arping: fix arping hang if SIGALRM is blocked

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.

Upstream-Status: Backport

Reported-by: Rui Prior <rprior@dcc.fc.up.pt>
RH-Bugzilla: #1085971
Signed-off-by: Jan Synacek <jsynacek@redhat.com>
Signed-off-by: Zhenbo Gao <zhenbo.gao@windriver.com>

diff --git a/arping.c.orig b/arping.c
index 35408c1..2098159 100644
--- a/arping.c.orig
+++ b/arping.c
@@ -1215,16 +1215,22 @@ main(int argc, char **argv)
 		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);
+
 		if ((cc = recvfrom(s, packet, sizeof(packet), 0,
 				   (struct sockaddr *)&from, &alen)) < 0) {
 			perror("arping: recvfrom");
 			continue;
 		}
 
-		sigemptyset(&sset);
-		sigaddset(&sset, SIGALRM);
-		sigaddset(&sset, SIGINT);
-		sigprocmask(SIG_BLOCK, &sset, &osset);
+		sigprocmask(SIG_BLOCK, &sset, NULL);
 		recv_pack(packet, cc, (struct sockaddr_ll *)&from);
 		sigprocmask(SIG_SETMASK, &osset, NULL);
 	}