- Patch from Vadim Fedorenko that adds MSG_DONTWAIT to receive

operations, so that instruction reordering does not cause mistakenly
  blocking socket operations.

dnstap/dtstream.c

@@ -954,7 +954,7 @@ static int dtio_write_more(struct dt_io_thread* dtio)
  * -1: continue, >0: number of bytes read into buffer */
 static ssize_t receive_bytes(struct dt_io_thread* dtio, void* buf, size_t len) {
 	ssize_t r;
-	r = recv(dtio->fd, (void*)buf, len, 0);
+	r = recv(dtio->fd, (void*)buf, len, MSG_DONTWAIT);
 	if(r == -1) {
 		char* to = dtio->socket_path;
 		if(!to) to = dtio->ip_str;

dnstap/unbound-dnstap-socket.c

@@ -617,7 +617,7 @@ static void log_data_frame(uint8_t* pkt, size_t len)
 static ssize_t receive_bytes(struct tap_data* data, int fd, void* buf,
 	size_t len)
 {
-	ssize_t ret = recv(fd, buf, len, 0);
+	ssize_t ret = recv(fd, buf, len, MSG_DONTWAIT);
 	if(ret == 0) {
 		/* closed */
 		if(verbosity) log_info("dnstap client stream closed from %s",

doc/Changelog

@@ -1,6 +1,9 @@
 31 August 2022: Wouter
 	- Fix to avoid process wide fcntl calls mixed with nonblocking
 	  operations after a blocked write.
+	- Patch from Vadim Fedorenko that adds MSG_DONTWAIT to receive
+	  operations, so that instruction reordering does not cause mistakenly
+	  blocking socket operations.
 
 22 August 2022: Wouter
 	- Fix #741: systemd socket activation fails on IPv6.

util/net_help.h

@@ -96,6 +96,11 @@ extern uint16_t EDNS_ADVERTISED_SIZE;
 /** return a random 16-bit number given a random source */
 #define GET_RANDOM_ID(rnd) (((unsigned)ub_random(rnd)>>8) & 0xffff)
 
+/** define MSG_DONTWAIT for unsupported platforms */
+#ifndef MSG_DONTWAIT
+#define MSG_DONTWAIT 0
+#endif
+
 /** minimal responses when positive answer */
 extern int MINIMAL_RESPONSES;
 

util/netevent.c

@@ -701,7 +701,7 @@ comm_point_udp_ancil_callback(int fd, short event, void* arg)
 		msg.msg_controllen = sizeof(ancil.buf);
 #endif /* S_SPLINT_S */
 		msg.msg_flags = 0;
-		rcv = recvmsg(fd, &msg, 0);
+		rcv = recvmsg(fd, &msg, MSG_DONTWAIT);
 		if(rcv == -1) {
 			if(errno != EAGAIN && errno != EINTR
 				&& udp_recv_needs_log(errno)) {
@@ -781,7 +781,7 @@ comm_point_udp_callback(int fd, short event, void* arg)
 		log_assert(fd != -1);
 		log_assert(sldns_buffer_remaining(rep.c->buffer) > 0);
 		rcv = recvfrom(fd, (void*)sldns_buffer_begin(rep.c->buffer), 
-			sldns_buffer_remaining(rep.c->buffer), 0, 
+			sldns_buffer_remaining(rep.c->buffer), MSG_DONTWAIT,
 			(struct sockaddr*)&rep.addr, &rep.addrlen);
 		if(rcv == -1) {
 #ifndef USE_WINSOCK
@@ -1703,7 +1703,7 @@ comm_point_tcp_handle_read(int fd, struct comm_point* c, int short_ok)
 	if(c->tcp_byte_count < sizeof(uint16_t)) {
 		/* read length bytes */
 		r = recv(fd,(void*)sldns_buffer_at(c->buffer,c->tcp_byte_count),
-			sizeof(uint16_t)-c->tcp_byte_count, 0);
+			sizeof(uint16_t)-c->tcp_byte_count, MSG_DONTWAIT);
 		if(r == 0) {
 			if(c->tcp_req_info)
 				return tcp_req_info_handle_read_close(c->tcp_req_info);
@@ -1794,7 +1794,7 @@ comm_point_tcp_handle_read(int fd, struct comm_point* c, int short_ok)
 	if(sldns_buffer_remaining(c->buffer) == 0)
 		log_err("in comm_point_tcp_handle_read buffer_remaining is not > 0 as expected, continuing with (harmless) 0 length recv");
 	r = recv(fd, (void*)sldns_buffer_current(c->buffer), 
-		sldns_buffer_remaining(c->buffer), 0);
+		sldns_buffer_remaining(c->buffer), MSG_DONTWAIT);
 	if(r == 0) {
 		if(c->tcp_req_info)
 			return tcp_req_info_handle_read_close(c->tcp_req_info);
@@ -2336,7 +2336,7 @@ http_read_more(int fd, struct comm_point* c)
 	ssize_t r;
 	log_assert(sldns_buffer_remaining(c->buffer) > 0);
 	r = recv(fd, (void*)sldns_buffer_current(c->buffer), 
-		sldns_buffer_remaining(c->buffer), 0);
+		sldns_buffer_remaining(c->buffer), MSG_DONTWAIT);
 	if(r == 0) {
 		return 0;
 	} else if(r == -1) {
@@ -2774,7 +2774,7 @@ ssize_t http2_recv_cb(nghttp2_session* ATTR_UNUSED(session), uint8_t* buf,
 	}
 #endif /* HAVE_SSL */
 
-	ret = recv(h2_session->c->fd, buf, len, 0);
+	ret = recv(h2_session->c->fd, buf, len, MSG_DONTWAIT);
 	if(ret == 0) {
 		return NGHTTP2_ERR_EOF;
 	} else if(ret < 0) {