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

[Marcel Apfelbaum <marcel.a@redhat.com>]

On Wed, 2014-05-14 at 15:38 -0500, Michael Roth wrote:
> Quoting Luiz Capitulino (2014-05-14 13:25:16)
> > On Wed, 14 May 2014 20:29:37 +0300
> > Marcel Apfelbaum <marcel.a@redhat.com> wrote:
> > 
> > > On Wed, 2014-05-14 at 19:00 +0200, Andreas Färber wrote:
> > > > Am 13.05.2014 21:08, schrieb Eric Blake:
> > > > > On 05/13/2014 11:36 AM, Andreas Färber wrote:
> > > > >> Am 07.05.2014 16:42, schrieb Marcel Apfelbaum:
> > > > >>> 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>
> > > > >> 
> > > > >> Where does the Rb come from on this v1? Is it in any tree
> > > > >> already?
> > > > >> 
> > > > > 
> > > > > The (weak) R-b was here: 
> > > > > https://lists.gnu.org/archive/html/qemu-devel/2014-02/msg02861.html
> > > > 
> > > > Thanks.
> > > > > 
> > > > So Luiz was okay with it too, but his last message seems to be
> > > > indicating this needs to be fixed somewhere else, too:
> > > > 
> > > > https://lists.gnu.org/archive/html/qemu-devel/2014-02/msg05228.html
> > > > https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg00217.html
> > > > 
> > > > Can/should that be addressed as a follow-up? Or is there a test case
> > > > that breaks?
> > > Simple and "popular" test case: the user does not use the -kernel-cmdline parameter.
> > > The patch is needed because otherwise the main function will fail
> > > if no value is passed by the user to string parameters. 
> > > 
> > > Regarding Luiz's concern, it can be a follow-up as I am not aware of
> > > any problem with that.
> > 
> > My concern was that I wasn't sure if this is the right fix for the issue
> > or if it's papering over the real bug. I quickly checked the code and it
> > seemed to make sense, but I didn't have time to study it deeper.
> 
> Not sure the fix is bad or not, but the cause might be a little more subtle
> than NULL string values as mentioned in the other thread. QmpOutputVisitor
> encodes NULL strings as "" via qmp_output_type_str(), so the problem doesn't
> seem to lie there: it shouldn't generate NULL values on the stack.
> 
> I think the real issue is that object_property_get_str() actually calls an
> accessor via property_get_str to get the string, then explicitly *skips*
> the call to visit_type_str() if it is NULL (as it would be in the case of,
> say, kernel_cmdline option being NULL). So I wonder if maybe the real issue
> we're fixing is a corner case where you call qmp_output_get_qobject() on
> an "empty" QmpOutputVisitor.
> 
> Surprised that's not covered by tests, but didn't see any coverage doing
> a cursory glance. Actually, might as well just add one..
> 
> diff --git a/tests/test-qmp-output-visitor.c b/tests/test-qmp-output-visitor.c
> index e073d83..f190eaa 100644
> --- a/tests/test-qmp-output-visitor.c
> +++ b/tests/test-qmp-output-visitor.c
> @@ -434,6 +434,17 @@ static void test_visitor_out_union(TestOutputVisitorData *data,
>      QDECREF(qdict);
>  }
>  
> +static void test_visitor_out_empty(TestOutputVisitorData *data,
> +                                   const void *unused)
> +{
> +    QObject *arg;
> +    QDict *qdict;
> +
> +    arg = qmp_output_get_qobject(data->qov);
> +    qdict = qobject_to_qdict(arg);
> +    QDECREF(qdict);
> +}
> +
>  static void init_native_list(UserDefNativeListUnion *cvalue)
>  {
>      int i;
> @@ -782,6 +793,8 @@ int main(int argc, char **argv)
>                              &out_visitor_data, test_visitor_out_list_qapi_free);
>      output_visitor_test_add("/visitor/output/union",
>                              &out_visitor_data, test_visitor_out_union);
> +    output_visitor_test_add("/visitor/output/empty",
> +                            &out_visitor_data, test_visitor_out_empty);
>      output_visitor_test_add("/visitor/output/native_list/int",
>                              &out_visitor_data, test_visitor_out_native_list_int);
>      output_visitor_test_add("/visitor/output/native_list/int8",
> 
> mdroth@loki:~/w/qemu-build$ tests/test-qmp-output-visitor 
> /visitor/output/int: OK
> /visitor/output/bool: OK
> /visitor/output/number: OK
> /visitor/output/string: OK
> /visitor/output/no-string: OK
> /visitor/output/enum: OK
> /visitor/output/enum-errors: OK
> /visitor/output/struct: OK
> /visitor/output/struct-nested: OK
> /visitor/output/struct-errors: OK
> /visitor/output/list: OK
> /visitor/output/list-qapi-free: OK
> /visitor/output/union: OK
> /visitor/output/empty: Segmentation fault (core dumped)
> 
> So I guess the question is whether we should support converting an empty
> QmpOutputVisitor to a QObject. I would say yes, and that a NULL value is
> probably the most reasonable value.
> 
> I would ask that commit/code is a little more explicit about what corner case
> is being handled though, and that something like the above unit test be
> included with the series.
Hi Michael,

Thanks for the test, of course I'll add it, may I add your Signed-off-by?
Marcel

> 
> > 
> > We could ask Michael Roth or Anthony, but I wouldn't hold this series
> > because of that. Here's my ACK if you need it:
> > 
> > Acked-by: Luiz Capitulino <lcapitulino@redhat.com>