Re: [PATCH] xfs: fix attr leaf header freemap.size underflow

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Nov 15, 2019 at 06:41:37AM -0500, Brian Foster wrote:
> The leaf format xattr addition helper xfs_attr3_leaf_add_work()
> adjusts the block freemap in a couple places. The first update drops
> the size of the freemap that the caller had already selected to
> place the xattr name/value data. Before the function returns, it
> also checks whether the entries array has encroached on a freemap
> range by virtue of the new entry addition. This is necessary because
> the entries array grows from the start of the block (but end of the
> block header) towards the end of the block while the name/value data
> grows from the end of the block in the opposite direction. If the
> associated freemap is already empty, however, size is zero and the
> subtraction underflows the field and causes corruption.
> 
> This is reproduced rarely by generic/070. The observed behavior is
> that a smaller sized freemap is aligned to the end of the entries
> list, several subsequent xattr additions land in larger freemaps and
> the entries list expands into the smaller freemap until it is fully
> consumed and then underflows. Note that it is not otherwise a
> corruption for the entries array to consume an empty freemap because
> the nameval list (i.e. the firstused pointer in the xattr header)
> starts beyond the end of the corrupted freemap.
> 
> Update the freemap size modification to account for the fact that
> the freemap entry can be empty and thus stale.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>

Hm.  freemap.size == 0 means the freemap entry is stale and therefore
anything looking for free space in the leaf will ignore the entry, right?

If so,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

(Urk, there are still a lot of ASSERT-on-metadata in the dir/attr
code...)

--D

> ---
>  fs/xfs/libxfs/xfs_attr_leaf.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_attr_leaf.c b/fs/xfs/libxfs/xfs_attr_leaf.c
> index 85ec5945d29f..86155260d8b9 100644
> --- a/fs/xfs/libxfs/xfs_attr_leaf.c
> +++ b/fs/xfs/libxfs/xfs_attr_leaf.c
> @@ -1510,7 +1510,9 @@ xfs_attr3_leaf_add_work(
>  	for (i = 0; i < XFS_ATTR_LEAF_MAPSIZE; i++) {
>  		if (ichdr->freemap[i].base == tmp) {
>  			ichdr->freemap[i].base += sizeof(xfs_attr_leaf_entry_t);
> -			ichdr->freemap[i].size -= sizeof(xfs_attr_leaf_entry_t);
> +			ichdr->freemap[i].size -=
> +				min_t(uint16_t, ichdr->freemap[i].size,
> +						sizeof(xfs_attr_leaf_entry_t));
>  		}
>  	}
>  	ichdr->usedbytes += xfs_attr_leaf_entsize(leaf, args->index);
> -- 
> 2.20.1
>