From: Roy Marples <roy@marples.name>
Date: Sun, 16 Aug 2020 17:52:17 +0000 (+0100)
Subject: privsep: Set a zero length receive buffer for write only sockets
X-Git-Tag: v9.2.0~28
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e5679c5f411e561c31a30fb9cf730ff54b9242ac;p=thirdparty%2Fdhcpcd.git

privsep: Set a zero length receive buffer for write only sockets

We cannot use shutdown(2) because they are not connected.
Constantly draining would be a waste of CPU time, so just let
the buffer overflow. To ease the kernel as much as we can, set
a zero length buffer.

The kernel may still allocate a small buffer, but this is kernel
dependant and we're just trying to be helpful.
---

diff --git a/src/privsep-root.c b/src/privsep-root.c
index f1b40745..fe6b1aa6 100644
--- a/src/privsep-root.c
+++ b/src/privsep-root.c
@@ -638,27 +638,46 @@ ps_root_startcb(void *arg)
 	/* Open network sockets for sending.
 	 * This is a small bit wasteful for non sandboxed OS's
 	 * but makes life very easy for unicasting DHCPv6 in non master
-	 * mode as we no longer care about address selection. */
+	 * mode as we no longer care about address selection.
+	 * We can't call shutdown SHUT_RD on the socket because it's
+	 * not connectd. All we can do is try and set a zero sized
+	 * receive buffer and just let it overflow.
+	 * Reading from it just to drain it is a waste of CPU time. */
 #ifdef INET
 	if (ctx->options & DHCPCD_IPV4) {
+		int buflen = 0;
+
 		ctx->udp_wfd = xsocket(PF_INET,
 		    SOCK_RAW | SOCK_CXNB, IPPROTO_UDP);
 		if (ctx->udp_wfd == -1)
 			logerr("%s: dhcp_openraw", __func__);
+		else if (setsockopt(ctx->udp_wfd, SOL_SOCKET, SO_RCVBUF,
+		    &buflen, sizeof(buflen)) == -1)
+			logerr("%s: setsockopt SO_RCVBUF DHCP", __func__);
 	}
 #endif
 #ifdef INET6
 	if (ctx->options & DHCPCD_IPV6) {
+		int buflen = 0;
+
 		ctx->nd_fd = ipv6nd_open(false);
 		if (ctx->nd_fd == -1)
 			logerr("%s: ipv6nd_open", __func__);
+		else if (setsockopt(ctx->nd_fd, SOL_SOCKET, SO_RCVBUF,
+		    &buflen, sizeof(buflen)) == -1)
+			logerr("%s: setsockopt SO_RCVBUF ND", __func__);
 	}
 #endif
 #ifdef DHCP6
 	if (ctx->options & DHCPCD_IPV6) {
+		int buflen = 0;
+
 		ctx->dhcp6_wfd = dhcp6_openraw();
 		if (ctx->dhcp6_wfd == -1)
 			logerr("%s: dhcp6_openraw", __func__);
+		else if (setsockopt(ctx->dhcp6_wfd, SOL_SOCKET, SO_RCVBUF,
+		    &buflen, sizeof(buflen)) == -1)
+			logerr("%s: setsockopt SO_RCVBUF DHCP6", __func__);
 	}
 #endif