linux-kernel - Re: [PATCH] selftests/user_events: Avoid taking address of packed member in perf

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Message-ID: <aQD2Igc3svAF3klc@fedora>
Date: Tue, 28 Oct 2025 22:28:10 +0530
From: Ankit Khushwaha <ankitkhushwaha.linux@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kselftest@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
	Bala-Vignesh-Reddy <reddybalavignesh9979@...il.com>,
	Wei Yang <richard.weiyang@...il.com>, linux-kernel@...r.kernel.org,
	linux-kernel-mentees@...ts.linuxfoundation.org,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] selftests/user_events: Avoid taking address of packed
 member in perf_test

On Mon, Oct 27, 2025 at 04:25:21PM -0700, Andrew Morton wrote:
> On Mon, 27 Oct 2025 17:04:39 +0530 Ankit Khushwaha <ankitkhushwaha.linux@...il.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?

yes, this hides the type mismatch from the compiler. But i think
casting to void * will not fix the alignment mismatch for packed struct.
It works on x86, but might break on other platform.

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

Thanks
Ankit