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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 13 May 2020 08:51:27 +0200
Message-ID: <CACT4Y+Zv3rCZs8z56NHM0hHWMwQr_2AT8nx0vUigzMG2v3Rt8Q@mail.gmail.com>
Subject: Re: [PATCH v2 1/3] rcu/kasan: record and print call_rcu() call stack
To:     Walter Wu <walter-zh.wu@mediatek.com>
Cc:     Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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On Wed, May 13, 2020 at 3:48 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?

I would assume it should not increase meta-data size. In both cases we
store exactly the same information inside of the object: quarantine
link and free track.
I see it more as a question of code organization. We already have a
concept of "this data is placed inside of the freed object", we
already have a name for it (kasan_free_meta), we already have code to
choose where to place it, we already have helper functions to access
it. And your change effectively duplicates all of this to place the
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.
>
> I see.