selftests/bpf: Mask socket type flags in mptcpify prog

The mptcpify BPF prog upgrades eligible TCP sockets to MPTCP, but only
when the socket type is exactly SOCK_STREAM. Its update_socket_protocol()
hook runs on the raw type from userspace, before the socket core masks
it with SOCK_TYPE_MASK, so the type may still carry SOCK_CLOEXEC or
SOCK_NONBLOCK in its upper bits and the equality check fails.

As a result, a socket(AF_INET, SOCK_STREAM | SOCK_CLOEXEC, 0) -- what
common libraries do by default -- is silently left as plain TCP. This
was hit in practice with curl. Since mptcpify.c is referenced as example
code for enabling MPTCP transparently, the same mistake is likely to be
copied into real deployments where it fails the same way and is hard to
diagnose.

Mask the type before comparing, mirroring the socket core. Extend the
test to also create the server with SOCK_CLOEXEC set; the same masking
is applied to start_server_addr() so a flagged type still listens.

Fixes: ddba122428 ("selftests/bpf: Add mptcpify test")
Signed-off-by: Guillaume Maudoux <layus.on@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/bpf/20260630095723.564392-1-layus.on@gmail.com

tools/testing/selftests/bpf/network_helpers.c

@@ -111,7 +111,7 @@ int start_server_addr(int type, const struct sockaddr_storage *addr, socklen_t a
 	if (settimeo(fd, opts->timeout_ms))
 		goto error_close;
 
-	if (type == SOCK_STREAM &&
+	if ((type & SOCK_TYPE_MASK) == SOCK_STREAM &&
 	    setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on))) {
 		log_err("Failed to enable SO_REUSEADDR");
 		goto error_close;
@@ -128,7 +128,7 @@ int start_server_addr(int type, const struct sockaddr_storage *addr, socklen_t a
 		goto error_close;
 	}
 
-	if (type == SOCK_STREAM) {
+	if ((type & SOCK_TYPE_MASK) == SOCK_STREAM) {
 		if (listen(fd, opts->backlog ? MAX(opts->backlog, 0) : 1) < 0) {
 			log_err("Failed to listed on socket");
 			goto error_close;

tools/testing/selftests/bpf/network_helpers.h

@@ -25,6 +25,11 @@ typedef __u16 __sum16;
 #define VIP_NUM 5
 #define MAGIC_BYTES 123
 
+/* include/linux/net.h */
+#ifndef SOCK_TYPE_MASK
+#define SOCK_TYPE_MASK 0xf
+#endif
+
 struct network_helper_opts {
 	int timeout_ms;
 	int proto;

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

@@ -264,7 +264,7 @@ static int verify_mptcpify(int server_fd, int client_fd)
 	return err;
 }
 
-static int run_mptcpify(int cgroup_fd)
+static int run_mptcpify(int cgroup_fd, int type)
 {
 	int server_fd, client_fd, err = 0;
 	struct mptcpify *mptcpify_skel;
@@ -280,7 +280,7 @@ static int run_mptcpify(int cgroup_fd)
 		goto out;
 
 	/* without MPTCP */
-	server_fd = start_server(AF_INET, SOCK_STREAM, NULL, 0, 0);
+	server_fd = start_server(AF_INET, type, NULL, 0, 0);
 	if (!ASSERT_GE(server_fd, 0, "start_server")) {
 		err = -EIO;
 		goto out;
@@ -317,7 +317,14 @@ static void test_mptcpify(void)
 	if (!ASSERT_OK_PTR(netns, "netns_new"))
 		goto fail;
 
-	ASSERT_OK(run_mptcpify(cgroup_fd), "run_mptcpify");
+	ASSERT_OK(run_mptcpify(cgroup_fd, SOCK_STREAM), "run_mptcpify");
+	/* userspace sets flags such as SOCK_CLOEXEC together with the type;
+	 * the BPF prog must still upgrade the socket to MPTCP. See
+	 * update_socket_protocol() in net/socket.c, which runs before the
+	 * type is masked with SOCK_TYPE_MASK.
+	 */
+	ASSERT_OK(run_mptcpify(cgroup_fd, SOCK_STREAM | SOCK_CLOEXEC),
+		  "run_mptcpify_cloexec");
 
 fail:
 	netns_free(netns);

tools/testing/selftests/bpf/progs/bpf_tracing_net.h

@@ -8,6 +8,9 @@
 #define AF_INET			2
 #define AF_INET6		10
 
+/* include/linux/net.h */
+#define SOCK_TYPE_MASK		0xf
+
 #define SOL_SOCKET		1
 #define SO_REUSEADDR		2
 #define SO_SNDBUF		7

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

@@ -15,7 +15,7 @@ int BPF_PROG(mptcpify, int family, int type, int protocol)
 		return protocol;
 
 	if ((family == AF_INET || family == AF_INET6) &&
-	    type == SOCK_STREAM &&
+	    (type & SOCK_TYPE_MASK) == SOCK_STREAM &&
 	    (!protocol || protocol == IPPROTO_TCP)) {
 		return IPPROTO_MPTCP;
 	}