This happens on 64-bit big-endian machines.
SO_RCVLOWAT requires an int parameter. However, instead of int, the test
uses unsigned long in one place and size_t in another. Both are 8 bytes
long on 64-bit machines. The kernel, having received the 8 bytes, doesn't
test for the exact size of the parameter, it only cares that it's >=
sizeof(int), and casts the 4 lower-addressed bytes to an int, which, on
a big-endian machine, contains 0. 0 doesn't trigger an error, SO_RCVLOWAT
returns with success and the socket stays with the default SO_RCVLOWAT = 1,
which results in test failures.

Fixes: b1346338fbae ("vsock_test: POLLIN + SO_RCVLOWAT test")
Fixes: 542e893fbadc ("vsock/test: two tests to check credit update logic")
Signed-off-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
---

Notes:
    The problem was found on s390 (big endian), while x86-64 didn't show it. After this fix, all tests pass on s390.
Changes for v2:
- add "Fixes:" lines to the commit message

 tools/testing/vsock/vsock_test.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c
index 8d38dbf8f41f..7fd25b814b4b 100644
--- a/tools/testing/vsock/vsock_test.c
+++ b/tools/testing/vsock/vsock_test.c
@@ -835,7 +835,7 @@ static void test_stream_poll_rcvlowat_server(const struct test_opts *opts)
 
 static void test_stream_poll_rcvlowat_client(const struct test_opts *opts)
 {
-	unsigned long lowat_val = RCVLOWAT_BUF_SIZE;
+	int lowat_val = RCVLOWAT_BUF_SIZE;
 	char buf[RCVLOWAT_BUF_SIZE];
 	struct pollfd fds;
 	short poll_flags;
@@ -1357,7 +1357,7 @@ static void test_stream_rcvlowat_def_cred_upd_client(const struct test_opts *opt
 static void test_stream_credit_update_test(const struct test_opts *opts,
 					   bool low_rx_bytes_test)
 {
-	size_t recv_buf_size;
+	int recv_buf_size;
 	struct pollfd fds;
 	size_t buf_size;
 	void *buf;
-- 
2.34.1

On Thu, Oct 24, 2024 at 11:10:58AM -0500, Konstantin Shkolnyy wrote:
>This happens on 64-bit big-endian machines.
>SO_RCVLOWAT requires an int parameter. However, instead of int, the test
>uses unsigned long in one place and size_t in another. Both are 8 bytes
>long on 64-bit machines. The kernel, having received the 8 bytes, doesn't
>test for the exact size of the parameter, it only cares that it's >=
>sizeof(int), and casts the 4 lower-addressed bytes to an int, which, on
>a big-endian machine, contains 0. 0 doesn't trigger an error, SO_RCVLOWAT
>returns with success and the socket stays with the default SO_RCVLOWAT = 1,
>which results in test failures.
>
>Fixes: b1346338fbae ("vsock_test: POLLIN + SO_RCVLOWAT test")
>Fixes: 542e893fbadc ("vsock/test: two tests to check credit update logic")
>Signed-off-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
>---
>
>Notes:
>    The problem was found on s390 (big endian), while x86-64 didn't show it. After this fix, all tests pass on s390.
>Changes for v2:
>- add "Fixes:" lines to the commit message

LGTM!

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

>
> tools/testing/vsock/vsock_test.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
>diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c
>index 8d38dbf8f41f..7fd25b814b4b 100644
>--- a/tools/testing/vsock/vsock_test.c
>+++ b/tools/testing/vsock/vsock_test.c
>@@ -835,7 +835,7 @@ static void test_stream_poll_rcvlowat_server(const struct test_opts *opts)
>
> static void test_stream_poll_rcvlowat_client(const struct test_opts *opts)
> {
>-	unsigned long lowat_val = RCVLOWAT_BUF_SIZE;
>+	int lowat_val = RCVLOWAT_BUF_SIZE;
> 	char buf[RCVLOWAT_BUF_SIZE];
> 	struct pollfd fds;
> 	short poll_flags;
>@@ -1357,7 +1357,7 @@ static void test_stream_rcvlowat_def_cred_upd_client(const struct test_opts *opt
> static void test_stream_credit_update_test(const struct test_opts *opts,
> 					   bool low_rx_bytes_test)
> {
>-	size_t recv_buf_size;
>+	int recv_buf_size;
> 	struct pollfd fds;
> 	size_t buf_size;
> 	void *buf;
>-- 
>2.34.1
>