release any held locks.
Task 5599 is doing `getdents64`, and the backtrace shows it *is* stuck in `__bfs`. It was preempted, and then when scheduled back in, called `hrtimer_start`, which triggered lockdep. `finish_task_switch` is called when a task is scheduled in, so it seems like task 5599's being scheduled in triggered lockdep. Since the task isn't making any progress, the lock is held. So, is task 5599 holding the `inode_lock` that task 6435 needs? It could be because `getdents64` is for directories, while `fat_direct_IO` is for files. The question now is can `fat_direct_IO` be done on directories, and in FAT, it can. So that's the deadlock. Task 5599 holds the `inode_lock` of the directory in `__bfs` and is pre-empted, while task 6435 is trying to operate on the corresponding file. Because task 5599 is stuck in `__bfs`, it's not releasing the directory lock.
|
| 15/2 |
2026/07/02 00:24 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/fat/inode.c Name:fat_direct_IO]
Results: map[SourceCode: 256: static ssize_t fat_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
257: {
258: struct file *file = iocb->ki_filp;
259: struct address_space *mapping = file->f_mapping;
260: struct inode *inode = mapping->host;
261: size_t count = iov_iter_count(iter);
262: loff_t offset = iocb->ki_pos;
263: ssize_t ret;
264:
265: if (iov_iter_rw(iter) == WRITE) {
266: /*
267: * FIXME: blockdev_direct_IO() doesn't use ->write_begin(),
268: * so we need to update the ->mmu_private to block boundary.
269: *
270: * But we must fill the remaining area or hole by nul for
271: * updating ->mmu_private.
272: *
273: * Return 0, and fallback to normal buffered write.
274: */
275: loff_t size = offset + count;
276: if (MSDOS_I(inode)->mmu_private < size)
277: return 0;
278: }
279:
280: /*
281: * FAT need to use the DIO_LOCKING for avoiding the race
282: * condition of fat_get_block() and ->truncate().
283: */
284: ret = blockdev_direct_IO(iocb, inode, iter, fat_get_block);
285: if (ret < 0 && iov_iter_rw(iter) == WRITE)
286: fat_write_failed(mapping, offset + count);
287:
288: return ret;
289: }
SourceFile:fs/fat/inode.c]
|
| 16/2 |
2026/07/02 00:24 |
llm |
expert |
0m
Model:
Tokens:
input: 28567
output: 36
thoughts: 63
Thoughts: **Investigating `fat_direct_IO` and Directory Reads**
Okay, so I'm looking at this `fat_direct_IO` function, and it seems like it's just a wrapper, ultimately delegating to `blockdev_direct_IO`. That's standard practice, but it's always worth confirming. Now, something's sparked my curiosity β I'm thinking about the edge cases. Can you even *open* a directory for reading and then try to use the `read` system call on it? I'm pretty sure, under normal circumstances, you'd get back an `-EISDIR` error. That's the expected behavior, but I need to make *absolutely* sure that it's implemented correctly within this filesystem's structure.
Therefore, I'll need to check the function `fat_dir_operations` to see how it handles things like directory traversal and what that implementation looks like. It is important to know if the `fat` filesystem itself does or does not allow this behavior, even though the OS itself might prevent it. This will clarify how the system responds if someone tries something unusual.
|
| 17/2 |
2026/07/02 00:24 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/fat/dir.c Name:fat_dir_operations]
Results: map[SourceCode: 872: const struct file_operations fat_dir_operations = {
873: .llseek = generic_file_llseek,
874: .read = generic_read_dir,
875: .iterate_shared = fat_readdir,
876: .unlocked_ioctl = fat_dir_ioctl,
877: #ifdef CONFIG_COMPAT
878: .compat_ioctl = fat_compat_dir_ioctl,
879: #endif
880: .fsync = fat_file_fsync,
881: .setlease = generic_setlease,
882: };
SourceFile:fs/fat/dir.c]
|
| 18/2 |
2026/07/02 00:24 |
llm |
expert |
0m
Model:
Tokens:
input: 29115
output: 34
thoughts: 23
Thoughts: Okay, here's my interpretation of those thoughts, tailored for an expert audience, presented in the first person:
**Decoding the `read` Assignment**
Right, so I see `.read = generic_read_dir`. That immediately sparks a specific train of thought. Let's see... the assignment of a function named `generic_read_dir` to the `.read` member of whatever object we're currently scrutinizing implies a very particular operational flow. My immediate instinct is to dissect the implications. My understanding is that this signals that my system is employing some abstraction layer for directory reading, which isn't always my preferred implementation. Iβm especially interested in whether thatβs a `std::filesystem` wrapper, a custom implementation, or something else entirely. Therefore, I need to understand `generic_read_dir`'s function. Let's dig into that function's details and get a handle on the behavior. I need to understand the characteristics of `generic_read_dir`!
|
| 19/2 |
2026/07/02 00:24 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/readdir.c Name:generic_read_dir]
Results: map[SourceCode: 223: ssize_t generic_read_dir(struct file *filp, char __user *buf, size_t siz, loff_t *ppos)
224: {
225: return -EISDIR;
226: }
SourceFile:fs/libfs.c]
|
| 20/2 |
2026/07/02 00:24 |
llm |
expert |
0m
Model:
Tokens:
input: 29481
output: 36
thoughts: 62
Thoughts: Okay, let me think this through.
**Investigating a Curious Direct I/O Call in FAT Filesystem**
Alright, hang on a second. I'm seeing `-EISDIR` returned from `generic_read_dir`, which means it's hitting a directory. But if `read` on a FAT directory returns `-EISDIR`, then how in the world did we even get to `fat_direct_IO` in the first place? That's the part that's tripping me up. It means whatever we were trying to read *wasn't* a directory to begin with. So, it *must* have been a regular file. Now, I need to confirm this quickly. Let's pull up the `fat_file_operations` structure and see how we're set up. This will tell me what file operations are supported.
|
| 21/2 |
2026/07/02 00:24 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/fat/file.c Name:fat_file_operations]
Results: map[SourceCode: 206: const struct file_operations fat_file_operations = {
207: .llseek = generic_file_llseek,
208: .read_iter = generic_file_read_iter,
209: .write_iter = generic_file_write_iter,
210: .mmap_prepare = generic_file_mmap_prepare,
211: .release = fat_file_release,
212: .unlocked_ioctl = fat_generic_ioctl,
213: .compat_ioctl = compat_ptr_ioctl,
214: .fsync = fat_file_fsync,
215: .splice_read = filemap_splice_read,
216: .splice_write = iter_file_splice_write,
217: .fallocate = fat_fallocate,
218: .setlease = generic_setlease,
219: };
SourceFile:fs/fat/file.c]
|
| 22/2 |
2026/07/02 00:24 |
llm |
expert |
0m
Model:
Tokens:
input: 30051
output: 39
thoughts: 2399
Thoughts: **Analysis of a Hung Task in a Lockdep Scenario**
Okay, let's break this down. From the traces, I see that `.read_iter = generic_file_read_iter` indicating task 6435 is reading a regular file. Task 5599 is doing a `getdents64` on a directory. The puzzling piece is that task 6435 is blocked on `inode_lock`. I'm immediately trying to figure out if these tasks are acting on the *same* inode. That seems like the logical starting point. But, `getdents64` inherently checks if it's a directory, so a file can't be opened as a directory, and `iterate_dir` wouldn't return anything unless it was a directory to begin with.
So, since 5599 is calling `touch_atime`, which is called from `iterate_dir`, task 5599 *must* be operating on a directory. Task 6435 is on a regular file. Why would there be lock contention here? Maybe 6435 is blocked on the inode lock of the file and somebody else holds it. I need to check the other tasks.
I'm checking the tasks and their locks. Task 7899 holds `i_mutex_dir_key` in `filename_create`. Task 5926 holds `&sb->s_type->i_mutex_key#9` in `blkdev_read_iter`. Task 6435 is blocked on `&sb->s_type->i_mutex_key#33`. Now, `&sb->s_type->i_mutex_key#33` represents the file's `inode_lock`. Okay, is it possible task 5599 is holding the file's `inode_lock`? No, since it's operating on a directory and the file is different. The `getdents64` is a syscall so it can't hold locks across. Wait, can lockdep be running a graph search? No, lockdep doesn't *acquire* the locks itself.
Now, if 5599 is stuck in `__bfs`, it's spinning with interrupts disabled, meaning it's hogging a CPU. If there are only 2 CPUs and the other CPU is running `khungtaskd`, then 6435 *will* hang if it needs a lock held by the task on CPU 0. YES! This is a classic priority inversion / CPU starvation scenario due to lockdep.
Let me explain this. Task A holds a lock and gets preempted. Task B (syz-executor/5599) starts running on CPU 0. Lockdep gets triggered and is stuck in a loop. Task C (syz.0.110/6435) tries to get the lock that task A has and blocks. Because task B is hogging CPU 0, task A will never run, and never releases the lock. Task C is stuck for 143 seconds and hits the hung task timeout. So the root cause is lockdep taking too long in `__bfs`.
Now, is this a *real* bug? Or just a lockdep artifact? Lockdep is for debugging and disabled in production builds, so it's not a security issue, thankfully.
But, I need to make sure there is not another reason for the hang. Could it be a deadlock? If it was a deadlock, lockdep would usually detect and splat a "deadlock detected" report. But it's stuck in `__bfs`, so that's out. Why is lockdep stuck? Syzkaller made the lockdep graph huge. `__bfs` is O(V + E) which can be slow with a massive graph. Could be an infinite loop in lockdep itself.
I'll search for "syzkaller __bfs lockdep" or "syzkaller lockdep infinite loop". I know these issues exist in the past, causing RCU stalls or hung tasks. Like "rcu: INFO: rcu_preempt detected stalls on CPUs/tasks" with `__bfs`. Our hung task is `syz.0.110/6435`, but the NMI backtrace has CPU 0 in `__bfs`, which fits the CPU hogging scenario.
Are there other tasks holding the lock? Task 6435 is doing `fat_direct_IO`. Is some other syzkaller thread writing to the same file? It is common for syzkaller to have threads competing, one holding `inode_lock`, another blocked. If the first thread is preempted and CPU 0 is hogged
|
| 23/2 |
2026/07/02 00:25 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/locking/lockdep.c Name:debug_show_all_locks]
Results: map[SourceCode:6765: void debug_show_all_locks(void)
6766: {
6767: struct task_struct *g, *p;
6768:
6769: if (unlikely(!debug_locks)) {
6770: pr_warn("INFO: lockdep is turned off.\n");
6771: return;
6772: }
6773: pr_warn("\nShowing all locks held in the system:\n");
6774:
6775: rcu_read_lock();
6776: for_each_process_thread(g, p) {
6777: if (!p->lockdep_depth)
6778: continue;
6779: lockdep_print_held_locks(p);
6780: touch_nmi_watchdog();
6781: touch_all_softlockup_watchdogs();
6782: }
6783: rcu_read_unlock();
6784:
6785: pr_warn("\n");
6786: pr_warn("=============================================\n\n");
6787: }
SourceFile:kernel/locking/lockdep.c]
|
| 24/2 |
2026/07/02 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 33858
output: 40
thoughts: 57
Thoughts: **Debugging a Lockdep Mystery**
Okay, so `lockdep_print_held_locks` *should* be the function that spits out the held locks, right? That's what it sounds like. But...why am I seeing "3 locks held by syz-executor/5599:" and *no* actual lock information? That's just...weird. It's like it's teasing me! It's like it's saying "I *could* tell you, but I won't." Time to dig into the code and see what's actually going on inside `lockdep_print_held_locks`. Let's see what's happening. Maybe there's a filtering issue, a bug, or perhaps some condition is being met and the printing is being skipped. Let's see...
|
| 25/2 |
2026/07/02 00:25 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/locking/lockdep.c Name:lockdep_print_held_locks]
Results: map[SourceCode: 786: static void lockdep_print_held_locks(struct task_struct *p)
787: {
788: int i, depth = READ_ONCE(p->lockdep_depth);
789:
790: if (!depth)
791: printk("no locks held by %s/%d.\n", p->comm, task_pid_nr(p));
792: else
793: printk("%d lock%s held by %s/%d:\n", depth,
794: str_plural(depth), p->comm, task_pid_nr(p));
795: /*
796: * It's not reliable to print a task's held locks if it's not sleeping
797: * and it's not the current task.
798: */
799: if (p != current && task_is_running(p))
800: return;
801: for (i = 0; i < depth; i++) {
802: printk(" #%d: ", i);
803: print_lock(p->held_locks + i);
804: }
805: }
SourceFile:kernel/locking/lockdep.c]
|
| 26/2 |
2026/07/02 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 34490
output: 37
thoughts: 1715
Thoughts: **Analysis of a Lockdep-Induced CPU Stall**
Okay, so looking at this, my initial thought is that the reason we're not seeing the held locks for certain tasks is because they're currently running, not sleeping. The code explicitly checks for that, which makes sense. Task 5599 is running and it's stuck in `__bfs` on CPU 0. We've got other tasks running as well: 5603, 5722, 6422, 7884, 7890, 7896. This explains why their locks aren't being printed β they are active.
Aha! So, Task 5599 is actively running on CPU 0 and holds three locks. Since it's stuck within `__bfs`, which is part of the lockdep subsystem, it's effectively hogging the CPU. This is a critical point. If it's hogging the CPU, other tasks might never get a chance to run. If 5599 holds a lock that task 6435 needs, then 6435 will hang. The evidence confirms this is active and we are looking at a spinning CPU, interrupts disabled. It has to be in the `__bfs` function. This means CPU 0 is completely unresponsive. No timer interrupts, no scheduling of other tasks. It's spinning in `__bfs` with interrupts disabled via `_raw_spin_lock_irqsave`.
CPU 1 is running `khungtaskd`, our helpful hung task detector. If CPU 0 is unresponsive, any task needing that CPU, or a lock held by 5599, will hang. Even if a needed lock is held by some other task, it might never run if it is also stuck on CPU 0 or just waiting for CPU time. In short, the root cause is CPU 0 being stuck in `__bfs` with interrupts disabled.
Why is `__bfs` running so long? Because the lockdep graph search is taking too long. It's a known performance issue specific to this debugging tool. This graph search is part of deadlock detection. Remember, in a production build, lockdep is disabled. Specifically, `CONFIG_PROVE_LOCKING` is not set.
Therefore, `__bfs` won't be compiled in, so this hang would not occur. It's a lockdep artifact. This is not exploitable and doesn't cause a denial of service in a production system.
Let's quickly check there's no other possibility. `__bfs` is only in `kernel/locking/lockdep.c`. It's only called by lockdep. The hang is not caused by lockdep reporting a real deadlock because it would print a report and disable itself, not just hang. The stall is because the graph is too large, and the BFS takes a long time. Syzkaller, in particular, is notorious for creating these huge lockdep graphs by creating a complex filesystem and various locking scenarios.
The graph search is for deadlock detection. Lockdep runs `__bfs` when a new dependency is added, to check for cycles. When the graph is huge, this can take a long time, with interrupts disabled by `_raw_spin_lock_irqsave`, causing RCU stalls and hung tasks. This is a well-reported issue, e.g., "lockdep: lockdep graph search takes too long." The kernel has some mitigations, but syzkaller can still hit it.
Let's confirm the stack trace. Yep, adding a new dependency, it's a lockdep graph search and is 100% a lockdep-induced stall.
Now, the security implications. Because lockdep is disabled in production, the relevant code path doesn't exist. This cannot be triggered by an attacker. Therefore, it is not exploitable, does not cause a denial of service, and is not accessible from unprivileged processes or user namespaces. It cannot be triggered from VM guests or hosts, networks, peripherals, or filesystems.
We need to double-check. Maybe the `inode_lock` hang is a real deadlock that would happen even without lockdep? Task 6435 is doing `fat_direct_IO`. It calls `inode_lock(inode)` at `fs/direct-io.c:1136`. It is possible that it already holds the `inode_lock`, or possibly that it gets the lock only after a certain condition. Let's trace through `generic_file_read_iter`.
|
| 27/2 |
2026/07/02 00:25 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/filemap.c Name:generic_file_read_iter]
Results: map[SourceCode:2943: /**
2944: * generic_file_read_iter - generic filesystem read routine
2945: * @iocb: kernel I/O control block
2946: * @iter: destination for the data read
2947: *
2948: * This is the "read_iter()" routine for all filesystems
2949: * that can use the page cache directly.
2950: *
2951: * The IOCB_NOWAIT flag in iocb->ki_flags indicates that -EAGAIN shall
2952: * be returned when no data can be read without waiting for I/O requests
2953: * to complete; it doesn't prevent readahead.
2954: *
2955: * The IOCB_NOIO flag in iocb->ki_flags indicates that no new I/O
2956: * requests shall be made for the read or for readahead. When no data
2957: * can be read, -EAGAIN shall be returned. When readahead would be
2958: * triggered, a partial, possibly empty read shall be returned.
2959: *
2960: * Return:
2961: * * number of bytes copied, even for partial reads
2962: * * negative error code (or 0 if IOCB_NOIO) if nothing was read
2963: */
2964: ssize_t
2965: generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
2966: {
2967: size_t count = iov_iter_count(iter);
2968: ssize_t retval = 0;
2969:
2970: if (!count)
2971: return 0; /* skip atime */
2972:
2973: if (iocb->ki_flags & IOCB_DIRECT) {
2974: struct file *file = iocb->ki_filp;
2975: struct address_space *mapping = file->f_mapping;
2976: struct inode *inode = mapping->host;
2977:
2978: retval = kiocb_write_and_wait(iocb, count);
2979: if (retval < 0)
2980: return retval;
2981: file_accessed(file);
2982:
2983: retval = mapping->a_ops->direct_IO(iocb, iter);
2984: if (retval >= 0) {
2985: iocb->ki_pos += retval;
2986: count -= retval;
2987: }
2988: if (retval != -EIOCBQUEUED)
2989: iov_iter_revert(iter, count - iov_iter_count(iter));
2990:
2991: /*
2992: * Btrfs can have a short DIO read if we encounter
2993: * compressed extents, so if there was an error, or if
2994: * we've already read everything we wanted to, or if
2995: * there was a short read because we hit EOF, go ahead
2996: * and return. Otherwise fallthrough to buffered io for
2997: * the rest of the read. Buffered reads will not work for
2998: * DAX files, so don't bother trying.
2999: */
3000: if (retval < 0 || !count || IS_DAX(inode))
3001: return retval;
3002: if (iocb->ki_pos >= i_size_read(inode))
3003: return retval;
3004: }
3005:
3006: return filemap_read(iocb, iter, retval);
3007: }
SourceFile:mm/filemap.c]
|
| 28/2 |
2026/07/02 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 38294
output: 66
thoughts: 1192
Thoughts: **Analysis of a Lockdep-Related Hang**
Okay, let me break this down. I'm looking at a situation where a task (6435) appears to be blocked on `inode_lock`. Initially, I traced the execution path through `generic_file_read_iter`, `fat_direct_IO`, and eventually to `__blockdev_direct_IO`, where `inode_lock` *is* acquired. However, lockdep is reporting the lock as held when the task is actually blocked trying to acquire it. This threw me for a moment, but then I remembered that `down_write` (used by `inode_lock`) uses `rwsem_acquire` before blocking, which causes lockdep to register the lock as "held" even before the actual wait. This is standard behavior for lockdep and sleeping locks.
So, 6435 isn't *holding* the lock; it's *waiting* for it. The question then becomes, who *is* holding it? Initially, I considered the possibility of recursion, but the code doesn't support that here.
My thoughts drifted to the possibility that the task holding the lock might be starved of CPU time because CPU 0 is hogged by another task (5599) spinning in `__bfs`. Or, task 5599 itself, which I've established is doing `getdents64` on a directory, might be holding *that* directory's `inode_lock`. But then I had a breakthrough! It *is* possible that another syzkaller task is performing a `write` operation and holds the file's `inode_lock`. If that task is preempted and stuck on CPU 0's runqueue, it's blocked, potentially creating a bottleneck. The other possibility is that if the lock dependency graph search is slow, it might block multiple tasks.
The key observation is that the NMI backtrace confirms that CPU 0 is hung in `__bfs`. And I know that `__bfs` can be slow and cause hung tasks, which is documented. Since lockdep is a debugging tool, this isn't a bug in the production kernel.
My analysis then moved to checking alternative possibilities. Could `__bfs` be stuck in an infinite loop due to a lockdep bug? Even if that's the case, it is a bug in lockdep which is not enabled in production. Could there be a genuine deadlock in the FAT filesystem? If there were a real deadlock, lockdep would detect it and print a deadlock warning, not hang within `__bfs`. The hang occurs during the process of adding a *new* dependency (`check_prevs_add`), when it's searching for a cyclic path. If the graph is large, this search takes a long time. So it's just a performance issue of lockdep.
Based on my understanding of this entire sequence of events, I'm confident that the root cause is the time-consuming lockdep graph search in `__bfs`. Therefore, I'm classifying this as a non-exploitable performance issue within the debugging tool, lockdep. I'm setting all results to false.
|
| 29/2 |
2026/07/02 00:25 |
tool |
set-results |
0m
Args: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 30/2 |
2026/07/02 00:25 |
llm |
expert |
0m
Model:
Tokens:
input: 40294
output: 550
thoughts: 405
Thoughts: **My Analysis of the Hung Task**
Okay, the data is in, time to wrap this up. Looking at this bug report, I see a classic lockdep-induced hang. The `syz.0.110` task is stuck waiting on `inode_lock` within `__blockdev_direct_IO`, which traces back to `fat_direct_IO`. Simultaneously, CPU 0 is buried deep in `lockdep.c`, specifically executing `__bfs` β a breadth-first search within the lock dependency graph. The NMI backtrace confirms this, showing the execution path from `__lock_acquire` down to `__bfs`.
The problem is immediately clear to me. `syz-executor/5599`, running on CPU 0, has the `lockdep` spinlock held, which means interrupts and preemption are disabled. This is the root cause. Syzkaller is known for generating intricate lock dependency graphs with an immense number of locks acquired in complex sequences. When lockdep's graph becomes excessively large, the `O(V + E)` complexity of `__bfs` means it can bog down the CPU. Since this graph search, which is intended to validate a new lock dependency, is performed with interrupts disabled, the CPU gets completely stalled, a not uncommon issue with syzkaller-generated workloads, particularly under heavy lock stress.
Because the stalled CPU is running `__bfs`, it can't release the lock on CPU 0, and this prevents progress by the blocked task on `inode_lock`. This cascading effect leads to the 143-second timeout and the `hung_task_timeout` panic.
Now, here's the kicker: `lockdep` is a debugging tool, a classic `CONFIG_PROVE_LOCKING` feature. It's not enabled in production builds. Therefore, this stall and subsequent hung task are purely artifacts of debugging instrumentation. There's zero security impact here. The analysis dimensions are all negative. This is a classic case of debugging overhead manifesting itself, but it's not a functional issue. It's a "feature" of the debug tool, not a failure of the system.
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Results: map[Explanation:Based on the provided crash report and source code analysis, here is the
assessment of the bug:
**Analysis:**
The bug report indicates a `hung_task_timeout` where the task `syz.0.110` is
blocked for more than 143 seconds trying to acquire the `inode_lock` in
`__blockdev_direct_IO` (called via `fat_direct_IO`).
Looking at the NMI backtrace for the other CPU (CPU 0), we can see that task
`syz-executor/5599` is actively executing `__bfs` inside
`kernel/locking/lockdep.c`:
```
RIP: 0010:__bfs kernel/locking/lockdep.c:1815 [inline]
...
RIP: 0010:check_prevs_add kernel/locking/lockdep.c:3284 [inline]
RIP: 0010:validate_chain kernel/locking/lockdep.c:3908 [inline]
RIP: 0010:__lock_acquire+0x18fa/0x2ce0 kernel/locking/lockdep.c:5237
```
The task on CPU 0 is holding the internal `lockdep` raw spinlock (which disables
interrupts and preemption) and is performing a breadth-first search (`__bfs`) on
the lock dependency graph to validate a newly observed lock dependency.
Syzkaller frequently generates massive lock dependency graphs by creating many
files/directories and acquiring their locks in various complex orders. When the
lockdep graph becomes extremely large, the O(V + E) `__bfs` algorithm can take
an excessively long time to complete. Because this search is performed with
interrupts disabled, it completely stalls the CPU.
This CPU stall prevents other tasksβincluding whichever task is currently
holding the `inode_lock` that `syz.0.110` is waiting forβfrom being scheduled
and making progress. Consequently, the waiting task hangs for over 143 seconds,
triggering the `hung_task_timeout` panic.
**Conclusion:**
This issue is entirely caused by the overhead of the kernel lock validator
(`lockdep`), which is a debugging tool (`CONFIG_PROVE_LOCKING`). In production
kernel builds, `lockdep` is disabled, meaning the `__bfs` graph search code is
not compiled in and this CPU stall cannot occur. Therefore, this is a debugging
artifact and poses no security risk or denial-of-service threat in production
environments.]
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