Re: BUG #16456: Implicit unsigned integer truncation at multixact.c:2626

Hello.

At Sat, 23 May 2020 14:21:38 +0000, PG Bug reporting form <noreply(at)postgresql(dot)org> wrote in
> REPRODUCTION && ERROR MESSAGE:
>
> compile the postgres project with UndefinedBehaviorSanitizer and start a
> start a server with "postgress" (command: "./postgres -Ddata -p23333"), and
> Ubsan reported an error message:

I'm not sure how the UndefinedBehaviorSanitizer works in details, but
the output of such kind of tools should need examiniations considering
the context of the code around.

> multixact.c:2626:19: runtime error: implicit conversion from type 'unsigned
> long' of value 18446744073709499264 (64-bit, unsigned) to type
> 'MultiXactOffset' (aka 'unsigned int') changed the value to 42949
> 14944 (32-bit, unsigned)
...
> ROOT CAUSE CODE
>
> The root cause code are shown bellow (multixact.c:2626). I believe that
> *offsetStopLimit* is a uint32 and *MULTIXACT_MEMBERS_PER_PAGE *
> SLRU_PAGES_PER_SEGMENT* cannot presented as uint32.

The variable is used in a wrap-arounded way. Although the
intermediate result tentatively becomes 64bits long because of size_t
contained in the expression, the result logically doesn't go beyond
32bit range and no bits of information are not lost through the down
conversion.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center