Re: BUG #14722: Segfault in tuplesort_heap_siftup, 32 bit overflow

On 2017-07-05 18:03:56 -0400, Tom Lane wrote:
> Sergey Koposov <skoposov(at)cmu(dot)edu> writes:
> > On Thu, 2017-06-29 at 10:00 -0700, Peter Geoghegan wrote:
> >> This is an oversight in commit 263865a. The fix is to use a variable
> >> that won't overflow in tuplesort_heap_siftup() -- this is probably a
> >> one-liner, because when the variable overflows today, the correct
> >> behavior would be for control to break out of the loop that declares
> >> the overflowing variable "j", and, I don't see any similar problem in
> >> other heap maintenance routines. It's a very isolated problem.
> >>
> >> I could write a patch.
>
> > Just to avoid being forgotten, I attach a trivial patch against 9.5
> > branch as well as have created a commitfest submission
> > https://commitfest.postgresql.org/14/1189/
>
> I don't like s/int/int64/g as a fix for this. That loop is probably
> a hot spot, and this fix is going to be expensive on any machine where
> int64 isn't the native word width. How about something like this instead:
>
> - int j = 2 * i + 1;
> + int j;
>
> + if (unlikely(i > INT_MAX / 2))
> + break; /* if j would overflow, we're done */
> + j = 2 * i + 1;
> if (j >= n)
> break;

Isn't an added conditional likely going to be more costly than the
s/32/64/ bit calculations on the majority of machines pg runs on? I'm
quite doubtful that it's worth catering for the few cases where that's
really slow.

- Andres