selftests/bpf: Verify optval=NULL case

Make sure we get optlen exported instead of getting EFAULT.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230418225343.553806-3-sdf@google.com

tools/testing/selftests/bpf/prog_tests/sockopt_sk.c

@@ -3,6 +3,7 @@
 #include "cgroup_helpers.h"
 
 #include <linux/tcp.h>
+#include <linux/netlink.h>
 #include "sockopt_sk.skel.h"
 
 #ifndef SOL_TCP
@@ -183,6 +184,33 @@ static int getsetsockopt(void)
 		goto err;
 	}
 
+	/* optval=NULL case is handled correctly */
+
+	close(fd);
+	fd = socket(AF_NETLINK, SOCK_RAW, 0);
+	if (fd < 0) {
+		log_err("Failed to create AF_NETLINK socket");
+		return -1;
+	}
+
+	buf.u32 = 1;
+	optlen = sizeof(__u32);
+	err = setsockopt(fd, SOL_NETLINK, NETLINK_ADD_MEMBERSHIP, &buf, optlen);
+	if (err) {
+		log_err("Unexpected getsockopt(NETLINK_ADD_MEMBERSHIP) err=%d errno=%d",
+			err, errno);
+		goto err;
+	}
+
+	optlen = 0;
+	err = getsockopt(fd, SOL_NETLINK, NETLINK_LIST_MEMBERSHIPS, NULL, &optlen);
+	if (err) {
+		log_err("Unexpected getsockopt(NETLINK_LIST_MEMBERSHIPS) err=%d errno=%d",
+			err, errno);
+		goto err;
+	}
+	ASSERT_EQ(optlen, 4, "Unexpected NETLINK_LIST_MEMBERSHIPS value");
+
 	free(big_buf);
 	close(fd);
 	return 0;

tools/testing/selftests/bpf/progs/sockopt_sk.c

@@ -32,6 +32,12 @@ int _getsockopt(struct bpf_sockopt *ctx)
 	__u8 *optval_end = ctx->optval_end;
 	__u8 *optval = ctx->optval;
 	struct sockopt_sk *storage;
+	struct bpf_sock *sk;
+
+	/* Bypass AF_NETLINK. */
+	sk = ctx->sk;
+	if (sk && sk->family == AF_NETLINK)
+		return 1;
 
 	/* Make sure bpf_get_netns_cookie is callable.
 	 */
@@ -131,6 +137,12 @@ int _setsockopt(struct bpf_sockopt *ctx)
 	__u8 *optval_end = ctx->optval_end;
 	__u8 *optval = ctx->optval;
 	struct sockopt_sk *storage;
+	struct bpf_sock *sk;
+
+	/* Bypass AF_NETLINK. */
+	sk = ctx->sk;
+	if (sk && sk->family == AF_NETLINK)
+		return 1;
 
 	/* Make sure bpf_get_netns_cookie is callable.
 	 */