Re: [PATCH] xfs_file_last_byte() needs to acquire ilock

Felix Blyakher
2009-04-24T04:25:09Z

On Apr 23, 2009, at 9:18 PM, Lachlan McIlroy wrote:

> We had some systems crash with this stack:
>
> [<a00000010000cb20>] ia64_leave_kernel+0x0/0x280
> [<a00000021291ca00>] xfs_bmbt_get_startoff+0x0/0x20 [xfs]
> [<a0000002129080b0>] xfs_bmap_last_offset+0x210/0x280 [xfs]
> [<a00000021295b010>] xfs_file_last_byte+0x70/0x1a0 [xfs]
> [<a00000021295b200>] xfs_itruncate_start+0xc0/0x1a0 [xfs]
> [<a0000002129935f0>] xfs_inactive_free_eofblocks+0x290/0x460 [xfs]
> [<a000000212998fb0>] xfs_release+0x1b0/0x240 [xfs]
> [<a0000002129ad930>] xfs_file_release+0x70/0xa0 [xfs]
> [<a000000100162ea0>] __fput+0x1a0/0x420
> [<a000000100163160>] fput+0x40/0x60
>
> The problem here is that xfs_file_last_byte() does not acquire the
> inode lock and can therefore race with another thread that is
> modifying
> the extext list. While xfs_bmap_last_offset() is trying to lookup
> what was the last extent some extents were merged and the extent list
> shrunk so the index we lookup is now beyond the end of the extent list
> and potentially in a freed buffer.
>
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index e7ae08d..cf62d9d 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -1258,8 +1258,10 @@ xfs_file_last_byte(
> * necessary.
> */
> if (ip->i_df.if_flags & XFS_IFEXTENTS) {
> + xfs_ilock(ip, XFS_ILOCK_SHARED);
> error = xfs_bmap_last_offset(NULL, ip, &last_block,
> XFS_DATA_FORK);
> + xfs_iunlock(ip, XFS_ILOCK_SHARED);
> if (error) {
> last_block = 0;
> }

My understanding from the original patch was that this is one part
of the fix, and the other was the following change:

@@ -3206,6 +3208,8 @@ xfs_bmap_del_extent(
*/
XFS_BMAP_TRACE_DELETE("3", ip, idx, 1, whichfork);
xfs_iext_remove(ifp, idx, 1);
+ if (idx >= (ifp->if_bytes /
(uint)sizeof(xfs_bmbt_rec_t)))
+ idx--;
ifp->if_lastex = idx;
if (delay)
break;

You don't think it's still needed, do you?

Felix