syzbot

map[ReproOpts:{ "procs": 2, "repeat": true, "threaded": true, "collide": true } ReproSyz:r0 = memfd_create(&(0x7f0000000000)='fat\x00', 0x0)
fallocate(r0, 0x0, 0x0, 0x10000000)
write(r0, &(0x7f0000000100)='\xeb\x58\x90\x4d\x53\x57\x49\x4e\x34\x2e\x31\x00\x02\x01\x20\x00\x02\x00\x00\x00\x00\xf8\x00\x00\x20\x00\x40\x00\x00\x00\x00\x00\x00\x08\x00\x00\x10\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00\x01\x00\x06\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x29\x12\x34\x56\x78\x4e\x4f\x20\x4e\x41\x4d\x45\x20\x20\x20\x20\x46\x41\x54\x33\x32\x20\x20\x20', 0x5a)
pwrite64(r0, &(0x7f0000000200)='\x55\xaa', 0x2, 0x1fe)
pwrite64(r0, &(0x7f0000000300)='\xf8\xff\xff\x0f\xff\xff\xff\x0f', 0x8, 0x4000)
pwrite64(r0, &(0x7f0000000300)='\xf8\xff\xff\x0f\xff\xff\xff\x0f', 0x8, 0x204000)
r1 = syz_open_dev(&(0x7f0000000400)='loop\x00', 0x0, 0x0)
ioctl$LOOP_SET_FD(r1, 0x4c00, r0)
mount(&(0x7f0000000500)='/dev/loop0\x00', &(0x7f0000000600)='./mnt\x00', &(0x7f0000000700)='vfat\x00', 0x0, 0x0)
r2 = openat(0xffffffffffffff9c, &(0x7f0000000800)='./mnt/file\x00', 0x42, 0x1ff)
r3 = openat(0xffffffffffffff9c, &(0x7f0000000500)='/dev/loop0\x00', 0x0, 0x0)
fallocate(r2, 0x0, 0x0, 0x100000)
pread64(r3, &(0x7f0000000900), 0x10000, 0x0)
 SyzkallerCommit:31e9c887f7dc24e04b3ca70d0d54fc34141844b0]

map[DescriptionFiles:[acpi_thermal_rel.txt aio.txt auto.txt binfmt.txt bpf.txt bpf_prog.txt bpf_trace.txt cgroup.txt damon.txt dev_ashmem.txt dev_bifrost.txt dev_binder.txt dev_binderfs.txt dev_block.txt dev_bsg.txt dev_bus_usb.txt dev_camx.txt dev_cdrom.txt dev_cec.txt dev_char_usb.txt dev_comedi.txt dev_dma_heap.txt dev_dri.txt dev_dsp.txt dev_dvb_demux.txt dev_dvb_dvr.txt dev_dvb_frontend.txt dev_fb.txt dev_floppy.txt dev_hidraw.txt dev_i2c.txt dev_i915.txt dev_img_rogue.txt dev_infiniband_rdma.txt dev_infiniband_rdma_cm.txt dev_input.txt dev_iommu.txt dev_kvm.txt dev_kvm_amd64.txt dev_kvm_arm64.txt dev_kvm_extra.txt dev_kvm_riscv64.txt dev_loop.txt dev_mali.txt dev_media.txt dev_msm.txt dev_msr.txt dev_nbd.txt dev_net_tun.txt dev_panthor.txt dev_ppp.txt dev_ptmx.txt dev_ptp.txt dev_qat_adf_ctl.txt dev_qrtr_tun.txt dev_random.txt dev_rfkill.txt dev_rtc.txt dev_sequencer.txt dev_sg.txt dev_snapshot.txt dev_snd_control.txt dev_snd_hw.txt dev_snd_midi.txt dev_snd_pcm.txt dev_snd_seq.txt dev_snd_timer.txt dev_sr.txt dev_sw_sync.txt dev_tlk_device.txt dev_trusty.txt dev_udmabuf.txt dev_uhid.txt dev_uinput.txt dev_usb_hiddev.txt dev_usbmon.txt dev_userio.txt dev_vfio.txt dev_vga_arbiter.txt dev_vhci.txt dev_video4linux.txt dev_video4linux_vim2m.txt dev_virtual_nci.txt dev_vtpm.txt fanotify.txt filesystem.txt fs_9p.txt fs_fuse.txt fs_incfs.txt fs_ioctl.txt fs_ioctl_autofs.txt fs_ioctl_btrfs.txt fs_ioctl_ext4.txt fs_ioctl_f2fs.txt fs_ioctl_fat.txt fs_ioctl_fscrypt.txt fs_ioctl_fsverity.txt fs_ioctl_xfs.txt futex.txt hafnium.txt inotify.txt io_uring.txt ipc.txt key.txt kfuzztest.txt l2cap.txt landlock.txt lsm.txt namespaces.txt net_80211.txt netfilter.txt netfilter_arp.txt netfilter_bridge.txt netfilter_ipv4.txt netfilter_ipv6.txt netfilter_ipvs.txt netfilter_targets.txt pagemap_ioctl.txt perf.txt prctl.txt quotactl.txt seccomp.txt security_apparmor.txt security_selinux.txt security_smack.txt socket.txt socket_alg.txt socket_ax25.txt socket_bluetooth.txt socket_caif.txt socket_can.txt socket_ieee802154.txt socket_inet.txt socket_inet6.txt socket_inet_dccp.txt socket_inet_icmp.txt socket_inet_igmp.txt socket_inet_l2tp.txt socket_inet_sctp.txt socket_inet_tcp.txt socket_inet_udp.txt socket_ip_tunnel.txt socket_isdn.txt socket_kcm.txt socket_key.txt socket_llc.txt socket_netlink.txt socket_netlink_audit.txt socket_netlink_crypto.txt socket_netlink_generic.txt socket_netlink_generic_80211.txt socket_netlink_generic_batadv.txt socket_netlink_generic_devlink.txt socket_netlink_generic_ethtool.txt socket_netlink_generic_fou.txt socket_netlink_generic_gtp.txt socket_netlink_generic_mptcp.txt socket_netlink_generic_net_dm.txt socket_netlink_generic_netlabel.txt socket_netlink_generic_nfc.txt socket_netlink_generic_seg6.txt socket_netlink_generic_smc.txt socket_netlink_generic_team.txt socket_netlink_generic_wireguard.txt socket_netlink_netfilter.txt socket_netlink_netfilter_acct.txt socket_netlink_netfilter_conntrack.txt socket_netlink_netfilter_conntrack_exp.txt socket_netlink_netfilter_conntrack_helper.txt socket_netlink_netfilter_ipset.txt socket_netlink_netfilter_log.txt socket_netlink_netfilter_nft_compat.txt socket_netlink_netfilter_nftables.txt socket_netlink_netfilter_osf.txt socket_netlink_netfilter_queue.txt socket_netlink_netfilter_timeout.txt socket_netlink_rdma.txt socket_netlink_route.txt socket_netlink_route_sched.txt socket_netlink_route_sched_retired.txt socket_netlink_sock_diag.txt socket_netlink_xfrm.txt socket_netrom.txt socket_nfc.txt socket_nvme_of_tcp.txt socket_packet.txt socket_phonet.txt socket_pppox.txt socket_qipcrtr.txt socket_rds.txt socket_rose.txt socket_rxrpc.txt socket_tipc.txt socket_tipc_netlink.txt socket_unix.txt socket_vnet.txt socket_x25.txt socket_xdp.txt sys.txt uffd.txt usbip.txt vmw_vmci.txt vnet.txt vnet_mptcp.txt vusb.txt watch_queue.txt xattr.txt] SyzkallerCommit:31e9c887f7dc24e04b3ca70d0d54fc34141844b0]

map[KernelSrc:/usr/local/google/home/dvyukov/syzkaller/agent/workdir/cache/src/2642fa7629c5424fce8f203f307b063c2f008d9f]

map[KernelObj:/usr/local/google/home/dvyukov/syzkaller/agent/workdir/cache/build/8a8d223043183454fcb320f80eb129c08706d7cc]

gemini-3-pro-preview

map[ReproOpts:{ "procs": 2, "repeat": true, "threaded": true, "collide": true }]

You are an expert in linux kernel fuzzing. Your goal is to write a syzkaller program to trigger a specific bug.
Print only the syz program that could be executed directly, without backticks.




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.

Bug Title: KCSAN: data-race in _copy_to_iter / fat32_ent_put

Original Crash Report:
==================================================================
BUG: KCSAN: data-race in _copy_to_iter / fat32_ent_put

read-write to 0xffff888139888070 of 4 bytes by task 5836 on cpu 0:
 fat32_ent_put+0x4e/0x90 fs/fat/fatent.c:191
 fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:417
 fat_chain_add+0x16c/0x490 fs/fat/misc.c:136
 fat_add_cluster fs/fat/inode.c:113 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x46c/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
 block_write_begin fs/buffer.c:2256 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2522 [inline]
 cont_write_begin+0x1b0/0x970 fs/buffer.c:2584
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2485
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:223
 fat_fallocate+0x177/0x1c0 fs/fat/file.c:301
 vfs_fallocate+0x3b6/0x400 fs/open.c:339
 ksys_fallocate fs/open.c:363 [inline]
 __do_sys_fallocate fs/open.c:368 [inline]
 __se_sys_fallocate fs/open.c:366 [inline]
 __x64_sys_fallocate+0x7a/0xd0 fs/open.c:366
 x64_sys_call+0x1cd4/0x3000 arch/x86/include/generated/asm/syscalls_64.h:286
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888139888000 of 512 bytes by task 3314 on cpu 1:
 instrument_copy_to_user include/linux/instrumented.h:113 [inline]
 copy_to_user_iter lib/iov_iter.c:24 [inline]
 iterate_ubuf include/linux/iov_iter.h:30 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:302 [inline]
 iterate_and_advance include/linux/iov_iter.h:330 [inline]
 _copy_to_iter+0x130/0xea0 lib/iov_iter.c:197
 copy_page_to_iter+0x18f/0x2d0 lib/iov_iter.c:374
 copy_folio_to_iter include/linux/uio.h:204 [inline]
 filemap_read+0x407/0xa10 mm/filemap.c:2851
 blkdev_read_iter+0x22d/0x2e0 block/fops.c:856
 new_sync_read fs/read_write.c:491 [inline]
 vfs_read+0x6c0/0x7f0 fs/read_write.c:572
 ksys_read+0xdc/0x1a0 fs/read_write.c:715
 __do_sys_read fs/read_write.c:724 [inline]
 __se_sys_read fs/read_write.c:722 [inline]
 __x64_sys_read+0x40/0x50 fs/read_write.c:722
 x64_sys_call+0x2889/0x3000 arch/x86/include/generated/asm/syscalls_64.h:1
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 3314 Comm: udevd Not tainted syzkaller #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


The list of existing description files:
acpi_thermal_rel.txt
aio.txt
auto.txt
binfmt.txt
bpf.txt
bpf_prog.txt
bpf_trace.txt
cgroup.txt
damon.txt
dev_ashmem.txt
dev_bifrost.txt
dev_binder.txt
dev_binderfs.txt
dev_block.txt
dev_bsg.txt
dev_bus_usb.txt
dev_camx.txt
dev_cdrom.txt
dev_cec.txt
dev_char_usb.txt
dev_comedi.txt
dev_dma_heap.txt
dev_dri.txt
dev_dsp.txt
dev_dvb_demux.txt
dev_dvb_dvr.txt
dev_dvb_frontend.txt
dev_fb.txt
dev_floppy.txt
dev_hidraw.txt
dev_i2c.txt
dev_i915.txt
dev_img_rogue.txt
dev_infiniband_rdma.txt
dev_infiniband_rdma_cm.txt
dev_input.txt
dev_iommu.txt
dev_kvm.txt
dev_kvm_amd64.txt
dev_kvm_arm64.txt
dev_kvm_extra.txt
dev_kvm_riscv64.txt
dev_loop.txt
dev_mali.txt
dev_media.txt
dev_msm.txt
dev_msr.txt
dev_nbd.txt
dev_net_tun.txt
dev_panthor.txt
dev_ppp.txt
dev_ptmx.txt
dev_ptp.txt
dev_qat_adf_ctl.txt
dev_qrtr_tun.txt
dev_random.txt
dev_rfkill.txt
dev_rtc.txt
dev_sequencer.txt
dev_sg.txt
dev_snapshot.txt
dev_snd_control.txt
dev_snd_hw.txt
dev_snd_midi.txt
dev_snd_pcm.txt
dev_snd_seq.txt
dev_snd_timer.txt
dev_sr.txt
dev_sw_sync.txt
dev_tlk_device.txt
dev_trusty.txt
dev_udmabuf.txt
dev_uhid.txt
dev_uinput.txt
dev_usb_hiddev.txt
dev_usbmon.txt
dev_userio.txt
dev_vfio.txt
dev_vga_arbiter.txt
dev_vhci.txt
dev_video4linux.txt
dev_video4linux_vim2m.txt
dev_virtual_nci.txt
dev_vtpm.txt
fanotify.txt
filesystem.txt
fs_9p.txt
fs_fuse.txt
fs_incfs.txt
fs_ioctl.txt
fs_ioctl_autofs.txt
fs_ioctl_btrfs.txt
fs_ioctl_ext4.txt
fs_ioctl_f2fs.txt
fs_ioctl_fat.txt
fs_ioctl_fscrypt.txt
fs_ioctl_fsverity.txt
fs_ioctl_xfs.txt
futex.txt
hafnium.txt
inotify.txt
io_uring.txt
ipc.txt
key.txt
kfuzztest.txt
l2cap.txt
landlock.txt
lsm.txt
namespaces.txt
net_80211.txt
netfilter.txt
netfilter_arp.txt
netfilter_bridge.txt
netfilter_ipv4.txt
netfilter_ipv6.txt
netfilter_ipvs.txt
netfilter_targets.txt
pagemap_ioctl.txt
perf.txt
prctl.txt
quotactl.txt
seccomp.txt
security_apparmor.txt
security_selinux.txt
security_smack.txt
socket.txt
socket_alg.txt
socket_ax25.txt
socket_bluetooth.txt
socket_caif.txt
socket_can.txt
socket_ieee802154.txt
socket_inet.txt
socket_inet6.txt
socket_inet_dccp.txt
socket_inet_icmp.txt
socket_inet_igmp.txt
socket_inet_l2tp.txt
socket_inet_sctp.txt
socket_inet_tcp.txt
socket_inet_udp.txt
socket_ip_tunnel.txt
socket_isdn.txt
socket_kcm.txt
socket_key.txt
socket_llc.txt
socket_netlink.txt
socket_netlink_audit.txt
socket_netlink_crypto.txt
socket_netlink_generic.txt
socket_netlink_generic_80211.txt
socket_netlink_generic_batadv.txt
socket_netlink_generic_devlink.txt
socket_netlink_generic_ethtool.txt
socket_netlink_generic_fou.txt
socket_netlink_generic_gtp.txt
socket_netlink_generic_mptcp.txt
socket_netlink_generic_net_dm.txt
socket_netlink_generic_netlabel.txt
socket_netlink_generic_nfc.txt
socket_netlink_generic_seg6.txt
socket_netlink_generic_smc.txt
socket_netlink_generic_team.txt
socket_netlink_generic_wireguard.txt
socket_netlink_netfilter.txt
socket_netlink_netfilter_acct.txt
socket_netlink_netfilter_conntrack.txt
socket_netlink_netfilter_conntrack_exp.txt
socket_netlink_netfilter_conntrack_helper.txt
socket_netlink_netfilter_ipset.txt
socket_netlink_netfilter_log.txt
socket_netlink_netfilter_nft_compat.txt
socket_netlink_netfilter_nftables.txt
socket_netlink_netfilter_osf.txt
socket_netlink_netfilter_queue.txt
socket_netlink_netfilter_timeout.txt
socket_netlink_rdma.txt
socket_netlink_route.txt
socket_netlink_route_sched.txt
socket_netlink_route_sched_retired.txt
socket_netlink_sock_diag.txt
socket_netlink_xfrm.txt
socket_netrom.txt
socket_nfc.txt
socket_nvme_of_tcp.txt
socket_packet.txt
socket_phonet.txt
socket_pppox.txt
socket_qipcrtr.txt
socket_rds.txt
socket_rose.txt
socket_rxrpc.txt
socket_tipc.txt
socket_tipc_netlink.txt
socket_unix.txt
socket_vnet.txt
socket_x25.txt
socket_xdp.txt
sys.txt
uffd.txt
usbip.txt
vmw_vmci.txt
vnet.txt
vnet_mptcp.txt
vusb.txt
watch_queue.txt
xattr.txt

r0 = memfd_create(&(0x7f0000000000)='fat\x00', 0x0)
fallocate(r0, 0x0, 0x0, 0x10000000)
write(r0, &(0x7f0000000100)='\xeb\x58\x90\x4d\x53\x57\x49\x4e\x34\x2e\x31\x00\x02\x01\x20\x00\x02\x00\x00\x00\x00\xf8\x00\x00\x20\x00\x40\x00\x00\x00\x00\x00\x00\x08\x00\x00\x10\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00\x01\x00\x06\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x29\x12\x34\x56\x78\x4e\x4f\x20\x4e\x41\x4d\x45\x20\x20\x20\x20\x46\x41\x54\x33\x32\x20\x20\x20', 0x5a)
pwrite64(r0, &(0x7f0000000200)='\x55\xaa', 0x2, 0x1fe)
pwrite64(r0, &(0x7f0000000300)='\xf8\xff\xff\x0f\xff\xff\xff\x0f', 0x8, 0x4000)
pwrite64(r0, &(0x7f0000000300)='\xf8\xff\xff\x0f\xff\xff\xff\x0f', 0x8, 0x204000)
r1 = syz_open_dev(&(0x7f0000000400)='loop\x00', 0x0, 0x0)
ioctl$LOOP_SET_FD(r1, 0x4c00, r0)
mount(&(0x7f0000000500)='/dev/loop0\x00', &(0x7f0000000600)='./mnt\x00', &(0x7f0000000700)='vfat\x00', 0x0, 0x0)
r2 = openat(0xffffffffffffff9c, &(0x7f0000000800)='./mnt/file\x00', 0x42, 0x1ff)
r3 = openat(0xffffffffffffff9c, &(0x7f0000000500)='/dev/loop0\x00', 0x0, 0x0)
fallocate(r2, 0x0, 0x0, 0x100000)
pread64(r3, &(0x7f0000000900), 0x10000, 0x0)

gemini-3-pro-preview

							input: 3253
							output: 40
							thoughts: 24096
						

**Reproducing the KCSAN Data Race in `_copy_to_iter` / `fat32_ent_put`**

Okay, here's the plan to nail down this data race. The goal is to create a syzkaller program that consistently triggers the KCSAN report.  We're looking at a race between `_copy_to_iter` and `fat32_ent_put`, which boils down to a conflict where one thread reads block data directly while another updates the FAT metadata that underlies that very data.

First, the analysis: The crash occurs when one thread modifies the FAT (specifically, via `fat32_ent_put` which gets called during `fallocate`) and another reads the raw block device containing the FAT filesystem. `fallocate` is the key writer here, because it modifies FAT entries in the filesystem (a vfat/fat32 in this instance). `read` is the key reader from the block device, potentially reading the very sectors where the FAT entries are stored.

We need a syzkaller program that will set this up.  We will need:

1.  A loopback device – a block device we can format and mount as a FAT filesystem.  I think syzkaller will usually create one, and we can make sure it is FAT32.
2.  Formatting the Loopback Image: We need to set it up so it is formatted as FAT32, which is required for `fat32_ent_put`.
3.  Mounting: We will mount the loopback device as a vfat filesystem.
4.  Opening the Block Device: We need to open the loopback device *itself* for raw reads.  This is essential.
5.  Writer Thread: This thread will open a file *inside* the mounted FAT filesystem and run `fallocate`.  This will allocate disk space and, critically, modify the FAT entries.
6.  Reader Thread: This thread will read from the opened block device *directly*.  This is our contender, reading in the background.

The core idea is to create a conflict: Writer modifies FAT entries, reader reads those entries (or nearby sectors) directly from the block device.

Here is the plan to ensure a FAT32 image is used:

1.  We'll use `memfd_create` and `fallocate` to set up the backing storage (we need control over the image).  This is safer than relying on `syz_mount_image`'s potentially limited formatting capabilities (it might default to FAT16, and this crash report is about FAT32).
2.  Next, we'll write a *minimal* FAT32 Boot Sector (BPB) and FAT data to the memfd.  I will construct a string to write a minimal BPB, which will set up the FAT as we intend. I will also write valid FAT entries (initial).
3.  We'll use `syz_open_dev` to get a file descriptor for a loop device to make it easy. We will make sure this works.
4.  We will use `ioctl` and `LOOP_SET_FD` to associate our memfd (the FAT32 image) with the loop device.
5.  Then, we will mount the loop device using `mount`.
6.  Next, we'll open a file in that mounted directory with `O_CREAT|O_RDWR` which we can `fallocate` into.
7.  We'll open the loop device a second time for reading.
8.  Finally, we'll launch the race: one thread will `fallocate` (writer), the other will read from the loop device (reader).  `fallocate` updates the FAT; `read` potentially reads those same FAT sectors, triggering the race condition.  The `pread` syscall is important here as it lets us specify an offset to read and control what we are reading, and `fat32_ent_put` has been observed to happen, the race is in progress.

Key details:

*   **FAT32 Image:** The crash report mentions `fat32_ent_put`, so it *must* be a FAT32 filesystem.  If we get FAT16, we're likely dead in the water. That's why manual image creation is better.
*   **Loop Device:** We need to access the raw loop device *after* mounting.  This is how we'll trigger `blkdev_read_iter`.
*   **`fallocate`:**  This syscall is our weapon for updating the FAT. We want it to allocate clusters and trigger those writes.
*   **`pread64`**:  This is our reader, reading raw data from the block device.

map[ReproOpts:{ "procs": 2, "repeat": true, "threaded": true, "collide": true }]

map[ReproOpts:{ "procs": 2, "repeat": true, "threaded": true, "collide": true }]

gemini-3-pro-preview

							input: 28452
							output: 929
							thoughts: 0