Re: Undefined behavior detected by new clang's ubsan

On Thu, Jan 22, 2026 at 12:14 PM John Naylor <johncnaylorls(at)gmail(dot)com> wrote:
>
> On Thu, Jan 22, 2026 at 3:00 AM Alexander Lakhin <exclusion(at)gmail(dot)com> wrote:
> > Unfortunately, no -- I still got:
> > heaptoast.c:771:17: runtime error: addition of unsigned offset to 0x78120673fac6 overflowed to 0x78120673fa04
> > SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior heaptoast.c:771:17
>
> Okay, I'll refrain from guessing more then. I'm due for an OS upgrade
> anyway, and that'll have clang 21, so I'll come back to this one
> unless someone beats me to it.

Small update: With clang 21 I've confirmed the behavior Alexander
found, and confirmed my v1 patches fix the warnings except for the one
in heap_fetch_toast_slice() that I skipped over. I'd like to
understand that one better, in case a more principled fix can be
found.

--
John Naylor
Amazon Web Services