selftests/net: Add coverage for UDP GSO with IPv6 extension headers

After enabling UDP GSO for devices not offering checksum offload, we have
hit a regression where a bad offload warning can be triggered when sending
a datagram with IPv6 extension headers.

Extend the UDP GSO IPv6 tests to cover this scenario.

Reviewed-by: Willem de Bruijn <[e-mail]>
Signed-off-by: Jakub Sitnicki <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/udpgso.c

···
 	int gso_len;		/* mss after applying gso */
 	int r_num_mss;		/* recv(): number of calls of full mss */
 	int r_len_last;		/* recv(): size of last non-mss dgram, if any */
+	bool v6_ext_hdr;	/* send() dgrams with IPv6 extension headers */
 };
 
 const struct in6_addr addr6 = {
···
 const struct in_addr addr4 = {
 	__constant_htonl(0x0a000001), /* 10.0.0.1 */
 };
+
+static const char ipv6_hopopts_pad1[8] = { 0 };
 
 struct testcase testcases_v4[] = {
 	{
···
 		.r_num_mss = 2,
 	},
 	{
+		/* send 2 1B segments with extension headers */
+		.tlen = 2,
+		.gso_len = 1,
+		.r_num_mss = 2,
+		.v6_ext_hdr = true,
+	},
+	{
 		/* send 2B + 2B + 1B segments */
 		.tlen = 5,
 		.gso_len = 2,
···
 	int i, ret, val, mss;
 	bool sent;
 
-	fprintf(stderr, "ipv%d tx:%d gso:%d %s\n",
+	fprintf(stderr, "ipv%d tx:%d gso:%d %s%s\n",
 			addr->sa_family == AF_INET ? 4 : 6,
 			test->tlen, test->gso_len,
+			test->v6_ext_hdr ? "ext-hdr " : "",
 			test->tfail ? "(fail)" : "");
+
+	if (test->v6_ext_hdr) {
+		if (setsockopt(fdt, IPPROTO_IPV6, IPV6_HOPOPTS,
+			       ipv6_hopopts_pad1, sizeof(ipv6_hopopts_pad1)))
+			error(1, errno, "setsockopt ipv6 hopopts");
+	}
 
 	val = test->gso_len;
 	if (cfg_do_setsockopt) {
···
 		error(1, 0, "send succeeded while expecting failure");
 	if (!sent && !test->tfail)
 		error(1, 0, "send failed while expecting success");
+
+	if (test->v6_ext_hdr) {
+		if (setsockopt(fdt, IPPROTO_IPV6, IPV6_HOPOPTS, NULL, 0))
+			error(1, errno, "setsockopt ipv6 hopopts clear");
+	}
+
 	if (!sent)
 		return;