Re: [PATCH v2 1/3] rcu/kasan: record and print call_rcu() call stack

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, May 12, 2020 at 5:38 AM Walter Wu <walter-zh.wu@mediatek.com> wrote:
> > Are you sure it will increase object size?
> > I think we overlap kasan_free_meta with the object as well. The only
> > case we don't overlap kasan_free_meta with the object are
> > SLAB_TYPESAFE_BY_RCU || cache->ctor. But these are rare and it should
> > only affect small objects with small redzones.
> > And I think now we simply have a bug for these objects, we check
> > KASAN_KMALLOC_FREE and then assume object contains free stack, but for
> > objects with ctor, they still contain live object data, we don't store
> > free stack in them.
> > Such objects can be both free and still contain user data.
> >
>
> Overlay kasan_free_meta. I see. but overlay it only when the object was
> freed. kasan_free_meta will be used until free object.
> 1). When put object into quarantine, it need kasan_free_meta.
> 2). When the object exit from quarantine, it need kasan_free_meta
>
> If we choose to overlay kasan_free_meta, then the free stack will be
> stored very late. It may has no free stack in report.

Sorry, I don't understand what you mean.

Why will it be stored too late?
In __kasan_slab_free() putting into quarantine and recording free
stack are literally adjacent lines of code:

static bool __kasan_slab_free(struct kmem_cache *cache, void *object,
      unsigned long ip, bool quarantine)
{
    ...
    kasan_set_free_info(cache, object, tag);
    quarantine_put(get_free_info(cache, object), cache);


Just to make sure, what I meant is that we add free_track to kasan_free_meta:

struct kasan_free_meta {
    struct qlist_node quarantine_link;
+  struct kasan_track free_track;
};

And I think its life-time and everything should be exactly what we need.

Also it should help to fix the problem with ctors: kasan_free_meta is
already allocated on the side for such objects, and that's exactly
what we need for objects with ctor's.