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

[Stefan Weil]

Am 17.08.2012 17:02, schrieb Luiz Capitulino:
> On Fri, 17 Aug 2012 16:41:34 +0200
> Markus Armbruster <address@hidden> wrote:
>
> > Luiz Capitulino <address@hidden> writes:
> >
> > > On Fri, 17 Aug 2012 16:10:12 +0200
> > > Markus Armbruster <address@hidden> wrote:
> > >
> > > > Stefan Weil <address@hidden> 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 <address@hidden>
> > > > > ---
> > > > >   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?


Adding QEMU_NORETURN to function expr_error also
fixes the warning from ccc-analyzer.

I'll send a patch series which adds this and some more
QEMU_NORETURN attributes.

What about using above patch in addition? IMHO it
improves readability, and it fixes the coding style.

Regards,

Stefan W.