Btrfs: fail on mismatched subvol and subvolid mount options

There's nothing to stop a user from passing both subvol= and subvolid=
to mount, but if they don't refer to the same subvolume, someone is
going to be surprised at some point. Error out on this case, but allow
users to pass in both if they do match (which they could, for example,
get out of /proc/mounts).
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/super.c

@@ -1167,8 +1167,9 @@ static char *setup_root_args(char *args)
 	return buf;
 }
 
-static struct dentry *mount_subvol(const char *subvol_name, int flags,
-				   const char *device_name, char *data)
+static struct dentry *mount_subvol(const char *subvol_name, u64 subvol_objectid,
+				   int flags, const char *device_name,
+				   char *data)
 {
 	struct dentry *root;
 	struct vfsmount *mnt = NULL;
@@ -1214,12 +1215,27 @@ static struct dentry *mount_subvol(const char *subvol_name, int flags,
 	/* mount_subtree() drops our reference on the vfsmount. */
 	mnt = NULL;
 
-	if (!IS_ERR(root) && !is_subvolume_inode(d_inode(root))) {
+	if (!IS_ERR(root)) {
 		struct super_block *s = root->d_sb;
+		struct inode *root_inode = d_inode(root);
+		u64 root_objectid = BTRFS_I(root_inode)->root->root_key.objectid;
+
+		ret = 0;
+		if (!is_subvolume_inode(root_inode)) {
+			pr_err("BTRFS: '%s' is not a valid subvolume\n",
+			       subvol_name);
+			ret = -EINVAL;
+		}
+		if (subvol_objectid && root_objectid != subvol_objectid) {
+			pr_err("BTRFS: subvol '%s' does not match subvolid %llu\n",
+			       subvol_name, subvol_objectid);
+			ret = -EINVAL;
+		}
+		if (ret) {
 			dput(root);
-		root = ERR_PTR(-EINVAL);
+			root = ERR_PTR(ret);
 			deactivate_locked_super(s);
-		pr_err("BTRFS: '%s' is not a valid subvolume\n", subvol_name);
+		}
 	}
 
 out:
@@ -1312,7 +1328,8 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 
 	if (subvol_name) {
 		/* mount_subvol() will free subvol_name. */
-		return mount_subvol(subvol_name, flags, device_name, data);
+		return mount_subvol(subvol_name, subvol_objectid, flags,
+				    device_name, data);
 	}
 
 	security_init_mnt_opts(&new_sec_opts);