Re: [PATCH v3] selftests: af_unix: Add tests for ECONNRESET and EOF semantics

Kuniyuki Iwashima
2025-11-07T07:07:15Z

From: Sunday Adelodun <adelodunolaoluwa@yahoo.com>
Date: Tue, 4 Nov 2025 11:42:47 +0100
> On 11/4/25 01:30, Kuniyuki Iwashima wrote:
> > On Mon, Nov 3, 2025 at 4:08 PM Sunday Adelodun
> > <adelodunolaoluwa@yahoo.com> wrote:
> >> On 11/2/25 08:32, Kuniyuki Iwashima wrote:
> >>> On Sat, Nov 1, 2025 at 10:23 AM 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/af_unix/Makefile | 1 +
> >>>> .../selftests/net/af_unix/unix_connreset.c | 179 ++++++++++++++++++
> >>>> 2 files changed, 180 insertions(+)
> >>>> create mode 100644 tools/testing/selftests/net/af_unix/unix_connreset.c
> >>>>
> >>>> 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 \
> >>> patchwork caught this test is not added to .gitignore.
> >>> https://patchwork.kernel.org/project/netdevbpf/patch/20251101172230.10179-1-adelodunolaoluwa@yahoo.com/
> >>>
> >>> Could you add it to this file ?
> >>>
> >>> tools/testing/selftests/net/.gitignore
> >> Oh, thank you for this. will add it
> >>>
> >>>> # 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..6f43435d96e2
> >>>> --- /dev/null
> >>>> +++ b/tools/testing/selftests/net/af_unix/unix_connreset.c
> >>>> @@ -0,0 +1,179 @@
> >>>> +// 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;
> >>>> +};
> >>>> +
> >>>> +/* Define variants: stream and datagram */
> >>> nit: outdated, maybe simply remove ?
> >> oh..skipped me.
> >> will do so.
> >>>> +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) {
> >>> patchwork caught mis-alignment here and other places.
> >>>
> >>> I'm using this for emacs, and other editors will have a similar config.
> >>>
> >>> (setq-default c-default-style "linux")
> >>>
> >>> You can check if lines are aligned properly by
> >>>
> >>> $ git show --format=email | ./scripts/checkpatch.pl
> >>>
> >>>
> >>>> + err = listen(self->server, 1);
> >>>> + ASSERT_EQ(0, err);
> >>>> +
> >>>> + self->client = socket(AF_UNIX, variant->socket_type, 0);
> >>> Could you add SOCK_NONBLOCK here too ?
> >> This is noted
> >>>> + ASSERT_LT(-1, self->client);
> >>>> +
> >>>> + err = connect(self->client, (struct sockaddr *)&addr, sizeof(addr));
> >>>> + ASSERT_EQ(0, err);
> >>>> +
> >>>> + self->child = accept(self->server, NULL, NULL);
> >>>> + ASSERT_LT(-1, self->child);
> >>>> + } else {
> >>>> + /* Datagram: bind and connect only */
> >>>> + self->client = socket(AF_UNIX, SOCK_DGRAM | 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;
> >>>> +
> >>>> + /* Peer closes normally */
> >>>> + if (variant->socket_type == SOCK_STREAM ||
> >>>> + variant->socket_type == SOCK_SEQPACKET)
> >>>> + close(self->child);
> >>>> + else
> >>>> + close(self->server);
> >>>> +
> >>>> + n = recv(self->client, buf, sizeof(buf), 0);
> >>>> + TH_LOG("%s: recv=%zd errno=%d (%s)", variant->name, n, errno, strerror(errno));
> >>> errno is undefined if not set, and same for strerror(errno).
> >>>
> >>> Also, if ASSERT_XXX() below fails, the same information
> >>> (test case, errno) is logged. So, TH_LOG() seems unnecessary.
> >>>
> >>> Maybe try modifying the condition below and see how the
> >>> assertion is logged.
> >> Oh..thank you. Didn't it through that way.
> >> I understand.
> >> I will remove the TH_LOG()'s
> >>>> + 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)
> >>>> +{
> >>>> + char buf[16] = {};
> >>>> + ssize_t n;
> >>>> +
> >>>> + /* Send data that will remain unread by client */
> >>>> + send(self->client, "hello", 5, 0);
> >>>> + close(self->child);
> >>>> +
> >>>> + n = recv(self->client, buf, sizeof(buf), 0);
> >>>> + TH_LOG("%s: recv=%zd errno=%d (%s)", variant->name, n, errno, strerror(errno));
> >>>> + if (variant->socket_type == SOCK_STREAM ||
> >>>> + variant->socket_type == SOCK_SEQPACKET) {
> >>>> + ASSERT_EQ(-1, n);
> >>>> + ASSERT_EQ(ECONNRESET, errno);
> >>>> + } else {
> >>>> + ASSERT_EQ(-1, n);
> >>>> + ASSERT_EQ(EAGAIN, errno);
> >>>> + }
> >>>> +}
> >>>> +
> >>>> +/* Test 3: SOCK_DGRAM peer close */
> >>>> Now Test 2 and Test 3 look identical ;)
> >> seems so, but the only difference is:
> >>
> >> close(self->child); is used in Test 2, while
> >> close(self->server); is used in Test 3.
> >> Maybe I should find a way to collapse Tests 2 and 3 (if statement might
> >> work)
> >>
> >> I am just afraid the tests to run will reduce to 6 from 9 and we will have 6
> >> cases passed as against 7 as before.
> >>
> >> What do you think?
> > The name of Test 2 is a bit confusing, which is not true
> > for SOCK_DGRAM. So, I'd use "if" to change which fd
> > to close() depending on the socket type.
> >
> > Also, close(self->server) does nothing for SOCK_STREAM
> > and SOCK_SEQPACKET after accept(). Rather, that close()
> > should have the same effect if self->child is not accept()ed.
> > (In this case, Skip() for SOCK_DGRAM makes sense)
> >
> > I think covering that scenario would be nicer.
> >
> > If interested, you can check the test coverage with this patch.
> > https://lore.kernel.org/linux-kselftest/20251028024339.2028774-1-kuniyu@google.com/
> >
> > Thanks!
> Thank you!
>
> kindly check these if any conforms to what it should be:
>
> TEST_F(unix_sock, reset_unread_behavior)
> {
> char buf[16] = {};
> ssize_t n;
>
> /* Send data that will remain unread by client */
> send(self->client, "hello", 5, 0);
>
> if (variant->socket_type == SOCK_DGRAM) {
> close(self->server);
> }
> else {
> if (!self->child)
> SKIP(return);
>
> 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)
> do { ASSERT_EQ(ECONNRESET, errno); } while (0);
> else
> ASSERT_EQ(EAGAIN, errno);
> }
>
> OR
>
> TEST_F(unix_sock, reset_unread_behavior)
> {
> char buf[16] = {};
> ssize_t n;
>
> /* Send data that will remain unread by client */
> send(self->client, "hello", 5, 0);
>
> if (variant->socket_type == SOCK_DGRAM) {
> close(self->server);
> }
> else {
> if (self->child)
> close(self->child);
> else
> close(self->server);
> }
>
> n = recv(self->client, buf, sizeof(buf), 0);
>
> ASSERT_EQ(-1, n);
>
> if (variant->socket_type == SOCK_STREAM ||
> variant->socket_type == SOCK_SEQPACKET)
> do { ASSERT_EQ(ECONNRESET, errno); } while (0);
> else
> ASSERT_EQ(EAGAIN, errno);
> }
>
> OR
>
> is there a better way to handle this?

Sorry for late!

What I had in mind is to move accept() in FIXTURE_SETUP() to
Test 1 & 2 (then, only listen() is conditional in FIXTURE_SETUP())
and rewrite Test 3 to cover the last ECONNRESET case caused by
close()ing un-accept()ed sockets:

TEST_F(unix_sock, reset_closed_embryo)
{
if (variant->socket_type == SOCK_DGRAM)
SKIP(return, "This test only applies to SOCK_STREAM and SOCK_SEQPACKET");

close(self->server);

n = recv(self->client, ...);
ASSERT_EQ(-1, n);
ASSERT_EQ(ECONNRESET, errno);
}

>
> I ran the KCOV_OUTPUT command using the first *TEST_F above* as the Test
> 2 and got the output below:
> *$ KCOV_OUTPUT=kcov KCOV_SLOTS=8192
> ./tools/testing/selftests/net/af_unix/unix_connreset*

You should be able to see line-by-line coverage by decoding
files under kcov with addr2line or vock/report.py.

Thanks!