selftests: af_unix: Add tests for ECONNRESET and EOF semantics

Add selftests to verify and document Linux’s intended behaviour for
UNIX domain sockets (SOCK_STREAM and SOCK_DGRAM) when a peer closes.
The tests verify that:

 1. SOCK_STREAM returns EOF when the peer closes normally.
 2. SOCK_STREAM returns ECONNRESET if the peer closes with unread data.
 3. SOCK_SEQPACKET returns EOF when the peer closes normally.
 4. SOCK_SEQPACKET returns ECONNRESET if the peer closes with unread data.
 5. SOCK_DGRAM does not return ECONNRESET when the peer closes.

This follows up on review feedback suggesting a selftest to clarify
Linux’s semantics.

Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Sunday Adelodun <adelodunolaoluwa@yahoo.com>
Link: https://patch.msgid.link/20251113112802.44657-1-adelodunolaoluwa@yahoo.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/.gitignore

@@ -65,3 +65,4 @@ udpgso
 udpgso_bench_rx
 udpgso_bench_tx
 unix_connect
+unix_connreset

tools/testing/selftests/net/af_unix/Makefile

@@ -7,6 +7,7 @@ TEST_GEN_PROGS := \
 	scm_pidfd \
 	scm_rights \
 	unix_connect \
+	unix_connreset \
 # end of TEST_GEN_PROGS
 
 include ../../lib.mk

tools/testing/selftests/net/af_unix/unix_connreset.c

@@ -0,0 +1,177 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Selftest for AF_UNIX socket close and ECONNRESET behaviour.
+ *
+ * This test verifies:
+ *  1. SOCK_STREAM returns EOF when the peer closes normally.
+ *  2. SOCK_STREAM returns ECONNRESET if peer closes with unread data.
+ *  3. SOCK_SEQPACKET returns EOF when the peer closes normally.
+ *  4. SOCK_SEQPACKET returns ECONNRESET if the peer closes with unread data.
+ *  5. SOCK_DGRAM does not return ECONNRESET when the peer closes.
+ *
+ * These tests document the intended Linux behaviour.
+ *
+ */
+
+#define _GNU_SOURCE
+#include <stdlib.h>
+#include <string.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include "../../kselftest_harness.h"
+
+#define SOCK_PATH "/tmp/af_unix_connreset.sock"
+
+static void remove_socket_file(void)
+{
+	unlink(SOCK_PATH);
+}
+
+FIXTURE(unix_sock)
+{
+	int server;
+	int client;
+	int child;
+};
+
+FIXTURE_VARIANT(unix_sock)
+{
+	int socket_type;
+	const char *name;
+};
+
+FIXTURE_VARIANT_ADD(unix_sock, stream) {
+	.socket_type = SOCK_STREAM,
+	.name = "SOCK_STREAM",
+};
+
+FIXTURE_VARIANT_ADD(unix_sock, dgram) {
+	.socket_type = SOCK_DGRAM,
+	.name = "SOCK_DGRAM",
+};
+
+FIXTURE_VARIANT_ADD(unix_sock, seqpacket) {
+	.socket_type = SOCK_SEQPACKET,
+	.name = "SOCK_SEQPACKET",
+};
+
+FIXTURE_SETUP(unix_sock)
+{
+	struct sockaddr_un addr = {};
+	int err;
+
+	addr.sun_family = AF_UNIX;
+	strcpy(addr.sun_path, SOCK_PATH);
+	remove_socket_file();
+
+	self->server = socket(AF_UNIX, variant->socket_type, 0);
+	ASSERT_LT(-1, self->server);
+
+	err = bind(self->server, (struct sockaddr *)&addr, sizeof(addr));
+	ASSERT_EQ(0, err);
+
+	if (variant->socket_type == SOCK_STREAM ||
+	    variant->socket_type == SOCK_SEQPACKET) {
+		err = listen(self->server, 1);
+		ASSERT_EQ(0, err);
+	}
+
+	self->client = socket(AF_UNIX, variant->socket_type | SOCK_NONBLOCK, 0);
+	ASSERT_LT(-1, self->client);
+
+	err = connect(self->client, (struct sockaddr *)&addr, sizeof(addr));
+	ASSERT_EQ(0, err);
+}
+
+FIXTURE_TEARDOWN(unix_sock)
+{
+	if (variant->socket_type == SOCK_STREAM ||
+	    variant->socket_type == SOCK_SEQPACKET)
+		close(self->child);
+
+	close(self->client);
+	close(self->server);
+	remove_socket_file();
+}
+
+/* Test 1: peer closes normally */
+TEST_F(unix_sock, eof)
+{
+	char buf[16] = {};
+	ssize_t n;
+
+	if (variant->socket_type == SOCK_STREAM ||
+	    variant->socket_type == SOCK_SEQPACKET) {
+		self->child = accept(self->server, NULL, NULL);
+		ASSERT_LT(-1, self->child);
+
+		close(self->child);
+	} else {
+		close(self->server);
+	}
+
+	n = recv(self->client, buf, sizeof(buf), 0);
+
+	if (variant->socket_type == SOCK_STREAM ||
+	    variant->socket_type == SOCK_SEQPACKET) {
+		ASSERT_EQ(0, n);
+	} else {
+		ASSERT_EQ(-1, n);
+		ASSERT_EQ(EAGAIN, errno);
+	}
+}
+
+/* Test 2: peer closes with unread data */
+TEST_F(unix_sock, reset_unread_behavior)
+{
+	char buf[16] = {};
+	ssize_t n;
+
+	/* Send data that will remain unread */
+	send(self->client, "hello", 5, 0);
+
+	if (variant->socket_type == SOCK_DGRAM) {
+		/* No real connection, just close the server */
+		close(self->server);
+	} else {
+		self->child = accept(self->server, NULL, NULL);
+		ASSERT_LT(-1, self->child);
+
+		/* Peer closes before client reads */
+		close(self->child);
+	}
+
+	n = recv(self->client, buf, sizeof(buf), 0);
+	ASSERT_EQ(-1, n);
+
+	if (variant->socket_type == SOCK_STREAM ||
+	    variant->socket_type == SOCK_SEQPACKET) {
+		ASSERT_EQ(ECONNRESET, errno);
+	} else {
+		ASSERT_EQ(EAGAIN, errno);
+	}
+}
+
+/* Test 3: closing unaccepted (embryo) server socket should reset client. */
+TEST_F(unix_sock, reset_closed_embryo)
+{
+	char buf[16] = {};
+	ssize_t n;
+
+	if (variant->socket_type == SOCK_DGRAM)
+		SKIP(return, "This test only applies to SOCK_STREAM and SOCK_SEQPACKET");
+
+	/* Close server without accept()ing */
+	close(self->server);
+
+	n = recv(self->client, buf, sizeof(buf), 0);
+
+	ASSERT_EQ(-1, n);
+	ASSERT_EQ(ECONNRESET, errno);
+}
+
+TEST_HARNESS_MAIN
+