Re: [Qemu-devel] [PATCH 2/4] qapi: output visitor crashes qemu if it encounters a NULL value

[Michael Roth <mdroth@linux.vnet.ibm.com>]

Quoting Markus Armbruster (2014-05-15 11:13:09)
> Marcel Apfelbaum <marcel.a@redhat.com> writes:
> 
> > 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.
> >
> > Reviewed-by: Eric Blake <eblake@redhat.com>
> > 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;
> > +    }
> > +
> >      return e->value;
> >  }
> 
> Let's see how this thing works.
> 
> The visitor's mutable state is a QStack, which is stack of (QObject,
> bool).  We can ignore the bool; it's just for qmp_output_next_list().
> 
> Visits start with an empty stack.  See qmp_output_visitor_new().
> 
> qmp_output_first() returns the object on the bottom of the stack.
> qmp_output_last() returns the object on the top of the stack.
> 
> <rant>
> When you implement a stack with a double-ended queue, you're totally
> free to pick either end of the queue for top of stack.  You're also free
> to name your functions accessing top and the bottom of the stack however
> you like.  "Of course" the author picked queue end and function names
> for maximum confusion:
> 
>     static QObject *qmp_output_first(QmpOutputVisitor *qov)
>     {
>         QStackEntry *e = QTAILQ_LAST(&qov->stack, QStack);
>         return e->value;
>     }
> 
>     static QObject *qmp_output_last(QmpOutputVisitor *qov)
>     {
>         QStackEntry *e = QTAILQ_FIRST(&qov->stack);
>         return e->value;
>     }
> 
> I hate you.
> </rant>
> 
> The result of the visit sits at the bottom of the stack.  Empty stack,
> null result.  See qmp_output_get_qobject().
> 
> Visiting a scalar type creates the appropriate scalar QObject, and
> "adds" it.  We'll find out what "adding" means shortly.  See
> qmp_output_type_{int,bool,str,number}().
> 
> Special case: null strings get converted to empty strings.  See
> qmp_output_type_str().
> 
> Starting a struct visit creates a QDict, adds it, and pushes it onto the
> stack.  Ending it pops it from the stack.  See
> qmp_output_{start,end}_struct().
> 
> Starting a list visit creates a QList, adds it, and pushes it onto the
> stack.  Ending it pops it from the stack.  See
> qmp_output_{start,end}_list().
> 
> Visiting a list member does nothing interesting; see
> qmp_output_next_list().  Aside: I suspect the GenericList traversal
> stuff now done in every next_list() method should be done in the visitor
> core instead.
> 
> Now let's figure out what it means to "add" an object.  This is
> qmp_output_add_obj().
> 
> If the stack is still empty, the object is the root object, and it gets
> pushed.
> 
> Else, if the object on top of the stack is a QDict, we're visiting a
> struct.  Enter the object into the QDict.
> 
> Else, if the object on top of the stack is a QList, we're visiting a
> list.  Append the object to the QList.
> 
> Else, the object on top of the stack must be scalar, and I think it must
> be the root object.  We replace it by the object being added.  WTF?
> 
> This feels more complicated than it could be.  Anyway, how could a null
> object end up at the bottom of the stack, so that qmp_output_first()
> chokes on it?  I can't see that.
> 
> If it can get added, then why can it be seen only by qmp_output_first(),
> but not by qmp_output_last() and qmp_output_pop()?

See my note above, the corner case we're hitting seems to be when there's
nothing in the stack at all: generating a QObject from an empty
QmpOutputVisitor.

This occurs with object_property_get_str skips visit_type_str if the
property-specific accessor returns NULL, but we still covert the
visitor to a QObject to pull the string out later.