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@cmu.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