Re: [Qemu-devel] [PATCH] monitor: Fix warning from clang

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 17 Aug 2012 16:41:34 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> Luiz Capitulino <lcapitulino@redhat.com> writes:
> 
> > On Fri, 17 Aug 2012 16:10:12 +0200
> > Markus Armbruster <armbru@redhat.com> wrote:
> >
> >> Stefan Weil <sw@weilnetz.de> writes:
> >> 
> >> > ccc-analyzer reports these warnings:
> >> >
> >> > monitor.c:3532:21: warning: Division by zero
> >> >                 val %= val2;
> >> >                     ^
> >> > monitor.c:3530:21: warning: Division by zero
> >> >                 val /= val2;
> >> >                     ^
> >> >
> >> > Rewriting the code fixes this (and also a style issue).
> >> 
> >> I'm afraid this doesn't actually fix anything, because...
> >> 
> >> > Signed-off-by: Stefan Weil <sw@weilnetz.de>
> >> > ---
> >> >  monitor.c |    7 ++++---
> >> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >> >
> >> > diff --git a/monitor.c b/monitor.c
> >> > index 0c34934..0ea2c14 100644
> >> > --- a/monitor.c
> >> > +++ b/monitor.c
> >> > @@ -3524,12 +3524,13 @@ static int64_t expr_prod(Monitor *mon)
> >> >              break;
> >> >          case '/':
> >> >          case '%':
> >> > -            if (val2 == 0)
> >> > +            if (val2 == 0) {
> >> >                  expr_error(mon, "division by zero");
> >> > -            if (op == '/')
> >> > +            } else if (op == '/') {
> >> >                  val /= val2;
> >> > -            else
> >> > +            } else {
> >> >                  val %= val2;
> >> > +            }
> >> >              break;
> >> >          }
> >> >      }
> >> 
> >> ... expr_error() longjmp()s out.  The expression evaluator commonly
> >> exploits that.
> >
> > And that's correct. As far far I understood it's fixing clang, not qemu.
> >
> >> If expr_error() returned, the code would be just as wrong after your
> >> patch as before.
> >
> > Hmm, how? It checks for val2 == 0 first.
> 
> It would evaluate A % 0 into A, which is wrong.

Oh, you're talking about the result that would be returned by expr_prod().
I thought you were saying that val2 == 0 was still possible.

> 
> >> Perhaps the checker can be shut up by making expr_error() QEMU_NORETURN.
> >
> > That's indeed a better solution.
> 
> Stefan, could you try that for us?
>