From e84122fd19d0e890c577b3f06e7e3e0e7b950266 Mon Sep 17 00:00:00 2001
From: minus <minus@mnus.de>
Date: Sun, 22 Aug 2021 12:09:06 +0200
Subject: Fix hanging arp_check

arp_check relied on select(2) decreasing the timeout. This doesn't seem
to be the case on Linux anymore, thus arp_check tends to hang when the
interface sees a lot of traffic. Calculating the timeout explicitly
solves the problem.
---
 src/firejail/arp.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/firejail/arp.c b/src/firejail/arp.c
index bbab9a6d9..c259fc0ad 100644
--- a/src/firejail/arp.c
+++ b/src/firejail/arp.c
@@ -20,6 +20,7 @@
 #include "firejail.h"
 #include <sys/socket.h>
 #include <sys/ioctl.h>
+#include <sys/time.h>
 #include <linux/if_ether.h>			  //TCP/IP Protocol Suite for Linux
 #include <net/if.h>
 #include <netinet/in.h>
@@ -188,9 +189,14 @@ int arp_check(const char *dev, uint32_t destaddr) {
 	FD_SET(sock, &fds);
 	int maxfd = sock;
 	struct timeval ts;
-	ts.tv_sec = 0; // 0.5 seconds wait time
-	ts.tv_usec = 500000;
+	gettimeofday(&ts, NULL);
+	double timerend = ts.tv_sec + ts.tv_usec / 1000000.0 + 0.5;
 	while (1) {
+		gettimeofday(&ts, NULL);
+		double now = ts.tv_sec + ts.tv_usec / 1000000.0;
+		double timeout = timerend - now;
+		ts.tv_sec = timeout;
+		ts.tv_usec = (timeout - ts.tv_sec) * 1000000;
 		int nready = select(maxfd + 1,  &fds, (fd_set *) 0, (fd_set *) 0, &ts);
 		if (nready < 0)
 			errExit("select");
@@ -201,8 +207,8 @@ int arp_check(const char *dev, uint32_t destaddr) {
 			}
 			if (sendto (sock, frame, 14 + sizeof(ArpHdr), 0, (struct sockaddr *) &addr, sizeof (addr)) <= 0)
 				errExit("send");
-			ts.tv_sec = 0; // 0.5 seconds wait time
-			ts.tv_usec = 500000;
+			gettimeofday(&ts, NULL);
+			timerend = ts.tv_sec + ts.tv_usec / 1000000.0 + 0.5;
 			fflush(0);
 		}
 		else {
-- 
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