CRED: Rename cred_exec_mutex to reflect that it's a guard against ptrace

Rename cred_exec_mutex to reflect that it's a guard against foreign
intervention on a process's credential state, such as is made by ptrace().  The
attachment of a debugger to a process affects execve()'s calculation of the new
credential state - _and_ also setprocattr()'s calculation of that state.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 0692ab5..27ac802 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -185,10 +185,11 @@
 	if (same_thread_group(task, current))
 		goto out;
 
-	/* Protect exec's credential calculations against our interference;
-	 * SUID, SGID and LSM creds get determined differently under ptrace.
+	/* Protect the target's credential calculations against our
+	 * interference; SUID, SGID and LSM creds get determined differently
+	 * under ptrace.
 	 */
-	retval = mutex_lock_interruptible(&task->cred_exec_mutex);
+	retval = mutex_lock_interruptible(&task->cred_guard_mutex);
 	if (retval  < 0)
 		goto out;
 
@@ -232,7 +233,7 @@
 bad:
 	write_unlock_irqrestore(&tasklist_lock, flags);
 	task_unlock(task);
-	mutex_unlock(&task->cred_exec_mutex);
+	mutex_unlock(&task->cred_guard_mutex);
 out:
 	return retval;
 }