sock: break up sock_cmsg_snd into __sock_cmsg_snd and loop

To process cmsg's of the SOL_SOCKET level in addition to
cmsgs of another level, protocols can call sock_cmsg_send().
This causes a double walk on the cmsghdr list, one for SOL_SOCKET
and one for the other level.

Extract the inner demultiplex logic from the loop that walks the list,
to allow having this called directly from a walker in the protocol
specific code.
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NSoheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1420,6 +1420,8 @@ struct sockcm_cookie {
 	u32 mark;
 };
 
+int __sock_cmsg_send(struct sock *sk, struct msghdr *msg, struct cmsghdr *cmsg,
+		     struct sockcm_cookie *sockc);
 int sock_cmsg_send(struct sock *sk, struct msghdr *msg,
 		   struct sockcm_cookie *sockc);
 

net/core/sock.c

@@ -1866,27 +1866,38 @@ struct sk_buff *sock_alloc_send_skb(struct sock *sk, unsigned long size,
 }
 EXPORT_SYMBOL(sock_alloc_send_skb);
 
+int __sock_cmsg_send(struct sock *sk, struct msghdr *msg, struct cmsghdr *cmsg,
+		     struct sockcm_cookie *sockc)
+{
+	switch (cmsg->cmsg_type) {
+	case SO_MARK:
+		if (!ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN))
+			return -EPERM;
+		if (cmsg->cmsg_len != CMSG_LEN(sizeof(u32)))
+			return -EINVAL;
+		sockc->mark = *(u32 *)CMSG_DATA(cmsg);
+		break;
+	default:
+		return -EINVAL;
+	}
+	return 0;
+}
+EXPORT_SYMBOL(__sock_cmsg_send);
+
 int sock_cmsg_send(struct sock *sk, struct msghdr *msg,
 		   struct sockcm_cookie *sockc)
 {
 	struct cmsghdr *cmsg;
+	int ret;
 
 	for_each_cmsghdr(cmsg, msg) {
 		if (!CMSG_OK(msg, cmsg))
 			return -EINVAL;
 		if (cmsg->cmsg_level != SOL_SOCKET)
 			continue;
-		switch (cmsg->cmsg_type) {
-		case SO_MARK:
-			if (!ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN))
-				return -EPERM;
-			if (cmsg->cmsg_len != CMSG_LEN(sizeof(u32)))
-				return -EINVAL;
-			sockc->mark = *(u32 *)CMSG_DATA(cmsg);
-			break;
-		default:
-			return -EINVAL;
-		}
+		ret = __sock_cmsg_send(sk, msg, cmsg, sockc);
+		if (ret)
+			return ret;
 	}
 	return 0;
 }