[PATCH 1/4] vfs: report change attribute in statx for IS_I_VERSION inodes

[Christian Brauner]

On Tue, Aug 16, 2022 at 09:27:56AM -0400, Jeff Layton wrote:
> From: Jeff Layton <jlayton at redhat.com>
> 
> Claim one of the spare fields in struct statx to hold a 64-bit change
> attribute. When statx requests this attribute, do an
> inode_query_iversion and fill the result in the field.
> 
> Also update the test-statx.c program to display the change attribute and
> the mountid as well.
> 
> Signed-off-by: Jeff Layton <jlayton at kernel.org>
> ---
>  fs/stat.c                 | 7 +++++++
>  include/linux/stat.h      | 1 +
>  include/uapi/linux/stat.h | 3 ++-
>  samples/vfs/test-statx.c  | 8 ++++++--
>  4 files changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/stat.c b/fs/stat.c
> index 9ced8860e0f3..7c3d063c31ba 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -17,6 +17,7 @@
>  #include <linux/syscalls.h>
>  #include <linux/pagemap.h>
>  #include <linux/compat.h>
> +#include <linux/iversion.h>
>  
>  #include <linux/uaccess.h>
>  #include <asm/unistd.h>
> @@ -118,6 +119,11 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
>  	stat->attributes_mask |= (STATX_ATTR_AUTOMOUNT |
>  				  STATX_ATTR_DAX);
>  
> +	if ((request_mask & STATX_CHANGE_ATTR) && IS_I_VERSION(inode)) {
> +		stat->result_mask |= STATX_CHANGE_ATTR;
> +		stat->change_attr = inode_query_iversion(inode);
> +	}
> +
>  	mnt_userns = mnt_user_ns(path->mnt);
>  	if (inode->i_op->getattr)
>  		return inode->i_op->getattr(mnt_userns, path, stat,
> @@ -611,6 +617,7 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
>  	tmp.stx_dev_major = MAJOR(stat->dev);
>  	tmp.stx_dev_minor = MINOR(stat->dev);
>  	tmp.stx_mnt_id = stat->mnt_id;
> +	tmp.stx_change_attr = stat->change_attr;
>  
>  	return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0;
>  }
> diff --git a/include/linux/stat.h b/include/linux/stat.h
> index 7df06931f25d..7b444c2ad0ad 100644
> --- a/include/linux/stat.h
> +++ b/include/linux/stat.h
> @@ -50,6 +50,7 @@ struct kstat {
>  	struct timespec64 btime;			/* File creation time */
>  	u64		blocks;
>  	u64		mnt_id;
> +	u64		change_attr;
>  };
>  
>  #endif
> diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
> index 1500a0f58041..fd839ec76aa4 100644
> --- a/include/uapi/linux/stat.h
> +++ b/include/uapi/linux/stat.h
> @@ -124,7 +124,7 @@ struct statx {
>  	__u32	stx_dev_minor;
>  	/* 0x90 */
>  	__u64	stx_mnt_id;
> -	__u64	__spare2;
> +	__u64	stx_change_attr; /* Inode change attribute */
>  	/* 0xa0 */
>  	__u64	__spare3[12];	/* Spare space for future expansion */
>  	/* 0x100 */
> @@ -152,6 +152,7 @@ struct statx {
>  #define STATX_BASIC_STATS	0x000007ffU	/* The stuff in the normal stat struct */
>  #define STATX_BTIME		0x00000800U	/* Want/got stx_btime */
>  #define STATX_MNT_ID		0x00001000U	/* Got stx_mnt_id */
> +#define STATX_CHANGE_ATTR	0x00002000U	/* Want/got stx_change_attr */

I'm a bit worried that STATX_CHANGE_ATTR isn't a good name for the flag
and field. Or I fail to understand what exact information this will
expose and how userspace will consume it.
To me the naming gives the impression that some set of generic
attributes have changed but given that statx is about querying file
attributes this becomes confusing.

Wouldn't it make more sense this time to expose it as what it is and
call this STATX_INO_VERSION and __u64 stx_ino_version?