Re: Undefined behavior detected by new clang's ubsan

On Wed, Feb 04, 2026 at 06:15:11PM +0700, John Naylor wrote:
> I logged the offending statement and found
>
> curchunk: 50
> TOAST_MAX_CHUNK_SIZE: 1996
> sliceoffset: 99994
> chcpystrt 194
>
> The expression for the offset to the pointer:
>
> (curchunk * TOAST_MAX_CHUNK_SIZE - sliceoffset) + chcpystrt
>
> is zero by way of
>
> (-194) + 194
>
> ...so the "+ (-194)" must get cast to "+ (size_t) (-194)" causing the
> overflow. So the parentheses are indeed at fault by forcing
> precedence. The entire offset expression is never negative for our
> regression tests (nor should it ever be), so it's kind of pointless to
> cast any subset of this to a signed type. I went ahead and pushed 0001
> to all versions. Thanks Alexander, for the report and the patch!

Ah. I've seen this one, actually, while playing with the TOAST patch
set for 64-bit values. The max chunk size varies as this code would
need to switch between two possible sizes, and I've been super puzzled
by the fact that I had to switch the code to use an unsigned integer
for the declaration of my max_chunk_size, and a signed integer quickly
failed.

Thanks for the fix!
--
Michael