On Sat, Jun 11, 2022 at 02:41:50AM -0700, Allison Henderson wrote:
> We need to define the parent pointer attribute format before we start
> adding support for it into all the code that needs to use it. The EA
> format we will use encodes the following information:
>
> name={parent inode #, parent inode generation, dirent offset}
> value={dirent filename}
>
> The inode/gen gives all the information we need to reliably identify the
> parent without requiring child->parent lock ordering, and allows
> userspace to do pathname component level reconstruction without the
> kernel ever needing to verify the parent itself as part of ioctl calls.
>
> By using the dirent offset in the EA name, we have a method of knowing
> the exact parent pointer EA we need to modify/remove in rename/unlink
> without an unbound EA name search.
>
> By keeping the dirent name in the value, we have enough information to
> be able to validate and reconstruct damaged directory trees. While the
> diroffset of a filename alone is not unique enough to identify the
> child, the {diroffset,filename,child_inode} tuple is sufficient. That
> is, if the diroffset gets reused and points to a different filename, we
> can detect that from the contents of EA. If a link of the same name is
> created, then we can check whether it points at the same inode as the
> parent EA we current have.
>
> [achender: rebased, changed __unint32_t to xfs_dir2_dataptr_t,
> changed p_ino to xfs_ino_t and p_namelen to uint8_t,
> moved to xfs_da_format for xfs_dir2_dataptr_t]
>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> Signed-off-by: Allison Henderson <allison.henderson@xxxxxxxxxx>
> Reviewed-by: Darrick J. Wong<darrick.wong@xxxxxxxxxx>
Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>
--D
> ---
> fs/xfs/libxfs/xfs_da_format.h | 25 +++++++++++++++++++++++++
> 1 file changed, 25 insertions(+)
>
> diff --git a/fs/xfs/libxfs/xfs_da_format.h b/fs/xfs/libxfs/xfs_da_format.h
> index 2d771e6429f2..6ac8c8dd4aab 100644
> --- a/fs/xfs/libxfs/xfs_da_format.h
> +++ b/fs/xfs/libxfs/xfs_da_format.h
> @@ -805,4 +805,29 @@ static inline unsigned int xfs_dir2_dirblock_bytes(struct xfs_sb *sbp)
> xfs_failaddr_t xfs_da3_blkinfo_verify(struct xfs_buf *bp,
> struct xfs_da3_blkinfo *hdr3);
>
> +/*
> + * Parent pointer attribute format definition
> + *
> + * EA name encodes the parent inode number, generation and the offset of
> + * the dirent that points to the child inode. The EA value contains the
> + * same name as the dirent in the parent directory.
> + */
> +struct xfs_parent_name_rec {
> + __be64 p_ino;
> + __be32 p_gen;
> + __be32 p_diroffset;
> +};
> +
> +/*
> + * incore version of the above, also contains name pointers so callers
> + * can pass/obtain all the parent pointer information in a single structure
> + */
> +struct xfs_parent_name_irec {
> + xfs_ino_t p_ino;
> + uint32_t p_gen;
> + xfs_dir2_dataptr_t p_diroffset;
> + const char *p_name;
> + uint8_t p_namelen;
> +};
> +
> #endif /* __XFS_DA_FORMAT_H__ */
> --
> 2.25.1
>
