[3.19.y-ckt stable] Patch "Btrfs: don't invalidate root dentry when subvolume deletion fails" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.y-ckt4.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From c1cd874ca158c87a5ccfbce3884159292fec75c4 Mon Sep 17 00:00:00 2001
From: Omar Sandoval <osandov at osandov.com>
Date: Tue, 2 Jun 2015 17:31:00 -0700
Subject: Btrfs: don't invalidate root dentry when subvolume deletion fails

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.

Fixes: bafc9b754f75 ("vfs: More precise tests in d_invalidate")
Reported-by: Markus Schauler <mschauler at gmail.com>
Signed-off-by: Omar Sandoval <osandov at osandov.com>
Signed-off-by: Chris Mason <clm at fb.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/btrfs/ioctl.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 334b0a9..0dc23cd 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2406,8 +2406,6 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 		goto out_unlock_inode;
 	}

-	d_invalidate(dentry);
-
 	down_write(&root->fs_info->subvol_sem);

 	err = may_destroy_subvol(dest);
@@ -2501,7 +2499,7 @@ 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);
--
1.9.1