Re: [PATCH 2/6] fs: FS_IOC_GETUUID

[Dave Chinner <david@fromorbit.com>]

On Mon, Feb 05, 2024 at 03:05:13PM -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; FS_IOC_SETFSUUID is left for individual
> filesystems to implement.
> 
> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> 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 | 16 ++++++++++++++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 76cf22ac97d7..858801060408 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 (WARN_ON(sb->s_uuid_len > sizeof(sb->s_uuid)))
> +		sb->s_uuid_len = sizeof(sb->s_uuid);

A "get"/read only ioctl should not be change superblock fields -
this is not the place for enforcing superblock filed constraints.
Make a helper function super_set_uuid(sb, uuid, uuid_len) for the
filesystems to call that does all the validity checking and then
sets the superblock fields appropriately.

> +
> +	struct fsuuid2 u = { .fsu_len = sb->s_uuid_len, };
> +	memcpy(&u.fsu_uuid[0], &sb->s_uuid, sb->s_uuid_len);

	if (!u.fsu_len)
		return -ENOENT;
	memcpy(&u.fsu_uuid[0], &sb->s_uuid, u.fsu_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..0389fea87db5 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -64,6 +64,20 @@ 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 {
> +	__u32       fsu_len;
> +	__u32       fsu_flags;
> +	__u8        fsu_uuid[16];
> +};

Nobody in userspace will care that this is "version 2" of the ext4
ioctl. I'd just name it "fs_uuid" as though the ext4 version didn't
ever exist.

> +
>  /* extent-same (dedupe) ioctls; these MUST match the btrfs ioctl definitions */
>  #define FILE_DEDUPE_RANGE_SAME		0
>  #define FILE_DEDUPE_RANGE_DIFFERS	1
> @@ -215,6 +229,8 @@ struct fsxattr {
>  #define FS_IOC_FSSETXATTR		_IOW('X', 32, struct fsxattr)
>  #define FS_IOC_GETFSLABEL		_IOR(0x94, 49, char[FSLABEL_MAX])
>  #define FS_IOC_SETFSLABEL		_IOW(0x94, 50, char[FSLABEL_MAX])
> +#define FS_IOC_GETFSUUID		_IOR(0x94, 51, struct fsuuid2)
> +#define FS_IOC_SETFSUUID		_IOW(0x94, 52, struct fsuuid2)

0x94 is the btrfs ioctl space, not the VFS space - why did you
choose that? That said, what is the VFS ioctl space identifier? 'v',
perhaps?

-Dave.

-- 
Dave Chinner
david@fromorbit.com