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.