Re: [PATCH v2] xfs: get rid of assert from xfs_btree_islastblock

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Nov 30, 2022 at 12:02:37PM +0800, Guo Xuenan wrote:
> xfs_btree_check_block contains debugging knobs. With XFS_DEBUG setting up,
> turn on the debugging knob can trigger the assert of xfs_btree_islastblock,
> test script as follows:
> 
> while true
> do
>     mount $disk $mountpoint
>     fsstress -d $testdir -l 0 -n 10000 -p 4 >/dev/null
>     echo 1 > /sys/fs/xfs/sda/errortag/btree_chk_sblk
>     sleep 10
>     umount $mountpoint
> done
> 
> Kick off fsstress and only *then* turn on the debugging knob. If it
> happens that the knob gets turned on after the cntbt lookup succeeds
> but before the call to xfs_btree_islastblock, then we *can* end up in
> the situation where a previously checked btree block suddenly starts
> returning EFSCORRUPTED from xfs_btree_check_block. Kaboom.
> 
> Darrick give a very detailed explanation as follows:
> Looking back at commit 27d9ee577dcce, I think the point of all this was
> to make sure that the cursor has actually performed a lookup, and that
> the btree block at whatever level we're asking about is ok.
> 
> If the caller hasn't ever done a lookup, the bc_levels array will be
> empty, so cur->bc_levels[level].bp pointer will be NULL.  The call to
> xfs_btree_get_block will crash anyway, so the "ASSERT(block);" part is
> pointless.
> 
> If the caller did a lookup but the lookup failed due to block
> corruption, the corresponding cur->bc_levels[level].bp pointer will also
> be NULL, and we'll still crash.  The "ASSERT(xfs_btree_check_block);"
> logic is also unnecessary.
> 
> If the cursor level points to an inode root, the block buffer will be
> incore, so it had better always be consistent.
> 
> If the caller ignores a failed lookup after a successful one and calls
> this function, the cursor state is garbage and the assert wouldn't have
> tripped anyway. So get rid of the assert.
> 
> Fixes: 27d9ee577dcc ("xfs: actually check xfs_btree_check_block return in xfs_btree_islastblock")
> Signed-off-by: Guo Xuenan <guoxuenan@huawei.com>

Seems fine to me, but what does everyone else think?

Tentatively,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  fs/xfs/libxfs/xfs_btree.h | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_btree.h b/fs/xfs/libxfs/xfs_btree.h
> index eef27858a013..29c4b4ccb909 100644
> --- a/fs/xfs/libxfs/xfs_btree.h
> +++ b/fs/xfs/libxfs/xfs_btree.h
> @@ -556,7 +556,6 @@ xfs_btree_islastblock(
>  	struct xfs_buf		*bp;
>  
>  	block = xfs_btree_get_block(cur, level, &bp);
> -	ASSERT(block && xfs_btree_check_block(cur, block, level, bp) == 0);
>  
>  	if (cur->bc_flags & XFS_BTREE_LONG_PTRS)
>  		return block->bb_u.l.bb_rightsib == cpu_to_be64(NULLFSBLOCK);
> -- 
> 2.31.1
>