Re: [PATCH v3 3/7] fs: FS_IOC_GETUUID

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Feb 7, 2024 at 4:57 AM Kent Overstreet
<kent.overstreet@linux.dev> wrote:
>
> Add a new generic ioctls for querying the filesystem UUID.
>
> These are lifted versions of the ext4 ioctls, with one change: we're not
> using a flexible array member, because UUIDs will never be more than 16
> bytes.
>
> This patch adds a generic implementation of FS_IOC_GETFSUUID, which
> reads from super_block->s_uuid. We're not lifting SETFSUUID from ext4 -
> that can be done on offline filesystems by the people who need it,
> trying to do it online is just asking for too much trouble.
>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Dave Chinner <dchinner@redhat.com>
> Cc: "Darrick J. Wong" <djwong@kernel.org>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Cc: linux-fsdevel@vger.kernel.or
> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> ---
>  .../userspace-api/ioctl/ioctl-number.rst         |  3 ++-
>  fs/ioctl.c                                       | 16 ++++++++++++++++
>  include/uapi/linux/fs.h                          | 16 ++++++++++++++++
>  3 files changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
> index 457e16f06e04..3731ecf1e437 100644
> --- a/Documentation/userspace-api/ioctl/ioctl-number.rst
> +++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
> @@ -82,8 +82,9 @@ Code  Seq#    Include File                                           Comments
>  0x10  00-0F  drivers/char/s390/vmcp.h
>  0x10  10-1F  arch/s390/include/uapi/sclp_ctl.h
>  0x10  20-2F  arch/s390/include/uapi/asm/hypfs.h
> -0x12  all    linux/fs.h
> +0x12  all    linux/fs.h                                              BLK* ioctls
>               linux/blkpg.h
> +0x15  all    linux/fs.h                                              FS_IOC_* ioctls
>  0x1b  all                                                            InfiniBand Subsystem
>                                                                       <http://infiniband.sourceforge.net/>
>  0x20  all    drivers/cdrom/cm206.h
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 76cf22ac97d7..74eab9549383 100644
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -763,6 +763,19 @@ static int ioctl_fssetxattr(struct file *file, void __user *argp)
>         return err;
>  }
>
> +static int ioctl_getfsuuid(struct file *file, void __user *argp)
> +{
> +       struct super_block *sb = file_inode(file)->i_sb;
> +       struct fsuuid2 u = { .len = sb->s_uuid_len, };
> +
> +       if (!sb->s_uuid_len)
> +               return -ENOIOCTLCMD;
> +
> +       memcpy(&u.uuid[0], &sb->s_uuid, sb->s_uuid_len);
> +
> +       return copy_to_user(argp, &u, sizeof(u)) ? -EFAULT : 0;
> +}
> +
>  /*
>   * do_vfs_ioctl() is not for drivers and not intended to be EXPORT_SYMBOL()'d.
>   * It's just a simple helper for sys_ioctl and compat_sys_ioctl.
> @@ -845,6 +858,9 @@ static int do_vfs_ioctl(struct file *filp, unsigned int fd,
>         case FS_IOC_FSSETXATTR:
>                 return ioctl_fssetxattr(filp, argp);
>
> +       case FS_IOC_GETFSUUID:
> +               return ioctl_getfsuuid(filp, argp);
> +
>         default:
>                 if (S_ISREG(inode->i_mode))
>                         return file_ioctl(filp, cmd, argp);
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 48ad69f7722e..d459f816cd50 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -64,6 +64,19 @@ struct fstrim_range {
>         __u64 minlen;
>  };
>
> +/*
> + * We include a length field because some filesystems (vfat) have an identifier
> + * that we do want to expose as a UUID, but doesn't have the standard length.
> + *
> + * We use a fixed size buffer beacuse this interface will, by fiat, never
> + * support "UUIDs" longer than 16 bytes; we don't want to force all downstream
> + * users to have to deal with that.
> + */
> +struct fsuuid2 {
> +       __u8    len;
> +       __u8    uuid[16];
> +};
> +
>  /* extent-same (dedupe) ioctls; these MUST match the btrfs ioctl definitions */
>  #define FILE_DEDUPE_RANGE_SAME         0
>  #define FILE_DEDUPE_RANGE_DIFFERS      1
> @@ -190,6 +203,9 @@ struct fsxattr {
>   * (see uapi/linux/blkzoned.h)
>   */
>
> +/* Returns the external filesystem UUID, the same one blkid returns */
> +#define FS_IOC_GETFSUUID               _IOR(0x15, 0, struct fsuuid2)

Please move that to the end of FS_IOC_* ioctls block.
The fact that it started a new vfs ioctl namespace does not justify starting
a different list IMO.

uapi readers don't care about the value of the ioctl.
locality to FS_IOC_GETFSLABEL is more important IMO.

Thanks,
Amir.


> +
>  #define BMAP_IOCTL 1           /* obsolete - kept for compatibility */
>  #define FIBMAP    _IO(0x00,1)  /* bmap access */
>  #define FIGETBSZ   _IO(0x00,2) /* get the block size used for bmap */
> --
> 2.43.0
>
>