audit: use struct_size() helper in kmalloc()

Make use of struct_size() helper instead of an open-coded calucation.

Link: KSPP/linux#160
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/audit.c

@@ -1446,7 +1446,7 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 			if (err)
 				return err;
 		}
-		sig_data = kmalloc(sizeof(*sig_data) + len, GFP_KERNEL);
+		sig_data = kmalloc(struct_size(sig_data, ctx, len), GFP_KERNEL);
 		if (!sig_data) {
 			if (audit_sig_sid)
 				security_release_secctx(ctx, len);

kernel/audit_tree.c

@@ -94,7 +94,7 @@ static struct audit_tree *alloc_tree(const char *s)
 {
 	struct audit_tree *tree;
 
-	tree = kmalloc(sizeof(struct audit_tree) + strlen(s) + 1, GFP_KERNEL);
+	tree = kmalloc(struct_size(tree, pathname, strlen(s) + 1), GFP_KERNEL);
 	if (tree) {
 		refcount_set(&tree->count, 1);
 		tree->goner = 0;

kernel/auditfilter.c

@@ -637,7 +637,7 @@ static struct audit_rule_data *audit_krule_to_data(struct audit_krule *krule)
 	void *bufp;
 	int i;
 
-	data = kmalloc(sizeof(*data) + krule->buflen, GFP_KERNEL);
+	data = kmalloc(struct_size(data, buf, krule->buflen), GFP_KERNEL);
 	if (unlikely(!data))
 		return NULL;
 	memset(data, 0, sizeof(*data));