Re: [RFC PATCH 05/13] xfs: define parent pointer xattr format

Darrick J. Wong
2017-08-14T23:59:43Z

On Wed, Aug 09, 2017 at 06:41:25PM -0700, Allison Henderson wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> 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]
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
> ---
> :100644 100644 d4d9bef... 91e2d5a... M fs/xfs/libxfs/xfs_format.h
> fs/xfs/libxfs/xfs_format.h | 27 +++++++++++++++++++++++++++
> 1 file changed, 27 insertions(+)
>
> diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
> index d4d9bef..91e2d5a 100644
> --- a/fs/xfs/libxfs/xfs_format.h
> +++ b/fs/xfs/libxfs/xfs_format.h
> @@ -18,6 +18,8 @@
> #ifndef __XFS_FORMAT_H__
> #define __XFS_FORMAT_H__
>
> +#include "xfs_da_format.h"
> +
> /*
> * XFS On Disk Format Definitions
> *
> @@ -1721,4 +1723,29 @@ struct xfs_acl {
> #define SGI_ACL_FILE_SIZE (sizeof(SGI_ACL_FILE)-1)
> #define SGI_ACL_DEFAULT_SIZE (sizeof(SGI_ACL_DEFAULT)-1)
>
> +/*
> + * 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 {
> + uint64_t p_ino;

Should use incore xfs types, i.e. xfs_ino_t.

> + uint32_t p_gen;
> + xfs_dir2_dataptr_t p_diroffset;
> + const char *p_name;
> + uint32_t p_namelen;

Pathname components are at most 255 bytes long, though I guess that
doesn't save us any space here.

--D

> +};
> +
> #endif /* __XFS_FORMAT_H__ */
> --
> 2.7.4
>
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