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

Walter Wu
2020-05-13T01:48:07Z

On Tue, 2020-05-12 at 16:03 +0200, Dmitry Vyukov wrote:
> 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;
> };
>

When I see above struct kasan_free_meta, I know why you don't understand
my meaning, because I thought you were going to overlay the
quarantine_link by free_track, but it seems like to add free_track to
kasan_free_meta. Does it enlarge meta-data size?

> 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.

I see.