linux-kernel - Re: [PATCH v2 1/3] rcu/kasan: record and print call

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Date:   Tue, 12 May 2020 16:03:06 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Walter Wu <walter-zh.wu@...iatek.com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 1/3] rcu/kasan: record and print call_rcu() call stack

On Tue, May 12, 2020 at 5:38 AM Walter Wu <walter-zh.wu@...iatek.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.