Re: [PATCH 0/2] ext4: Fix possible fs corruption due to xattr races

[Ritesh Harjani <ritesh.list@gmail.com>]

On 22/06/06 04:28PM, Jan Kara wrote:
> Hello,
>
> I've tracked down the culprit of the jbd2 assertion Ritesh reported to me. In

Hello Jan,

Thanks for working on the problem and identifying the race.

> the end it does not have much to do with jbd2 but rather points to a subtle
> race in xattr code between xattr block reuse and xattr block freeing that can
> result in fs corruption during journal replay. See patch 2/2 for more details.
> These patches fix the problem. I have to say I'm not too happy with the special

So while I was still reviewing this patch-set, I thought of giving a try with some
stress test for xattrs (given that this is some sort of race which is not always
easy to track down).

So it seems it is easy to recreate the crash with stress-ng xattr test (even
with your patches included).
	stress-ng --xattr 16 --timeout=10000s

Hope this might help further narrow down the problem.

root@qemu:/home/qemu# [  257.862064] ------------[ cut here ]------------
[  257.862834] kernel BUG at fs/jbd2/revoke.c:460!
[  257.863461] invalid opcode: 0000 [#1] PREEMPT SMP PTI
[  257.864084] CPU: 0 PID: 1499 Comm: stress-ng-xattr Not tainted 5.18.0-rc5+
#102
[  257.864973] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.13.0-1ubuntu1.1 04/01/2014
[  257.865972] RIP: 0010:jbd2_journal_cancel_revoke+0x12c/0x170
[  257.866606] Code: 49 89 44 24 08 e8 b4 bf d8 00 48 8b 3d 2d f9 29 02 4c 89 e6
e8 c5 81 ea ff 48 8b 73 18 4c 89 ef e8 39 f8
[  257.868547] RSP: 0018:ffffc9000170b9c0 EFLAGS: 00010286
[  257.869106] RAX: ffff888101cadb00 RBX: ffff888121cb9f08 RCX: 0000000000000000
[  257.869837] RDX: 0000000000000001 RSI: 000000000000242d RDI: 00000000ffffffff
[  257.870552] RBP: ffffc9000170b9e0 R08: ffffffff82cf2f20 R09: ffff888108831e10
[  257.871264] R10: 00000000000000bb R11: 000000000105d68a R12: ffff888108831e10
[  257.871977] R13: ffff888120937000 R14: ffff888108928500 R15: ffff888108831e18
[  257.872689] FS:  00007ffff6b4dc00(0000) GS:ffff88842fc00000(0000)
knlGS:0000000000000000
[  257.873528] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  257.874101] CR2: 0000555556361220 CR3: 00000001201dc000 CR4: 00000000000006f0
[  257.874813] Call Trace:
[  257.875077]  <TASK>
[  257.875313]  do_get_write_access+0x3d9/0x460
[  257.875753]  jbd2_journal_get_write_access+0x54/0x80
[  257.876260]  __ext4_journal_get_write_access+0x8b/0x1b0
[  257.876805]  ? ext4_dirty_inode+0x70/0x80
[  257.877255]  ext4_xattr_block_set+0x935/0xfb0
[  257.877709]  ext4_xattr_set_handle+0x5c8/0x680
[  257.878159]  ext4_xattr_set+0xd5/0x180
[  257.878540]  ext4_xattr_user_set+0x35/0x40
[  257.878957]  __vfs_removexattr+0x5a/0x70
[  257.879373]  __vfs_removexattr_locked+0xc5/0x160
[  257.879846]  vfs_removexattr+0x5b/0x100
[  257.880235]  removexattr+0x61/0x90
[  257.880611]  ? kvm_clock_read+0x18/0x30
[  257.881023]  ? kvm_clock_get_cycles+0x9/0x10
[  257.881492]  ? ktime_get+0x3e/0xa0
[  257.881856]  ? native_apic_msr_read+0x40/0x40
[  257.882302]  ? lapic_next_event+0x21/0x30
[  257.882716]  ? clockevents_program_event+0x8f/0xe0
[  257.883206]  ? hrtimer_update_next_event+0x4b/0x70
[  257.883698]  ? debug_smp_processor_id+0x17/0x20
[  257.884181]  ? preempt_count_add+0x4d/0xc0
[  257.884605]  __x64_sys_fremovexattr+0x82/0xb0
[  257.885063]  do_syscall_64+0x3b/0x90
[  257.885495]  entry_SYSCALL_64_after_hwframe+0x44/0xae
<...>
[  257.892816]  </TASK>

-ritesh

> mbcache interface I had to add because it just requires too deep knowledge of
> how things work internally to get things right. If you get it wrong, you'll
> have subtle races like above. But I didn't find a more transparent way to
> fix this race. If someone has ideas, suggestions are welcome!
>
> 								Honza