[PATCH] Prevent arping from freezing when no packet is received

[Ortmann, Michael]

Hello,

I ran into a case where a call like "arping -D -c 2 -I eth0 10.0.0.64" would freeze forever. It is caused by a blocking recfrom call at the end of the code. Turns out this was already fixed in the original iputils code. The following patch is just a port from iputils which can be found here: https://github.com/iputils/iputils/blob/master/arping.c
It also fixes another problem that can occur when the Ethernet connection is down.

I successfully tested the code change with busybox 1.24.1. It should also work with the current version since this part of the code has not changed since then. The following patch is for the current version.

Regards,
Michael

--- a/networking/arping.c            2018-02-09 11:50:56.551059773 +0100
+++ b/networking/arping.c         2018-02-09 11:54:37.494598160 +0100
@@ -423,16 +423,25 @@
                               struct sockaddr_ll from;
                               socklen_t alen = sizeof(from);
                               int cc;
+
+                             sigemptyset(&sset);
+                             sigaddset(&sset, SIGALRM);
+                             sigaddset(&sset, SIGINT);
+                             /* Unblock SIGALRM so that the previously called alarm()
+                             * can prevent recvfrom from blocking forever in case the
+                             * inherited procmask is blocking SIGALRM and no packet
+                             * is received. */
+                             sigprocmask(SIG_UNBLOCK, &sset, &osset);
                                cc = recvfrom(sock_fd, packet, 4096, 0, (struct sockaddr *) &from, &alen);
                               if (cc < 0) {
                                               bb_perror_msg("recvfrom");
+                                             if (errno == ENETDOWN)
+                                                             exit(2);
                                               continue;
                               }
-                              sigemptyset(&sset);
-                              sigaddset(&sset, SIGALRM);
-                              sigaddset(&sset, SIGINT);
-                              sigprocmask(SIG_BLOCK, &sset, &osset);
+
+                             sigprocmask(SIG_BLOCK, &sset, NULL);
                               recv_pack(packet, cc, &from);
                               sigprocmask(SIG_SETMASK, &osset, NULL);
               }
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.busybox.net/pipermail/busybox/attachments/20180209/fca0f242/attachment.html>