Annotate struct fs_struct's usage count restriction

Annotate struct fs_struct's usage count to indicate the restrictions upon it.
It may not be incremented, except by clone(CLONE_FS), as this affects the
check in check_unsafe_exec() in fs/exec.c.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/fs_struct.h

@@ -4,7 +4,10 @@
 #include <linux/path.h>
 
 struct fs_struct {
-	atomic_t count;
+	atomic_t count;	/* This usage count is used by check_unsafe_exec() for
+			 * security checking purposes - therefore it may not be
+			 * incremented, except by clone(CLONE_FS).
+			 */
 	rwlock_t lock;
 	int umask;
 	struct path root, pwd;