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Help debugging a use after free

To: xfs@xxxxxxxxxxx
Subject: Help debugging a use after free
From: Stuart MENEFY <stuart.menefy@xxxxxx>
Date: Mon, 13 Apr 2009 18:42:10 +0100
User-agent: Thunderbird 2.0.0.17 (X11/20080914)
Folks

For the last few days I've been trying to debug an apparent use after
free in XFS code. This is in a 2.6.23.17 kernel on an SH4 processor,
so this could easily be an XFS problem which has already been solved,
or an architecture problem. This started because I've been trying to
work around the D-cache aliasing problems which the SH4 suffers from
because of XFS's use of vmap, so its also possible this is an obscure
aliasing problem. I've also been unable to reproduce it on an x86_64,
albeit with a slightly later kernel.

The problem occurs when running a loop which is essentially:
  mount /dev/sda1 /mnt
  bonnie++  -u root -f -s0 -n16:1024:1024 -b -d /mnt
  umount /mnt
on an otherwise clean partition, and usually takes at least 24 hours
to appear.  The problem only occurs with SLAB poisoning enabled, and
results in a misaligned access because the poison value when treated
as a pointer is an illegal misaligned:

Unaligned kernel access on behalf of "bonnie++" pid=22419 pc=0x841a6c6c 
ins=0x8433
Pid : 22419, Comm:             bonnie++
PC is at xfs_trans_unlocked_item+0xc/0x60
PC  : 841a6c6c SP  : 87f29d3c SR  : 40008000 TEA : 00402ebe    Not tainted
R0  : 00000000 R1  : 841a6c60 R2  : 87f28000 R3  : 00000001
R4  : 6b6b6b6b R5  : 6b6b6b6b R6  : 843342ac R7  : 840139c0
R8  : 6b6b6b6b R9  : 6b6b6b6b R10 : 878cb000 R11 : 8452eb8c
R12 : 00003255 R13 : 00000625 R14 : 87f29d4c
MACH: 00000000 MACL: 0df75800 GBR : 297f9658 PR  : 84189d20

Call trace:
[<84189d20>] xfs_iunlock+0x60/0xc0
[<841915da>] xfs_inode_item_push+0x1a/0x40
[<841a6ff4>] xfs_trans_push_ail+0x1b4/0x240
[<841971b6>] xlog_grant_push_ail+0xf6/0x180
[<8419961c>] xfs_log_reserve+0x3c/0xc0
[<841a540a>] xfs_trans_reserve+0x8a/0x220
[<8418d6f0>] xfs_itruncate_finish+0x170/0x420
[<841a7de0>] xfs_trans_ihold+0x0/0x20
[<841a7de0>] xfs_trans_ihold+0x0/0x20
[<841a7e60>] xfs_trans_ijoin+0x0/0xa0
[<841b112a>] xfs_inactive+0x3ea/0x500
[<84189da0>] xfs_ilock+0x0/0xe0
[<841bf75c>] xfs_fs_clear_inode+0x3c/0xa0
[<843379a0>] _spin_unlock+0x0/0x60
[<840818da>] clear_inode+0x5a/0x140
[<84081a92>] generic_delete_inode+0xd2/0x120
[<84081bc6>] generic_drop_inode+0xe6/0x1c0
[<84080e2e>] iput+0x4e/0x80
[<840764ba>] do_unlinkat+0xfa/0x1a0
[<8408fdba>] do_fsync+0x7a/0xe0
[<8408fe40>] __do_fsync+0x20/0x60
[<8407656e>] sys_unlink+0xe/0x20
[<84076560>] sys_unlink+0x0/0x20
[<840088f8>] syscall_call+0xa/0xe

(Note that due to the way backtrace is implemented on SH architectures there
are spurious values in this backtrace, so it is probably safe to ignore any
entries of the form FN+0x0.)

The problem is caused by the "xfs_inode_t" which is passed into
xfs_iunlock() having already been freed at the SLAB level. Putting
some additional tracing code into the free path, this structure is
being freed by xfs_inode_item_destroy(), which is called from from
xfs_idestroy() which is called from xfs_ireclaim().

In between the free and use there appears to be some disk activity,
as I see an allocate from the SCSI layer, plus a few other kmalloc
and kfree's.

Running with XFS_DEBUG enabled hasn't shown any problems, and still
failed in the same way.

So, does this sound like a problem which has been seen before?
Alternatively can anyone suggest what would normally prevent this
happening, and I can look into that (although I know next to nothing
about the guts of the XFS code, so please don't be afraid to state
the obvious!). Or any other suggestions welcome.

Stuart

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