Patch "Btrfs: send, don't BUG_ON() when an empty symlink is found" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

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

    Btrfs: send, don't BUG_ON() when an empty symlink is found

to the 4.4-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-send-don-t-bug_on-when-an-empty-symlink-is-found.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From a879719b8c90e15c9e7fa7266d5e3c0ca962f9df Mon Sep 17 00:00:00 2001
From: Filipe Manana <fdmanana@suse.com>
Date: Thu, 31 Dec 2015 18:07:59 +0000
Subject: Btrfs: send, don't BUG_ON() when an empty symlink is found

From: Filipe Manana <fdmanana@suse.com>

commit a879719b8c90e15c9e7fa7266d5e3c0ca962f9df upstream.

When a symlink is successfully created it always has an inline extent
containing the source path. However if an error happens when creating
the symlink, we can leave in the subvolume's tree a symlink inode without
any such inline extent item - this happens if after btrfs_symlink() calls
btrfs_end_transaction() and before it calls the inode eviction handler
(through the final iput() call), the transaction gets committed and a
crash happens before the eviction handler gets called, or if a snapshot
of the subvolume is made before the eviction handler gets called. Sadly
we can't just avoid this by making btrfs_symlink() call
btrfs_end_transaction() after it calls the eviction handler, because the
later can commit the current transaction before it removes any items from
the subvolume tree (if it encounters ENOSPC errors while reserving space
for removing all the items).

So make send fail more gracefully, with an -EIO error, and print a
message to dmesg/syslog informing that there's an empty symlink inode,
so that the user can delete the empty symlink or do something else
about it.

Reported-by: Stephen R. van den Berg <srb@cuci.nl>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/btrfs/send.c |   16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -1469,7 +1469,21 @@ static int read_symlink(struct btrfs_roo
 	ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
 	if (ret < 0)
 		goto out;
-	BUG_ON(ret);
+	if (ret) {
+		/*
+		 * An empty symlink inode. Can happen in rare error paths when
+		 * creating a symlink (transaction committed before the inode
+		 * eviction handler removed the symlink inode items and a crash
+		 * happened in between or the subvol was snapshoted in between).
+		 * Print an informative message to dmesg/syslog so that the user
+		 * can delete the symlink.
+		 */
+		btrfs_err(root->fs_info,
+			  "Found empty symlink inode %llu at root %llu",
+			  ino, root->root_key.objectid);
+		ret = -EIO;
+		goto out;
+	}
 
 	ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
 			struct btrfs_file_extent_item);


Patches currently in stable-queue which might be from fdmanana@suse.com are

queue-4.4/btrfs-fix-transaction-handle-leak-on-failure-to-create-hard-link.patch
queue-4.4/btrfs-fix-number-of-transaction-units-required-to-create-symlink.patch
queue-4.4/btrfs-send-don-t-bug_on-when-an-empty-symlink-is-found.patch