apparmor: make fn_label_build() capable of handling not supported

Currently fn_label_build() callback fns must provide a transition or
failure. Change this so that a callback can indicate it should be
skipped/not be involved in the label being built.

This will be useful when building object labels based on mediation
flags, as to whether the label should be set.

Existing callers can keep treating NULL return as an error because
none of those callback fns support skipping, but instead of the old
error handling replace with AA_BUG.

Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

Author: jrjohansen

security/apparmor/include/lib.h

@@ -281,14 +281,15 @@ void aa_policy_destroy(struct aa_policy *policy);
  * @FN: fn to call for each profile transition. @P is set to the profile
  *
  * Returns: new label on success
+ *	    NULL if all callbacks decline to specify a transition
  *          ERR_PTR if build @FN fails
  *
- * @FN must return a label or ERR_PTR on failure. NULL is not allowed
+ * @FN must return a label or ERR_PTR on failure.
  */
 #define fn_label_build(L, P, GFP, FN)					\
 ({									\
 	__label__ __do_cleanup, __done;					\
-	struct aa_label *__new_;					\
+	struct aa_label *__new_ = NULL;					\
 									\
 	if ((L)->size > 1) {						\
 		/* TODO: add cache of transitions already done */	\
@@ -303,11 +304,15 @@ void aa_policy_destroy(struct aa_policy *policy);
 		__j = 0;						\
 		label_for_each(__i, (L), (P)) {				\
 			__new_ = (FN);					\
-			AA_BUG(!__new_);				\
+			if (!__new_)					\
+				continue;				\
 			if (IS_ERR(__new_))				\
 				goto __do_cleanup;			\
 			__lvec[__j++] = __new_;				\
 		}							\
+		if (__j == 0)						\
+			/* no components adding to build */		\
+			goto __do_cleanup;				\
 		for (__j = __count = 0; __j < (L)->size; __j++)		\
 			__count += __lvec[__j]->size;			\
 		if (!vec_setup(profile, __pvec, __count, (GFP))) {	\
@@ -331,12 +336,10 @@ __do_cleanup:								\
 	} else {							\
 		(P) = labels_profile(L);				\
 		__new_ = (FN);						\
-		AA_BUG(!__new_);					\
 	}								\
 __done:									\
 	if (PTR_ERR(__new_))						\
 		AA_DEBUG(DEBUG_LABEL, "label build failed\n");		\
-	AA_BUG(!__new_);						\
 	(__new_);							\
 })