Quoting Markus Armbruster (2014-05-15 11:13:09)
> Marcel Apfelbaum <marce...@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 <ebl...@redhat.com>
> > Signed-off-by: Marcel Apfelbaum <marce...@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.
