[PATCH] btrfs: do not allow mounting non-subvolumes via subvol option

[David Sterba <dsterba@suse.cz>]

There's a missing test whether the path passed to subvol=path option
during mount is a real subvolume, allowing any directory located in
default subovlume to be passed and accepted for mount.

(current btrfs progs prevent this early)
$ btrfs subvol snapshot . p1-snap
ERROR: '.' is not a subvolume

(with "is subvolume?" test bypassed)
$ btrfs subvol snapshot . p1-snap
Create a snapshot of '.' in './p1-snap'

$ btrfs subvol list -p .
ID 258 parent 5 top level 5 path subvol
ID 259 parent 5 top level 5 path subvol1
ID 260 parent 5 top level 5 path default-subvol1
ID 262 parent 5 top level 5 path p1/p1-snapshot
ID 263 parent 259 top level 5 path subvol1/subvol1-snap

The problem I see is that this makes a false impression of snapshotting the
given subvolume but in fact snapshots the default one: a user expects outcome
like ID 263 but in fact gets ID 262 .

This patch makes mount fail with EINVAL with a message in syslog.

Signed-off-by: David Sterba <dsterba@suse.cz>
---

I did not find a better errno than EINVAL, probably adding someting like
ENSUBVOL would be better so that other filesystems with such functionality may
use it in future.

 fs/btrfs/super.c |   19 +++++++++++++++++++
 1 files changed, 19 insertions(+), 0 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 15634d4..0c2a1d1 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -753,6 +753,15 @@ static int btrfs_set_super(struct super_block *s, void *data)
 	return set_anon_super(s, data);
 }
 
+/*
+ * subvolumes are identified by ino 256
+ */
+static inline int is_subvolume_inode(struct inode *inode)
+{
+	if (inode && inode->i_ino == BTRFS_FIRST_FREE_OBJECTID)
+		return 1;
+	return 0;
+}
 
 /*
  * Find a superblock for the given device / mount point.
@@ -873,6 +882,16 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 			error = -ENXIO;
 			goto error_free_subvol_name;
 		}
+
+		if (!is_subvolume_inode(new_root->d_inode)) {
+			dput(root);
+			dput(new_root);
+			deactivate_locked_super(s);
+			error = -EINVAL;
+			printk(KERN_ERR "btrfs: '%s' is not a valid subvolume\n",
+					subvol_name);
+			goto error_free_subvol_name;
+		}
 		dput(root);
 		root = new_root;
 	} else {
-- 
1.7.6