btrfs: add metadata_uuid to FS_INFO ioctl

Add retrieval of the filesystem's metadata UUID to the fsinfo ioctl.
This is driven by setting the BTRFS_FS_INFO_FLAG_METADATA_UUID flag in
btrfs_ioctl_fs_info_args::flags.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -3252,6 +3252,12 @@ static long btrfs_ioctl_fs_info(struct btrfs_fs_info *fs_info,
 		fi_args->flags |= BTRFS_FS_INFO_FLAG_GENERATION;
 	}
 
+	if (flags_in & BTRFS_FS_INFO_FLAG_METADATA_UUID) {
+		memcpy(&fi_args->metadata_uuid, fs_devices->metadata_uuid,
+		       sizeof(fi_args->metadata_uuid));
+		fi_args->flags |= BTRFS_FS_INFO_FLAG_METADATA_UUID;
+	}
+
 	if (copy_to_user(arg, fi_args, sizeof(*fi_args)))
 		ret = -EFAULT;
 

include/uapi/linux/btrfs.h

@@ -252,6 +252,8 @@ struct btrfs_ioctl_dev_info_args {
 
 /* Request information about filesystem generation */
 #define BTRFS_FS_INFO_FLAG_GENERATION			(1 << 1)
+/* Request information about filesystem metadata UUID */
+#define BTRFS_FS_INFO_FLAG_METADATA_UUID		(1 << 2)
 
 struct btrfs_ioctl_fs_info_args {
 	__u64 max_id;				/* out */
@@ -265,7 +267,8 @@ struct btrfs_ioctl_fs_info_args {
 	__u16 csum_size;			/* out */
 	__u64 flags;				/* in/out */
 	__u64 generation;			/* out */
-	__u8 reserved[960];			/* pad to 1k */
+	__u8 metadata_uuid[BTRFS_FSID_SIZE];	/* out */
+	__u8 reserved[944];			/* pad to 1k */
 };
 
 /*