On Thu, Sep 03, 2009 at 10:24:17AM -0400, Tom Lane wrote:
> We have seen several previous reports of regression test failures
> due to division by zero causing SIGFPE, even though the code should
> never reach the division command:
>
> http://archives.postgresql.org/pgsql-bugs/2006-11/msg00180.php
> http://archives.postgresql.org/pgsql-bugs/2007-11/msg00032.php
> http://archives.postgresql.org/pgsql-bugs/2008-05/msg00148.php
> http://archives.postgresql.org/pgsql-general/2009-05/msg00774.php
>
> It's always been on non-mainstream architectures so it was hard to
> investigate. But I have finally been able to reproduce this:
> https://bugzilla.redhat.com/show_bug.cgi?id=520916
>
> While s390x is still not quite mainstream, at least I can get
> access to one ;-).
Do you also have access to z/OS with Unix System Services? IBM's
compiler, c89, is amazingly strict, and should help us flush out bugs. :)
> What turns out to be the case is that
> "simple" test cases like
> if (y == 0)
> single_function_call(...);
> z = x / y;
> do not show the problem; you need something pretty complex in the
> if-command. Like, say, an ereport() construct. So that's why the gcc
> boys haven't already had visits from mobs of villagers about this.
>
> I hope that the bug will get fixed in due course, but even if they
> respond pretty quickly it will be years before the problem disappears
> from every copy of gcc in the field. So I'm thinking that it would
> behoove us to install a workaround, now that we've characterized the
> problem sufficiently. What I am thinking is that in the three
> functions known to exhibit the bug (int24div, int28div, int48div)
> we should do something like this:
>
>
> if (arg2 == 0)
> + {
> ereport(ERROR,
> (errcode(ERRCODE_DIVISION_BY_ZERO),
> errmsg("division by zero")));
> + /* ensure compiler realizes we don't reach the division */
> + PG_RETURN_NULL();
> + }
> /* No overflow is possible */
> PG_RETURN_INT64((int64) arg1 / arg2);
>
> Thoughts?
How big would this change be? How would people know to use that
construct everywhere it's appropriate?
Cheers,
David.
--
David Fetter <da...@fetter.org> http://fetter.org/
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