Re: [syzbot] [fuse?] WARNING in fuse_writepages

[Joanne Koong]

On Tue, Oct 1, 2024 at 12:24 PM syzbot
<syzbot+217a976dc26ef2fa8711@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    e32cde8d2bd7 Merge tag 'sched_ext-for-6.12-rc1-fixes-1' of..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=12e8bdd0580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=1b5201b91035a876
> dashboard link: https://syzkaller.appspot.com/bug?extid=217a976dc26ef2fa8711
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/a585cdb91cda/disk-e32cde8d.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/dbeec5d7b296/vmlinux-e32cde8d.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/000fd790e08a/bzImage-e32cde8d.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+217a976dc26ef2fa8711@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 5296 at fs/fuse/file.c:1989 fuse_write_file_get fs/fuse/file.c:1989 [inline]
> WARNING: CPU: 0 PID: 5296 at fs/fuse/file.c:1989 fuse_write_file_get fs/fuse/file.c:1986 [inline]
> WARNING: CPU: 0 PID: 5296 at fs/fuse/file.c:1989 fuse_writepages+0x497/0x5a0 fs/fuse/file.c:2368
> Modules linked in:
> CPU: 0 UID: 0 PID: 5296 Comm: kworker/u8:8 Not tainted 6.12.0-rc1-syzkaller-00031-ge32cde8d2bd7 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
> Workqueue: writeback wb_workfn (flush-0:52)
> RIP: 0010:fuse_write_file_get fs/fuse/file.c:1989 [inline]
> RIP: 0010:fuse_write_file_get fs/fuse/file.c:1986 [inline]
> RIP: 0010:fuse_writepages+0x497/0x5a0 fs/fuse/file.c:2368
> Code: 00 00 00 44 89 f8 5b 5d 41 5c 41 5d 41 5e 41 5f c3 cc cc cc cc e8 79 b6 90 fe 48 8b 7c 24 08 e8 af 6f 27 08 e8 6a b6 90 fe 90 <0f> 0b 90 41 bf fb ff ff ff eb 8b e8 59 b6 90 fe 48 8b 7c 24 18 be
> RSP: 0018:ffffc900044ff4a8 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: ffffc900044ff4f8 RCX: 0000000000000000
> RDX: ffff88802d42da00 RSI: ffffffff82fcd286 RDI: 0000000000000001
> RBP: ffff88805c994aa0 R08: 0000000000000000 R09: ffffed100b9329d7
> R10: ffff88805c994ebb R11: 0000000000000003 R12: ffffc900044ff840
> R13: ffff88805c994880 R14: ffff88805f330000 R15: ffff88805c994d50
> FS:  0000000000000000(0000) GS:ffff8880b8600000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020055000 CR3: 000000005df4a000 CR4: 00000000003526f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  do_writepages+0x1a3/0x7f0 mm/page-writeback.c:2683
>  __writeback_single_inode+0x166/0xfa0 fs/fs-writeback.c:1658
>  writeback_sb_inodes+0x603/0xfa0 fs/fs-writeback.c:1954
>  wb_writeback+0x199/0xb50 fs/fs-writeback.c:2134
>  wb_do_writeback fs/fs-writeback.c:2281 [inline]
>  wb_workfn+0x294/0xbc0 fs/fs-writeback.c:2321
>  process_one_work+0x9c5/0x1ba0 kernel/workqueue.c:3229
>  process_scheduled_works kernel/workqueue.c:3310 [inline]
>  worker_thread+0x6c8/0xf00 kernel/workqueue.c:3391
>  kthread+0x2c1/0x3a0 kernel/kthread.c:389
>  ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
>  </TASK>
>

#syz dup: [syzbot] [fuse?] WARNING in fuse_write_file_get (2)

This is the same warning reported in
https://lore.kernel.org/linux-fsdevel/66fbae38.050a0220.6bad9.0051.GAE@xxxxxxxxxx/T/#u

The warning is complaining about this WARN_ON here
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/fuse/file.c#n1989.
I think this warning can get triggered if there's a race between a
write() and a close() where the page is dirty in the cache after the
release has happened. Then when writeback (eg fuse_writepages()) is
triggered, we hit this warning. (this possibility has always existed,
it was surfaced after this refactoring commit 4046d3adcca4: "move fuse
file initialization to wpa allocation time" but the actual logic
hasn't been changed).

I think we can address this by instead calling "data.ff =
__fuse_write_file_get(fi);" in fuse_writepages(). I'll submit a fix
for this to Miklos's tree.


Thanks,
Joanne

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