1 files changed, 114 insertions, 0 deletions
diff --git a/tools/testing/selftests/net/ipv6_fragmentation.c b/tools/testing/selftests/net/ipv6_fragmentation.c
new file mode 100644
index 000000000000..267ef62b5c72
--- /dev/null
+++ b/tools/testing/selftests/net/ipv6_fragmentation.c
@@ -0,0 +1,114 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Author: Brett A C Sheffield <bacs@librecast.net>
+ *
+ * Kernel selftest for the IPv6 fragmentation regression which affected stable
+ * kernels:
+ *
+ *   https://lore.kernel.org/stable/aElivdUXqd1OqgMY@karahi.gladserv.com
+ *
+ * Commit: a18dfa9925b9 ("ipv6: save dontfrag in cork") was backported to stable
+ * without some prerequisite commits.
+ *
+ * This caused a regression when sending IPv6 UDP packets by preventing
+ * fragmentation and instead returning -1 (EMSGSIZE).
+ *
+ * This selftest demonstrates the issue by sending an IPv6 UDP packet to
+ * localhost (::1) on the loopback interface from the autoconfigured link-local
+ * address.
+ *
+ * sendmsg(2) returns bytes sent correctly on a working kernel, and returns -1
+ * (EMSGSIZE) when the regression is present.
+ *
+ * The regression was not present in the mainline kernel, but add this test to
+ * catch similar breakage in future.
+ */
+
+#define _GNU_SOURCE
+
+#include <error.h>
+#include <net/if.h>
+#include <netinet/in.h>
+#include <sched.h>
+#include <stdio.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <unistd.h>
+#include "../kselftest.h"
+
+#define MTU 1500
+#define LARGER_THAN_MTU 8192
+
+static void setup(void)
+{
+	struct ifreq ifr = {
+		.ifr_name = "lo"
+	};
+	int ctl;
+
+	/* we need to set MTU, so do this in a namespace to play nicely */
+	if (unshare(CLONE_NEWNET) == -1)
+		error(KSFT_FAIL, errno, "unshare");
+
+	ctl = socket(AF_LOCAL, SOCK_STREAM, 0);
+	if (ctl == -1)
+		error(KSFT_FAIL, errno, "socket");
+
+	/* ensure MTU is smaller than what we plan to send */
+	ifr.ifr_mtu = MTU;
+	if (ioctl(ctl, SIOCSIFMTU, &ifr) == -1)
+		error(KSFT_FAIL, errno, "ioctl: set MTU");
+
+	/* bring up interface */
+	if (ioctl(ctl, SIOCGIFFLAGS, &ifr) == -1)
+		error(KSFT_FAIL, errno, "ioctl SIOCGIFFLAGS");
+	ifr.ifr_flags = ifr.ifr_flags | IFF_UP;
+	if (ioctl(ctl, SIOCSIFFLAGS, &ifr) == -1)
+		error(KSFT_FAIL, errno, "ioctl: bring interface up");
+
+	if (close(ctl) == -1)
+		error(KSFT_FAIL, errno, "close");
+}
+
+int main(void)
+{
+	struct in6_addr addr = {
+		.s6_addr[15] = 0x01,  /* ::1 */
+	};
+	struct sockaddr_in6 sa = {
+		.sin6_family = AF_INET6,
+		.sin6_addr = addr,
+		.sin6_port = htons(9) /* port 9/udp (DISCARD) */
+	};
+	static char buf[LARGER_THAN_MTU] = {0};
+	struct iovec iov = { .iov_base = buf, .iov_len = sizeof(buf) };
+	struct msghdr msg = {
+		.msg_iov = &iov,
+		.msg_iovlen = 1,
+		.msg_name = (struct sockaddr *)&sa,
+		.msg_namelen = sizeof(sa),
+	};
+	ssize_t rc;
+	int s;
+
+	printf("Testing IPv6 fragmentation\n");
+	setup();
+	s = socket(AF_INET6, SOCK_DGRAM, 0);
+send_again:
+	rc = sendmsg(s, &msg, 0);
+	if (rc == -1) {
+		/* if interface wasn't ready, try again */
+		if (errno == EADDRNOTAVAIL) {
+			usleep(1000);
+			goto send_again;
+		}
+		error(KSFT_FAIL, errno, "sendmsg");
+	} else if (rc != LARGER_THAN_MTU) {
+		error(KSFT_FAIL, errno, "sendmsg returned %zi, expected %i",
+				rc, LARGER_THAN_MTU);
+	}
+	printf("[PASS] sendmsg() returned %zi\n", rc);
+	if (close(s) == -1)
+		error(KSFT_FAIL, errno, "close");
+	return KSFT_PASS;
+}