Re: Division by zero

[Ingo Schwarze <schwarze@usta.de>]

Hi Kristaps,

Kristaps Dzonsons wrote on Sun, Oct 19, 2014 at 11:02:27PM +0200:

> Enclosed is a patch to handle roff.c's division by zero.

Good point.

> This was found (indirectly) when playing with mandoc's new support
> for inline equations.
> 
> groff(1) seems to actually warn us when this happens--if we're to do
> the same, we'll need to pass the mparse function through to the
> offending function.

Makes sense to me.  Perhaps just hand the struct roff down into
roff_evalcond, roff_evalnum, and roff_evalpar?  That one is
already a usual argument for many functions, while struct mparse
is not.

I think the message should be an ERROR, not a WARNING, because it
can have arbitrarily severe consequences on formatting and content.

Two more nits:  You say "x / 0 = x".  Hum, maybe.  But groff prefers
a different lie:  "x / 0 = 0".  And operator2 is int, not float.
So i think you need something like

	case '/':
		if (operand2 == 0) {
			scream("WOLF!");
			*res = 0;
		} else
			*res /= operand2;
		break;

> By the way, upon an audit, I see that term_ps.c is vulnerable to
> division by zero if the "scale" parameter in termp_ps is zero.
> However, we don't ever seem to re-set that, so it's ok.

Seems true.

Yours,
  Ingo
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