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

[Luiz Capitulino <lcapitulino@redhat.com>]

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.

> Perhaps the checker can be shut up by making expr_error() QEMU_NORETURN.

That's indeed a better solution.