Re: Undefined behavior detected by new clang's ubsan

Hello John,

21.01.2026 12:05, John Naylor wrote:
> As for the rest of the proposed fixes, most seem okay, but I have some nits:

Thank you for spending time on this!
I agree with all of your changes (except for one noted below) -- didn't
mean to propose committable changes, just wanted to show the fixes that
allowed check-world to pass.

> heaptoast.c
> memcpy(VARDATA(result) +
> - (curchunk * TOAST_MAX_CHUNK_SIZE - sliceoffset) + chcpystrt,
> + (int)(curchunk * TOAST_MAX_CHUNK_SIZE - sliceoffset) + chcpystrt,
>
> Not sure about this one. It would be better if we reversing the
> operands allowed us to avoid overflow in the first place:
>
> - (curchunk * TOAST_MAX_CHUNK_SIZE - sliceoffset) + chcpystrt,
> + chcpystrt + (curchunk * TOAST_MAX_CHUNK_SIZE - sliceoffset)
>
> Does that silence the warning?

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

Best regards,
Alexander