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

[Dave Chinner <david@fromorbit.com>]

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