[PATCH net v2 2/3] selftests/net/af_unix: test listen() rejects wrong socket states

[John Ericson <John.Ericson@Obsidian.Systems>]

From: John Ericson <mail@johnericson.me>

Add a regression test for the `unix_listen()` state check. The key case
is `listen()` on a bound socket that has already been connected: it is
no longer in `TCP_CLOSE` or `TCP_LISTEN`, so it must fail with `EINVAL`.
A `prepare_peercred()` call slipped in ahead of that check once left
`err` at 0 and made `listen()` silently succeed there instead; this
guards against a repeat.

The neighbouring outcomes are covered too so they cannot regress the
same way: a bound socket in `TCP_CLOSE` listens fine, re-`listen()`ing a
socket already in `TCP_LISTEN` is allowed, and an unbound socket fails
with `EINVAL`.

Fixes: fd0a109a0f6b ("net, pidfs: prepare for handing out pidfds for reaped sk->sk_peer_pid")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: John Ericson <mail@johnericson.me>
---
 .../testing/selftests/net/af_unix/.gitignore  |   1 +
 tools/testing/selftests/net/af_unix/Makefile  |   1 +
 .../selftests/net/af_unix/unix_listen.c       | 126 ++++++++++++++++++
 3 files changed, 128 insertions(+)
 create mode 100644 tools/testing/selftests/net/af_unix/unix_listen.c

diff --git a/tools/testing/selftests/net/af_unix/.gitignore b/tools/testing/selftests/net/af_unix/.gitignore
index 240b26740c9e..973176644103 100644
--- a/tools/testing/selftests/net/af_unix/.gitignore
+++ b/tools/testing/selftests/net/af_unix/.gitignore
@@ -6,3 +6,4 @@ scm_rights
 so_peek_off
 unix_connect
 unix_connreset
+unix_listen
diff --git a/tools/testing/selftests/net/af_unix/Makefile b/tools/testing/selftests/net/af_unix/Makefile
index 4c0375e28bbe..57d159803a3a 100644
--- a/tools/testing/selftests/net/af_unix/Makefile
+++ b/tools/testing/selftests/net/af_unix/Makefile
@@ -14,6 +14,7 @@ TEST_GEN_PROGS := \
 	so_peek_off \
 	unix_connect \
 	unix_connreset \
+	unix_listen \
 # end of TEST_GEN_PROGS
 
 include ../../lib.mk
diff --git a/tools/testing/selftests/net/af_unix/unix_listen.c b/tools/testing/selftests/net/af_unix/unix_listen.c
new file mode 100644
index 000000000000..7b5264c97f52
--- /dev/null
+++ b/tools/testing/selftests/net/af_unix/unix_listen.c
@@ -0,0 +1,126 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Tests for the state checks in AF_UNIX listen().
+ *
+ * The central case is a regression test: listen() on a bound socket that
+ * is already connected (i.e. not in TCP_CLOSE or TCP_LISTEN state) must
+ * fail with EINVAL.  A prior change accidentally let it return success
+ * without doing anything, because a helper called in between reset the
+ * error code to 0.  The neighbouring checks (unbound, already listening)
+ * are tested too so they cannot silently regress the same way.
+ */
+#define _GNU_SOURCE
+
+#include <errno.h>
+#include <stddef.h>
+#include <string.h>
+#include <unistd.h>
+
+#include <sys/socket.h>
+#include <sys/un.h>
+
+#include "kselftest_harness.h"
+
+/* Fill @addr with an abstract-namespace address named @name. */
+static socklen_t unix_abstract(struct sockaddr_un *addr, const char *name)
+{
+	size_t len = strlen(name);
+
+	memset(addr, 0, sizeof(*addr));
+	addr->sun_family = AF_UNIX;
+	/* Leading NUL selects the abstract namespace (no filesystem entry). */
+	memcpy(addr->sun_path + 1, name, len);
+
+	return offsetof(struct sockaddr_un, sun_path) + 1 + len;
+}
+
+FIXTURE(unix_listen)
+{
+	int sk;			/* socket under test */
+	int server;		/* a listening peer, when a test needs one */
+	struct sockaddr_un addr, srv_addr;
+	socklen_t addrlen, srv_addrlen;
+};
+
+FIXTURE_SETUP(unix_listen)
+{
+	self->sk = -1;
+	self->server = -1;
+	self->addrlen = unix_abstract(&self->addr, "unix_listen.sk");
+	self->srv_addrlen = unix_abstract(&self->srv_addr, "unix_listen.srv");
+}
+
+FIXTURE_TEARDOWN(unix_listen)
+{
+	if (self->sk >= 0)
+		close(self->sk);
+	if (self->server >= 0)
+		close(self->server);
+	/* Abstract addresses are released automatically on close. */
+}
+
+/* A bound socket in TCP_CLOSE is the normal, allowed case. */
+TEST_F(unix_listen, bound_is_ok)
+{
+	self->sk = socket(AF_UNIX, SOCK_STREAM, 0);
+	ASSERT_LE(0, self->sk);
+	ASSERT_EQ(0, bind(self->sk, (struct sockaddr *)&self->addr,
+			  self->addrlen));
+
+	EXPECT_EQ(0, listen(self->sk, 8));
+}
+
+/* Listening again on an already-listening socket (TCP_LISTEN) is allowed. */
+TEST_F(unix_listen, relisten_is_ok)
+{
+	self->sk = socket(AF_UNIX, SOCK_STREAM, 0);
+	ASSERT_LE(0, self->sk);
+	ASSERT_EQ(0, bind(self->sk, (struct sockaddr *)&self->addr,
+			  self->addrlen));
+	ASSERT_EQ(0, listen(self->sk, 8));
+
+	EXPECT_EQ(0, listen(self->sk, 16));
+}
+
+/* listen() on an unbound socket fails: there is nothing to listen on. */
+TEST_F(unix_listen, unbound_is_einval)
+{
+	int ret;
+
+	self->sk = socket(AF_UNIX, SOCK_STREAM, 0);
+	ASSERT_LE(0, self->sk);
+
+	ret = listen(self->sk, 8);
+	EXPECT_EQ(-1, ret);
+	EXPECT_EQ(EINVAL, errno);
+}
+
+/*
+ * The regression: a bound socket that has already been connected is not in
+ * TCP_CLOSE or TCP_LISTEN, so listen() must reject it with EINVAL rather
+ * than quietly succeeding.
+ */
+TEST_F(unix_listen, connected_is_einval)
+{
+	int ret;
+
+	self->server = socket(AF_UNIX, SOCK_STREAM, 0);
+	ASSERT_LE(0, self->server);
+	ASSERT_EQ(0, bind(self->server, (struct sockaddr *)&self->srv_addr,
+			  self->srv_addrlen));
+	ASSERT_EQ(0, listen(self->server, 8));
+
+	self->sk = socket(AF_UNIX, SOCK_STREAM, 0);
+	ASSERT_LE(0, self->sk);
+	/* Bind first so the unbound check does not mask the state check. */
+	ASSERT_EQ(0, bind(self->sk, (struct sockaddr *)&self->addr,
+			  self->addrlen));
+	ASSERT_EQ(0, connect(self->sk, (struct sockaddr *)&self->srv_addr,
+			     self->srv_addrlen));
+
+	ret = listen(self->sk, 8);
+	EXPECT_EQ(-1, ret);
+	EXPECT_EQ(EINVAL, errno);
+}
+
+TEST_HARNESS_MAIN
-- 
2.54.0