invert logic in read_nl_sock for better clarity and reliability (#2101)

The seq and pid parameter are apparently used as a safety check in case
a stream of netlink messages is not properly terminated. I initially thought
that they represented the expected values of the incoming messages.
Instead seq is set to one-past the expected value and the loop aborts when
seq and pid match the message.

Inverting the logic so that reading continues as long as the seq and pid match
the incoming messages is, to me at least, more intuitive. It is also more
reliable since it does not rely on the next seq being strictly sequential. It
also catches unexpected messages preceding or interleaved with the expected
messages.

There seems to be a lot of confusion all across the internet over the
nlmsg_pid field. The changes here were made based on examination of the
kernel source and iproute2 (a common user of netlink).

The kernel does not read nlmsg_pid at all for routing messages.
Iproute2 sets it to zero unconditionally so I changed this code to do the same.

src/enum_net.cpp

@@ -160,13 +160,13 @@ namespace libtorrent {namespace {
 
 #if TORRENT_USE_NETLINK
 
-	int read_nl_sock(int sock, char *buf, int bufsize, std::uint32_t const seq, int const pid)
+	int read_nl_sock(int sock, char *buf, int bufsize, std::uint32_t const seq, std::uint32_t const pid)
 	{
 		nlmsghdr* nl_hdr;
 
 		int msg_len = 0;
 
-		do
+		for (;;)
 		{
 			int read_len = int(recv(sock, buf, std::size_t(bufsize - msg_len), 0));
 			if (read_len < 0) return -1;
@@ -184,14 +184,18 @@ namespace libtorrent {namespace {
 #pragma clang diagnostic pop
 #endif
 
+			// this function doesn't handle multiple requests at the same time
+			// so report an error if the message does not have the expected seq and pid
+			if (nl_hdr->nlmsg_seq != seq || nl_hdr->nlmsg_pid != pid)
+				return -1;
+
 			if (nl_hdr->nlmsg_type == NLMSG_DONE) break;
 
 			buf += read_len;
 			msg_len += read_len;
 
 			if ((nl_hdr->nlmsg_flags & NLM_F_MULTI) == 0) break;
-
-		} while((nl_hdr->nlmsg_seq != seq) || (int(nl_hdr->nlmsg_pid) != pid));
+		}
 		return msg_len;
 	}
 
@@ -1052,8 +1056,11 @@ namespace libtorrent {
 		nl_msg->nlmsg_len = NLMSG_LENGTH(sizeof(rtmsg));
 		nl_msg->nlmsg_type = RTM_GETROUTE;
 		nl_msg->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST;
-		nl_msg->nlmsg_seq = seq++;
-		nl_msg->nlmsg_pid = std::uint32_t(getpid());
+		nl_msg->nlmsg_seq = seq;
+		// in theory nlmsg_pid should be set to the netlink port ID (NOT the process ID)
+		// of the sender, but the kernel ignores this field so it is typically set to
+		// zero
+		nl_msg->nlmsg_pid = 0;
 
 		if (send(sock, nl_msg, nl_msg->nlmsg_len, 0) < 0)
 		{
@@ -1062,7 +1069,17 @@ namespace libtorrent {
 			return std::vector<ip_route>();
 		}
 
-		int len = read_nl_sock(sock, msg, BUFSIZE, seq, getpid());
+		// get the socket's port ID so that we can verify it in the repsonse
+		sockaddr_nl sock_addr;
+		socklen_t sock_addr_len = sizeof(sock_addr);
+		if (getsockname(sock, (sockaddr*)&sock_addr, &sock_addr_len) < 0)
+		{
+			ec = error_code(errno, system_category());
+			close(sock);
+			return std::vector<ip_route>();
+		}
+
+		int len = read_nl_sock(sock, msg, BUFSIZE, seq, sock_addr.nl_pid);
 		if (len < 0)
 		{
 			ec = error_code(errno, system_category());
@@ -1070,6 +1087,10 @@ namespace libtorrent {
 			return std::vector<ip_route>();
 		}
 
+		// seq should be incremented between requests so do it here
+		// just in case someone adds another send below
+		++seq;
+
 		int s = socket(AF_INET, SOCK_DGRAM, 0);
 		if (s < 0)
 		{