doc: Test behaviour of closing duplicate UDP sockets

To simplify lifetime management of "listening" UDP sockets, UDP flow
support needs to duplicate existing bound sockets.  Those duplicates will
be close()d when their corresponding flow expires, but we expect the
original to still receive datagrams as always.  That is, we expect the
close() on the duplicate to remove the duplicated fd, but not to close the
underlying UDP socket.

Add a test program to doc/platform-requirements to verify this requirement.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

doc/platform-requirements/.gitignore

@@ -1,2 +1,3 @@
 /reuseaddr-priority
 /recv-zero
+/udp-close-dup

doc/platform-requirements/Makefile

@@ -3,8 +3,8 @@
 # Copyright Red Hat
 # Author: David Gibson <david@gibson.dropbear.id.au>
 
-TARGETS = reuseaddr-priority recv-zero
-SRCS = reuseaddr-priority.c recv-zero.c
+TARGETS = reuseaddr-priority recv-zero udp-close-dup
+SRCS = reuseaddr-priority.c recv-zero.c udp-close-dup.c
 CFLAGS = -Wall
 
 all: cppcheck clang-tidy $(TARGETS:%=check-%)

doc/platform-requirements/udp-close-dup.c

@@ -0,0 +1,105 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/* udp-close-dup.c
+ *
+ * Verify that closing one dup() of a UDP socket won't stop other dups from
+ * receiving packets.
+ *
+ * Copyright Red Hat
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ */
+
+#include <arpa/inet.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <net/if.h>
+#include <netinet/in.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "common.h"
+
+#define DSTPORT	13257U
+
+/* 127.0.0.1:DSTPORT */
+static const struct sockaddr_in lo_dst = SOCKADDR_INIT(INADDR_LOOPBACK, DSTPORT);
+
+enum dup_method {
+	DUP_DUP,
+	DUP_FCNTL,
+	NUM_METHODS,
+};
+
+static void test_close_dup(enum dup_method method)
+{
+	long token;
+	int s1, s2, send_s;
+	ssize_t rc;
+
+	s1 = sock_reuseaddr();
+	if (bind(s1, (struct sockaddr *)&lo_dst, sizeof(lo_dst)) < 0)
+		die("bind(): %s\n", strerror(errno));
+
+	send_s = sock_reuseaddr();
+	if (connect(send_s, (struct sockaddr *)&lo_dst, sizeof(lo_dst)) < 0)
+		die("connect(): %s\n", strerror(errno));
+
+	/* Receive before duplicating */
+	token = random();
+	send_token(send_s, token);
+	recv_token(s1, token);
+
+	switch (method) {
+	case DUP_DUP:
+		/* NOLINTNEXTLINE(android-cloexec-dup) */
+		s2 = dup(s1);
+		if (s2 < 0)
+			die("dup(): %s\n", strerror(errno));
+		break;
+	case DUP_FCNTL:
+		s2 = fcntl(s1, F_DUPFD_CLOEXEC, 0);
+		if (s2 < 0)
+			die("F_DUPFD_CLOEXEC: %s\n", strerror(errno));
+		break;
+	default:
+		die("Bad method\n");
+	}
+
+	/* Receive via original handle */
+	token = random();
+	send_token(send_s, token);
+	recv_token(s1, token);
+
+	/* Receive via duplicated handle */
+	token = random();
+	send_token(send_s, token);
+	recv_token(s2, token);
+
+	/* Close duplicate */
+	rc = close(s2);
+	if (rc < 0)
+		die("close() dup: %s\n", strerror(errno));
+
+	/* Receive after closing duplicate */
+	token = random();
+	send_token(send_s, token);
+	recv_token(s1, token);
+}
+
+int main(int argc, char *argv[])
+{
+	enum dup_method method;
+
+	(void)argc;
+	(void)argv;
+
+	for (method = 0; method < NUM_METHODS; method++)
+		test_close_dup(method);
+
+	printf("Closing dup()ed UDP sockets seems to work as expected\n");
+
+	exit(0);
+}