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

[Ankit Khushwaha]

On Tue, Oct 28, 2025 at 01:26:05PM -0700, Andrew Morton wrote:
> On Tue, 28 Oct 2025 22:28:10 +0530 Ankit Khushwaha 
> <ankitkhushwaha.li...@gmail.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.

Hi Andrew,
Indeed 
`ASSERT_NE(-1, write(self->data_fd, (void *)&reg.write_index, 
                     sizeof(reg.write_index)));`

would work. However since `reg` is packed struct, directly taking the 
address of its member  `&reg.write_index` may lead to unaligned access 
on some architectures. as indicated by the compiler warning

        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,
            |                                             ^~~~~~~~~~~~~~~

Using `memcpy` avoids this by performing a byte-wise copy, which is safe 
to use for packed structures.

Thanks
-- Ankit