Patch "Btrfs: don't invalidate root dentry when subvolume deletion fails" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

Btrfs: don't invalidate root dentry when subvolume deletion fails

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
btrfs-don-t-invalidate-root-dentry-when-subvolume-deletion-fails.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Omar Sandoval <[email protected]>
Date: Tue, 2 Jun 2015 17:31:00 -0700
Subject: Btrfs: don't invalidate root dentry when subvolume deletion fails

From: Omar Sandoval <[email protected]>

commit 64ad6c488975d7516230cf7849190a991fd615ae upstream.

Since commit bafc9b754f75 ("vfs: More precise tests in d_invalidate"), mounted subvolumes can be deleted because d_invalidate() won't fail. However, we run into problems when we attempt to delete the default subvolume while it is mounted as the root filesystem:

	# btrfs subvol list /
	ID 257 gen 306 top level 5 path rootvol
	ID 267 gen 334 top level 5 path snap1
	# btrfs subvol get-default /
	ID 267 gen 334 top level 5 path snap1
	# btrfs inspect-internal rootid /
	267
	# mount -o subvol=/ /dev/vda1 /mnt
	# btrfs subvol del /mnt/snap1
	Delete subvolume (no-commit): '/mnt/snap1'
	ERROR: cannot delete '/mnt/snap1' - Operation not permitted
	# findmnt /
	findmnt: can't read /proc/mounts: No such file or directory
	# ls /proc
	#

Markus reported that this same scenario simply led to a kernel oops.

This happens because in btrfs_ioctl_snap_destroy(), we call d_invalidate() before we check may_destroy_subvol(), which means that we detach the submounts and drop the dentry before erroring out. Instead, we should only invalidate the dentry once the deletion has succeeded. Additionally, the shrink_dcache_sb() isn't necessary; d_invalidate() will prune the dcache for the deleted subvolume.

---
 fs/btrfs/ioctl.c |    4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

@@ -2413,8 +2413,6 @@ static noinline int btrfs_ioctl_snap_des
 		goto out_unlock_inode;
 	}
 
-	d_invalidate(dentry);
-
 	down_write(&root->fs_info->subvol_sem);
 
 	err = may_destroy_subvol(dest);

@@ -2508,7 +2506,7 @@ out_up_write:
 out_unlock_inode:
 	mutex_unlock(&inode->i_mutex);
 	if (!err) {
-		shrink_dcache_sb(root->fs_info->sb);
+		d_invalidate(dentry);
 		btrfs_invalidate_inodes(dest);
 		d_delete(dentry);
 		ASSERT(dest->send_in_progress == 0);

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/btrfs-don-t-invalidate-root-dentry-when-subvolume-deletion-fails.patch