Quoting Markus Armbruster (2014-05-15 12:19:08) > Michael Roth <mdr...@linux.vnet.ibm.com> writes: > > > 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. > > The other user of qmp_output_first() calls it like this: > > QObject *root = QTAILQ_EMPTY(&v->stack) ? NULL : qmp_output_first(v); > > Patching qmp_output_first() makes this check redundant. > > I suspect we should change both callers to test QTAILQ_EMPTY() instead.
Or remove the redundant check, don't have a strong preference either way, though personally I think qmp_output_first should handle it internally and just give us the (possibly NULL) QObject, since it exposes less internals to callers. > > > 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. > > Can't see visit_type_str() being called from object_property_get_str(). > Do you mean property_get_str()? Yah, I think it's something like: object_property_get_str->object_property_get->prop.get->property_get_str > > static void property_get_str(Object *obj, Visitor *v, void *opaque, > const char *name, Error **errp) > { > StringProperty *prop = opaque; > char *value; > > value = prop->get(obj, errp); > if (value) { > visit_type_str(v, &value, name, errp); > g_free(value); > } > } > > Why do we skip visit_type_str() when value is null? Not sure..., seems like an explicit fall through to the QERR_INVALID_PARAMETER_TYPE error in object_property_get_str that wasn't reachable prior to Marcel's patch (due to segfault), so I'm not sure this code path was in play until now.