Re: [PATCH v2 3/7] fs: FS_IOC_GETUUID

[Dave Chinner <david@fromorbit.com>]

On Tue, Feb 06, 2024 at 03:18:51PM -0500, Kent Overstreet 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>
> ---
>  fs/ioctl.c              | 16 ++++++++++++++++
>  include/uapi/linux/fs.h | 17 +++++++++++++++++
>  2 files changed, 33 insertions(+)
> 
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 76cf22ac97d7..046c30294a82 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;
> +
> +	if (!sb->s_uuid_len)
> +		return -ENOIOCTLCMD;
> +
> +	struct fsuuid2 u = { .len = sb->s_uuid_len, };
> +	memcpy(&u.uuid[0], &sb->s_uuid, sb->s_uuid_len);
> +
> +	return copy_to_user(argp, &u, sizeof(u)) ? -EFAULT : 0;
> +}

Can we please keep the declarations separate from the code? I always
find this sort of implicit scoping of variables both difficult to
read (especially in larger functions) and a landmine waiting to be
tripped over. This could easily just be:

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, };

	....

and then it's consistent with all the rest of the code...

> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 48ad69f7722e..16a6ecadfd8d 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(0x12, 142, struct fsuuid2)
> +

Can you add a comment somewhere in the file saying that new VFS
ioctls should use the "0x12" namespace in the range 142-255, and
mention that BLK ioctls should be kept within the 0x12 {0-141}
range?

Probably also document this clearly in
Documentation/userspace-api/ioctl/ioctl-number.rst, too?

-Dave.


-- 
Dave Chinner
david@fromorbit.com