Re: [PATCH] selftests/user_events: Avoid taking address of packed member in perf_test

Andrew Morton
2025-10-27T23:25:29Z

On Mon, 27 Oct 2025 17:04:39 +0530 Ankit Khushwaha <ankitkhushwaha.linux@gmail.com> wrote:

> Accessing 'reg.write_index' directly triggers a -Waddress-of-packed-member
> warning due to potential unaligned pointer access:
>
> perf_test.c:239:38: warning: taking address of packed member 'write_index'
> of class or structure 'user_reg' may result in an unaligned pointer value
> [-Waddress-of-packed-member]
> 239 | ASSERT_NE(-1, write(self->data_fd, &reg.write_index,
> | ^~~~~~~~~~~~~~~
>
> Use memcpy() instead to safely copy the value and avoid unaligned pointer
> access across architectures.
>
> ...
>
> --- a/tools/testing/selftests/user_events/perf_test.c
> +++ b/tools/testing/selftests/user_events/perf_test.c
> @@ -201,6 +201,7 @@ TEST_F(user, perf_empty_events) {
> struct perf_event_mmap_page *perf_page;
> int page_size = sysconf(_SC_PAGESIZE);
> int id, fd;
> + __u32 write_index;
> __u32 *val;
>
> reg.size = sizeof(reg);
> @@ -236,7 +237,8 @@ TEST_F(user, perf_empty_events) {
> ASSERT_EQ(1 << reg.enable_bit, self->check);
>
> /* Ensure write shows up at correct offset */
> - ASSERT_NE(-1, write(self->data_fd, &reg.write_index,
> + memcpy(&write_index, &reg.write_index, sizeof(reg.write_index));
> + ASSERT_NE(-1, write(self->data_fd, &write_index,
> sizeof(reg.write_index)));

Simply casting &write_index to void* would fix this?

> val = (void *)(((char *)perf_page) + perf_page->data_offset);
> ASSERT_EQ(PERF_RECORD_SAMPLE, *val);