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

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

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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