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

[Markus Armbruster]

Michael Roth <address@hidden> writes:

> Quoting Markus Armbruster (2014-05-15 11:13:09)
>> Marcel Apfelbaum <address@hidden> 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 <address@hidden>
>> > Signed-off-by: Marcel Apfelbaum <address@hidden>
>> > ---
>> >  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.

> 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()?

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?