Let's add a simple test to check the basic functionality of SO_INQ.

The test does the following:

  1. Create socketpair in self->fd[]
  2. Enable SO_INQ
  3. Send data via self->fd[0]
  4. Receive data from self->fd[1]
  5. Compare the SCM_INQ cmsg with ioctl(SIOCINQ)

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 tools/testing/selftests/net/.gitignore        |   1 +
 tools/testing/selftests/net/af_unix/Makefile  |   2 +-
 tools/testing/selftests/net/af_unix/scm_inq.c | 125 ++++++++++++++++++
 3 files changed, 127 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/net/af_unix/scm_inq.c

diff --git a/tools/testing/selftests/net/.gitignore b/tools/testing/selftests/net/.gitignore
index c6dd2a335cf4..47c293c2962f 100644
--- a/tools/testing/selftests/net/.gitignore
+++ b/tools/testing/selftests/net/.gitignore
@@ -34,6 +34,7 @@ reuseport_bpf_numa
 reuseport_dualstack
 rxtimestamp
 sctp_hello
+scm_inq
 scm_pidfd
 scm_rights
 sk_bind_sendto_listen
diff --git a/tools/testing/selftests/net/af_unix/Makefile b/tools/testing/selftests/net/af_unix/Makefile
index 50584479540b..a4b61c6d0290 100644
--- a/tools/testing/selftests/net/af_unix/Makefile
+++ b/tools/testing/selftests/net/af_unix/Makefile
@@ -1,4 +1,4 @@
 CFLAGS += $(KHDR_INCLUDES)
-TEST_GEN_PROGS := diag_uid msg_oob scm_pidfd scm_rights unix_connect
+TEST_GEN_PROGS := diag_uid msg_oob scm_inq scm_pidfd scm_rights unix_connect
 
 include ../../lib.mk
diff --git a/tools/testing/selftests/net/af_unix/scm_inq.c b/tools/testing/selftests/net/af_unix/scm_inq.c
new file mode 100644
index 000000000000..9d22561e7b8f
--- /dev/null
+++ b/tools/testing/selftests/net/af_unix/scm_inq.c
@@ -0,0 +1,125 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright 2025 Google LLC */
+
+#include <linux/sockios.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+
+#include "../../kselftest_harness.h"
+
+#define NR_CHUNKS	100
+#define MSG_LEN		256
+
+struct scm_inq {
+	struct cmsghdr cmsghdr;
+	int inq;
+};
+
+FIXTURE(scm_inq)
+{
+	int fd[2];
+};
+
+FIXTURE_VARIANT(scm_inq)
+{
+	int type;
+};
+
+FIXTURE_VARIANT_ADD(scm_inq, stream)
+{
+	.type = SOCK_STREAM,
+};
+
+FIXTURE_VARIANT_ADD(scm_inq, dgram)
+{
+	.type = SOCK_DGRAM,
+};
+
+FIXTURE_VARIANT_ADD(scm_inq, seqpacket)
+{
+	.type = SOCK_SEQPACKET,
+};
+
+FIXTURE_SETUP(scm_inq)
+{
+	int err;
+
+	err = socketpair(AF_UNIX, variant->type | SOCK_NONBLOCK, 0, self->fd);
+	ASSERT_EQ(0, err);
+}
+
+FIXTURE_TEARDOWN(scm_inq)
+{
+	close(self->fd[0]);
+	close(self->fd[1]);
+}
+
+static void send_chunks(struct __test_metadata *_metadata,
+			FIXTURE_DATA(scm_inq) *self)
+{
+	char buf[MSG_LEN] = {};
+	int i, ret;
+
+	for (i = 0; i < NR_CHUNKS; i++) {
+		ret = send(self->fd[0], buf, sizeof(buf), 0);
+		ASSERT_EQ(sizeof(buf), ret);
+	}
+}
+
+static void recv_chunks(struct __test_metadata *_metadata,
+			FIXTURE_DATA(scm_inq) *self)
+{
+	struct msghdr msg = {};
+	struct iovec iov = {};
+	struct scm_inq cmsg;
+	char buf[MSG_LEN];
+	int i, ret;
+	int inq;
+
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = &cmsg;
+	msg.msg_controllen = CMSG_SPACE(sizeof(cmsg.inq));
+
+	iov.iov_base = buf;
+	iov.iov_len = sizeof(buf);
+
+	for (i = 0; i < NR_CHUNKS; i++) {
+		memset(buf, 0, sizeof(buf));
+		memset(&cmsg, 0, sizeof(cmsg));
+
+		ret = recvmsg(self->fd[1], &msg, 0);
+		ASSERT_EQ(MSG_LEN, ret);
+		ASSERT_NE(NULL, CMSG_FIRSTHDR(&msg));
+		ASSERT_EQ(CMSG_LEN(sizeof(cmsg.inq)), cmsg.cmsghdr.cmsg_len);
+		ASSERT_EQ(SOL_SOCKET, cmsg.cmsghdr.cmsg_level);
+		ASSERT_EQ(SCM_INQ, cmsg.cmsghdr.cmsg_type);
+
+		ret = ioctl(self->fd[1], SIOCINQ, &inq);
+		ASSERT_EQ(0, ret);
+		ASSERT_EQ(cmsg.inq, inq);
+	}
+}
+
+TEST_F(scm_inq, basic)
+{
+	int err, inq;
+
+	err = setsockopt(self->fd[1], SOL_SOCKET, SO_INQ, &(int){1}, sizeof(int));
+	if (variant->type != SOCK_STREAM) {
+		ASSERT_EQ(-ENOPROTOOPT, -errno);
+		return;
+	}
+
+	ASSERT_EQ(0, err);
+
+	err = ioctl(self->fd[1], SIOCINQ, &inq);
+	ASSERT_EQ(0, err);
+	ASSERT_EQ(0, inq);
+
+	send_chunks(_metadata, self);
+	recv_chunks(_metadata, self);
+}
+
+TEST_HARNESS_MAIN
-- 
2.50.0.727.gbf7dc18ff4-goog