On Mon, Nov 24, 2014 at 06:34:30PM +0100, David Sterba wrote:

The btrfs_root_readonly checks in setxattr and removexattr are
unnecessary because they're done through xattr_permisssion. I'll send a
patch to remove them.

Does not work because the security.* and system.* namespaces do not call
the permission() hook, so no patch. However, if the proposed
inode_is_readonly callback is merged, we can replace the btrfs-specific
checks with is_readonly check in xattr_permission().