Re: Update on CPython Extension Module -fanalyzer plugin development

[Eric Feng <ef2648@columbia.edu>]

Hi Dave,

Thanks for the feedback!


On Wed, Aug 16, 2023 at 5:29 PM David Malcolm <dmalcolm@redhat.com> wrote:
>
> On Wed, 2023-08-16 at 15:17 -0400, Eric Feng via Gcc wrote:
> > Hi everyone,
>
> [fixing typo in my email address]
>
> Hi Eric, thanks for the update, and the WIP patch.
>
> >
> > After pushing the code that supports various known function classes last week,
> > I've turned my attention back to the core reference count checking
> > functionality. This functionality used to reside in region_model, which
> > wasn't ideal. To address this, I've introduced a hook to register callbacks
> > to pop_frame. Specifically, this allows the code that checks the reference
> > count and emits diagnostics to be housed within the plugin, rather than the
> > core analyzer.
> >
> > As of now, the parameters of pop_frame_callback are tailored specifically to
> > our needs. If the use of callbacks at the end of pop_frame becomes more
> > prevalent, we can revisit the setup to potentially make it more general.
> >
> > Moreover, the core reference count checking logic was previously somewhat
> > bloated, contained in one extensive function. I've since refactored it,
> > breaking it down into several helper functions to simplify and reduce
> > complexity. There are still some aspects that need refinement, especially
> > since the plugin has seen changes since I last worked on this logic. However,
> > I believe that there aren't any significant problems.
>
> Suggestion: introduce some more decls into analyzer-decls.h and
> known_functions for them into the plugin so that you can run/test/debug
> the helper functions independently (similar to the existing ones in kf-
> analyzer.cc).
>
> e.g.
>   extern void __analyzer_cpython_dump_real_refcounts (void);
>   extern void __analyzer_cpython_dump_ob_refcnt (void);
>
> >
Thanks for the suggestion. This will be even more helpful now that we
have split the logic into helper functions. I will look into these
when I come back to the "is there a refcount problem" side of the
equation.
> > Currently, I've started working a custom stmt_finder similar to leak_stmt_finder
> > to address the issue of m_stmt and m_stmt_finder being NULL at the time of
> > region_model::pop_frame. This approach was discussed as a viable solution in
> > a previous email, and I'll keep everyone posted on my progress. Afterwards, I
> > will go back to address the refinements necessary mentioned above.
>
> You might want to experiment with splitting out
> (a) "is there a refcount problem" from
> (b) "emit a refcount problem".
>
> For example, you could hardcode (a) to true, so we always complain with
> (b) on every heap-allocated object, just to debug the stmt_finder
> workaround.
>
>
> [...snip...]
>
> BTW, you don't need to bother to write ChangeLog entries if you're just
> sending a work-in-progress for me.
>
> > diff --git a/gcc/testsuite/gcc.dg/plugin/analyzer_cpython_plugin.c b/gcc/testsuite/gcc.dg/plugin/analyzer_cpython_plugin.c
> > index 7cd72e8a886..918bb5a5587 100644
> > --- a/gcc/testsuite/gcc.dg/plugin/analyzer_cpython_plugin.c
> > +++ b/gcc/testsuite/gcc.dg/plugin/analyzer_cpython_plugin.c
>
> [...]
>
> > +/* For PyListObjects: processes the ob_item field within the current region and
> > + * increments the reference count if conditions are met. */
> > +void
> > +process_ob_item_region (const region_model *model, region_model_manager *mgr,
> > +                       region_model_context *ctxt, const region *curr_region,
> > +                       const svalue *pylist_type_ptr, const region *base_reg,
> > +                       int &actual_refcnt)
>
> You seem to be special-casing PyListObject here; why?  That seems like
> it's not going to be scalable tothe general case.
>
> Am I right in thinking the intent of this code is to count the actual
> number of pointers in memory that point to a particular region?
>
> Doesn't the ob_item buffer show up in the store as another cluster?
> Can't you just look at the bindings in the clusters and tally up the
> pointers for each heap_allocated_region? (accumulating a result map
> from region to int of the actual reference counts).
Yes, you're correct, I hadn't thought of that ... This simplifies a
lot of things. Thanks for the great suggestion!
>
> Or am I missing something?
>
> What does
>   model->debug ();
> show in your examples?
>
>
> > +{
> > +  tree ob_item_field_tree = get_field_by_name (pylistobj_record, "ob_item");
> > +  const region *ob_item_field_reg
> > +      = mgr->get_field_region (curr_region, ob_item_field_tree);
> > +  const svalue *ob_item_ptr = model->get_store_value (ob_item_field_reg, ctxt);
> > +
> > +  if (const auto &cast_ob_item_reg = ob_item_ptr->dyn_cast_region_svalue ())
> > +    {
> > +      const region *ob_item_reg = cast_ob_item_reg->get_pointee ();
> > +      const svalue *allocated_bytes = model->get_dynamic_extents (ob_item_reg);
> > +      const region *ob_item_sized = mgr->get_sized_region (
> > +         ob_item_reg, pyobj_ptr_ptr, allocated_bytes);
> > +      const svalue *buffer_contents_sval
> > +         = model->get_store_value (ob_item_sized, ctxt);
> > +
> > +      if (const auto &buffer_contents
> > +         = buffer_contents_sval->dyn_cast_compound_svalue ())
> > +       {
> > +         for (const auto &buffer_content : buffer_contents->get_map ())
> > +           {
> > +                   const auto &content_value = buffer_content.second;
> > +                   if (const auto &content_region
> > +                       = content_value->dyn_cast_region_svalue ())
> > +                           if (content_region->get_pointee () == base_reg)
> > +                                   actual_refcnt++;
> > +           }
> > +       }
> > +    }
> > +}
> > +
> > +/* Counts the actual references from all clusters in the model's store. */
> > +int
> > +count_actual_references (const region_model *model, region_model_manager *mgr,
> > +                        region_model_context *ctxt, const region *base_reg,
> > +                        const svalue *pylist_type_ptr, tree ob_type_field)
> > +{
> > +  int actual_refcnt = 0;
> > +  for (const auto &other_cluster : *model->get_store ())
> > +    {
> > +      for (const auto &binding : other_cluster.second->get_map ())
> > +       {
> > +         const auto &sval = binding.second;
> > +         const auto &curr_region = sval->maybe_get_region ();
> > +
> > +         if (!curr_region || curr_region->get_kind () != RK_HEAP_ALLOCATED)
> > +           continue;
> > +
> > +         increment_count_if_base_matches (curr_region, base_reg,
> > +                                           actual_refcnt);
> > +
> > +         const region *ob_type_region
> > +             = mgr->get_field_region (curr_region, ob_type_field);
> > +         const svalue *stored_sval
> > +             = model->get_store_value (ob_type_region, ctxt);
> > +         const auto &remove_cast = stored_sval->dyn_cast_unaryop_svalue ();
> > +
> > +         if (!remove_cast)
> > +           continue;
> > +
> > +         const svalue *type = remove_cast->get_arg ();
> > +         if (type == pylist_type_ptr)
> > +           process_ob_item_region (model, mgr, ctxt, curr_region,
> > +                                   pylist_type_ptr, base_reg, actual_refcnt);
> > +       }
> > +    }
> > +  return actual_refcnt;
> > +}
> >
>
> Hope the above is constructive.
> Dave
>

Best,
Eric