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@sss.pgh.pa.us> wrote:
>
> [ redirecting to -hackers ]
>
> Alexander Korotkov <aekorotkov@gmail.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