64-bit integer subtraction bug on some platforms

On Wed, 2023-11-08 at 11:58 +0000, Dean Rasheed wrote:

One of the new tests in the infinite interval patch has revealed a bug
in our 64-bit integer subtraction code. Consider the following:

select 0::int8 - '-9223372036854775808'::int8;

This should overflow, since the correct result (+9223372036854775808)
is out of range. However, on platforms without integer overflow
builtins or 128-bit integers, pg_sub_s64_overflow() does the
following:

if ((a < 0 && b > 0 && a < PG_INT64_MIN + b) ||
(a > 0 && b < 0 && a > PG_INT64_MAX + b))
{
*result = 0x5EED; /* to avoid spurious warnings */
return true;
}
*result = a - b;
return false;

which fails to spot the fact that overflow is also possible when a ==
0. So on such platforms, it returns the wrong result.

Patch attached.

The patch looks good to me.

Yours,
Laurenz Albe

Laurenz Albe <laurenz.albe@cybertec.at> writes:

On Wed, 2023-11-08 at 11:58 +0000, Dean Rasheed wrote:

This should overflow, since the correct result (+9223372036854775808)
is out of range. However, on platforms without integer overflow
builtins or 128-bit integers, pg_sub_s64_overflow() does the
following:
...
which fails to spot the fact that overflow is also possible when a ==
0. So on such platforms, it returns the wrong result.

Patch attached.

The patch looks good to me.

+1: good catch, fix looks correct.

regards, tom lane