Re: Detecting pointer misalignment (was Re: pgsql: Implementation of subscripting for jsonb)

Hi, Tom!

Thank you for taking care of this.

On Mon, Feb 8, 2021 at 3:47 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> [ redirecting to -hackers ]
>
> Alexander Korotkov <aekorotkov(at)gmail(dot)com> writes:
> >> BTW, I managed to reproduce the issue by compiling with CFLAGS="-O0
> >> -fsanitize=alignment -fsanitize-trap=alignment" and the patch
> >> attached.
> >> I can propose the following to catch such issues earlier. We could
> >> finish (wrap attribute with macro and apply it to other places with
> >> misalignment access if any) and apply the attached patch and make
> >> commitfest.cputube.org check patches with CFLAGS="-O0
> >> -fsanitize=alignment -fsanitize-trap=alignment". What do you think?
>
> > The revised patch is attached. The attribute is wrapped into
> > pg_attribute_no_sanitize_alignment() macro. I've checked it works for
> > me with gcc-10 and clang-11.
>
> I found some time to experiment with this today. It is really nice
> to be able to detect these problems without using obsolete hardware.
> However, I have a few issues:
>
> * Why do you recommend -O0? Seems to me we want to test the code
> as we'd normally use it, ie typically -O2.

My idea was that with -O0 we can see some unaligned accesses, which
would be optimized away with -O2. I mean with -O2 we might completely
skip accessing some pointer, which would be accessed in -O0. However,
this situation is probably very rare.

> * I think the right place to run such a check is in some buildfarm
> animals. The cfbot only sees portions of what goes into our tree.

Could we have both cfbot + buildfarm animals?

------
Regards,
Alexander Korotkov