Re: [PATCH v4] selftests: af_unix: Add tests for ECONNRESET and EOF semantics

Kuniyuki Iwashima
2025-11-13T06:10:44Z

On Wed, Nov 12, 2025 at 9:25 PM Sunday Adelodun
<adelodunolaoluwa@yahoo.com> wrote:
>
> On 11/12/25 23:04, Kuniyuki Iwashima wrote:
> > On Wed, Nov 12, 2025 at 1:20 PM Sunday Adelodun
> > <adelodunolaoluwa@yahoo.com> wrote:
> >> 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>
> >> ---
> >> tools/testing/selftests/net/.gitignore | 1 +
> >> tools/testing/selftests/net/af_unix/Makefile | 1 +
> >> .../selftests/net/af_unix/unix_connreset.c | 178 ++++++++++++++++++
> >> 3 files changed, 180 insertions(+)
> >> create mode 100644 tools/testing/selftests/net/af_unix/unix_connreset.c
> >>
> >> diff --git a/tools/testing/selftests/net/.gitignore b/tools/testing/selftests/net/.gitignore
> >> index 439101b518ee..e89a60581a13 100644
> >> --- a/tools/testing/selftests/net/.gitignore
> >> +++ b/tools/testing/selftests/net/.gitignore
> >> @@ -65,3 +65,4 @@ udpgso
> >> udpgso_bench_rx
> >> udpgso_bench_tx
> >> unix_connect
> >> +unix_connreset
> >> diff --git a/tools/testing/selftests/net/af_unix/Makefile b/tools/testing/selftests/net/af_unix/Makefile
> >> index de805cbbdf69..5826a8372451 100644
> >> --- a/tools/testing/selftests/net/af_unix/Makefile
> >> +++ b/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
> >> diff --git a/tools/testing/selftests/net/af_unix/unix_connreset.c b/tools/testing/selftests/net/af_unix/unix_connreset.c
> >> new file mode 100644
> >> index 000000000000..9413f8a0814f
> >> --- /dev/null
> >> +++ b/tools/testing/selftests/net/af_unix/unix_connreset.c
> >> @@ -0,0 +1,178 @@
> >> +// 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) & self->child > 0)
> >> + 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;
> >> +
> >> + if (variant->socket_type == SOCK_DGRAM) {
> >> + /* No real connection, just close the server */
> >> + close(self->server);
> >> + } else {
> >> + /* Establish full connection first */
> >> + self->child = accept(self->server, NULL, NULL);
> >> + ASSERT_LT(-1, self->child);
> >> +
> >> + /* Send data that will remain unread */
> >> + send(self->client, "hello", 5, 0);
> > Could you move this send() before "if (...)" because we want
> > to test unread_data behaviour for SOCK_DGRAM too ?
> >
> > Otherwise looks good, so with that fixed:
> >
> > Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
> >
> > Thanks!
> Thank you for the prompt response.
> I thought of putting the send before the if the statement , but I was afraid
> STREAM and SEQPACKET connections won't be accepted before data sending.

connect() create the paired child socket and accept()
allocates a file descriptor to expose the socket to user
space.

In that sense, the comment above accept() sounds a
bit weird ;)

>
> I will start working on v5.
>
> That part will look like this now:
> /* 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 {
> /* Establish full connection first */
> 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);
> }
> }
>
> Thank you once again.
> >
> >
> >> +
> >> + /* 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
> >> +
> >> --
> >> 2.43.0
> >>
>