Re: [PATCH 1/2] xfs: fix rmap key comparison functions

Dave Chinner
2022-11-01T23:40:33Z

On Sun, Oct 02, 2022 at 11:20:12AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
>
> Keys for extent interval records in the reverse mapping btree are
> supposed to be computed as follows:
>
> (physical block, owner, fork, is_btree, offset)
>
> This provides users the ability to look up a reverse mapping from a file
> block mapping record -- start with the physical block; then if there are
> multiple records for the same block, move on to the owner; then the
> inode fork type; and so on to the file offset.
>
> However, the key comparison functions incorrectly remove the fork/bmbt
> information that's encoded in the on-disk offset. This means that
> lookup comparisons are only done with:
>
> (physical block, owner, offset)
>
> This means that queries can return incorrect results. On consistent
> filesystems this isn't an issue because bmbt blocks and blocks mapped to
> an attr fork cannot be shared, but this prevents us from detecting
> incorrect fork and bmbt flag bits in the rmap btree.
>
> A previous version of this patch forgot to keep the (un)written state
> flag masked during the comparison and caused a major regression in
> 5.9.x since unwritten extent conversion can update an rmap record
> without requiring key updates.
>
> Note that blocks cannot go directly from data fork to attr fork without
> being deallocated and reallocated, nor can they be added to or removed
> from a bmbt without a free/alloc cycle, so this should not cause any
> regressions.
>
> Found by fuzzing keys[1].attrfork = ones on xfs/371.
>
> Fixes: 4b8ed67794fe ("xfs: add rmap btree operations")
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
> fs/xfs/libxfs/xfs_rmap_btree.c | 25 +++++++++++++++++--------
> 1 file changed, 17 insertions(+), 8 deletions(-)
>
>
> diff --git a/fs/xfs/libxfs/xfs_rmap_btree.c b/fs/xfs/libxfs/xfs_rmap_btree.c
> index 7f83f62e51e0..e2e1f68cedf5 100644
> --- a/fs/xfs/libxfs/xfs_rmap_btree.c
> +++ b/fs/xfs/libxfs/xfs_rmap_btree.c
> @@ -219,6 +219,15 @@ xfs_rmapbt_init_ptr_from_cur(
> ptr->s = agf->agf_roots[cur->bc_btnum];
> }
>
> +/*
> + * Fork and bmbt are significant parts of the rmap record key, but written
> + * status is merely a record attribute.
> + */
> +static inline uint64_t offset_keymask(uint64_t offset)
> +{
> + return offset & ~XFS_RMAP_OFF_UNWRITTEN;
> +}

Ok. but doesn't that mean xfs_rmapbt_init_key_from_rec() and
xfs_rmapbt_init_high_key_from_rec() should be masking out the
XFS_RMAP_OFF_UNWRITTEN bit as well?

-Dave.

--
Dave Chinner
david@fromorbit.com