Re: [Qemu-devel] [PATCH] qapi: output visitor crashes qemu if it encounters a NULL value

[Marcel Apfelbaum <marcel.a@redhat.com>]

On Fri, 2014-02-28 at 11:37 -0500, Luiz Capitulino wrote:
> On Mon, 17 Feb 2014 20:01:42 +0200
> Marcel Apfelbaum <marcel.a@redhat.com> wrote:
> 
> > On Mon, 2014-02-17 at 10:38 -0700, Eric Blake wrote:
> > > On 02/17/2014 04:52 AM, Marcel Apfelbaum wrote:
> > > > A NULL value is not added to visitor's stack, but there
> > > > is no check for that when the visitor tries to return
> > > > that value, leading to Qemu crash.
> > > 
> > > Do you have an easy formula for reproducing the crash?
> > 
> > Hi Eric, thank you for your review!
> > 
> > In order to reproduce this you need to use object_property_get_str
> > on an object with a string property with a null value.
> 
> I tried looking into this but got a bit lost in the abstraction maze.
> Can you point me to your series and how to reproduce it?
Hi Luiz, thanks for the review!

I will send an RFC V2 soon and CC you to it. Basically if you have a string property
with a NULL value and you access it with obj_property_get_str, QEMU will crush.
You can see this in the series.

> 
> > I don't know if in the current code base we have this scenario, but
> > I am trying to QOMify the QemuMachine and properties as "kernel" may be NULL.
> > 
> > Either way (if NULL properties are not wanted), IMHO it is recommended to cover such cases in order to avoid QEMU crash. 
> 
> I agree, but I want to be sure we're fixing the right thing and not only
> papering over a bug. Also, Why is the check not needed in qmp_output_last()?
You lost me here, I debugged the above crush and this was the exact place the check
should be in order to avoid it. It is a "get" path.
> 
> Btw, sorry for the long delay.
No problem, I am going to send this as part of machine QOMify series.

Thanks,
Marcel

> > 
> > > 
> > > > 
> > > > Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
> > > > ---
> > > >  qapi/qmp-output-visitor.c | 5 +++++
> > > >  1 file changed, 5 insertions(+)
> > > > 
> > > > diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
> > > > index 74a5684..0562f49 100644
> > > > --- a/qapi/qmp-output-visitor.c
> > > > +++ b/qapi/qmp-output-visitor.c
> > > > @@ -66,6 +66,11 @@ static QObject *qmp_output_pop(QmpOutputVisitor *qov)
> > > >  static QObject *qmp_output_first(QmpOutputVisitor *qov)
> > > >  {
> > > >      QStackEntry *e = QTAILQ_LAST(&qov->stack, QStack);
> > > > +
> > > > +    if (!e) {
> > > > +        return NULL;
> > > > +    }
> > > > +
> > > 
> > > The code looks okay to me, but without a formula, my review is fairly weak:
> > Appreciated,
> > Marcel
> > 
> > > 
> > > Reviewed-by: Eric Blake <eblake@redhat.com>
> > > 
> > 
> > 
> > 
>