Re: BUG #17839: Heap-buffer overflow on float8_to_char with invalid template

PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> Heap-buffer overflow on float8_to_char when format exceeds max double
> digits. I noticed this when running tests with memory sanitiser (msan).
> The following example triggers the failure (considering max double digits
> `DBL_DIG` is 15):
> float8_to_char(12345678901, "FM9999999999D999990")

Thanks for the report! For reasons that aren't entirely clear to me,
valgrind didn't whine about this until I added a few more zeroes to the
format string, like

SELECT to_char('12345678901'::float8, 'FM9999999999D9999900000000000000000');

Nonetheless, it did see the issue at that point. I'm inclined to fix
it by making the code that adjusts Np->last_relevant more paranoid,
as attached.

regards, tom lane