fs: FS_IOC_GETUUID

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.org
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Link: https://lore.kernel.org/r/20240207025624.1019754-4-kent.overstreet@linux.devSigned-off-by: Christian Brauner <brauner@kernel.org>

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

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);

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
@@ -215,6 +228,8 @@ 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])
+/* Returns the external filesystem UUID, the same one blkid returns */
+#define FS_IOC_GETFSUUID		_IOR(0x15, 0, struct fsuuid2)
 
 /*
  * Inode flags (FS_IOC_GETFLAGS / FS_IOC_SETFLAGS)