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

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Message-Id: <20251028132605.2926d3ef5eb6ea60d22ceffe@linux-foundation.org>
Date: Tue, 28 Oct 2025 13:26:05 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Ankit Khushwaha <ankitkhushwaha.linux@...il.com>
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 Tue, 28 Oct 2025 22:28:10 +0530 Ankit Khushwaha <ankitkhushwaha.linux@...il.com> wrote:

> > > @@ -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.

It's the second argument to write(2)!  write(2) expects a const char *,
but void* will work.