linux-kernel - Re: [PATCH v8 09/20] kasan: preassign tags to objects with ctors or SLAB_TYPESAFE_BY

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Date:   Mon, 24 Sep 2018 11:19:17 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Dave Martin <dave.martin@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Ingo Molnar <mingo@...nel.org>,
        Paul Lawrence <paullawrence@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-sparse@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        "open list:KERNEL BUILD + fi..." <linux-kbuild@...r.kernel.org>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Jann Horn <jannh@...gle.com>,
        Mark Brand <markbrand@...gle.com>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Vishwath Mohan <vishwath@...gle.com>
Subject: Re: [PATCH v8 09/20] kasan: preassign tags to objects with ctors or SLAB_TYPESAFE_BY_RCU

On Fri, Sep 21, 2018 at 2:24 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> On Fri, Sep 21, 2018 at 1:25 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> On Wed, Sep 19, 2018 at 8:54 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
>
>>>         if (!shuffle) {
>>>                 for_each_object_idx(p, idx, s, start, page->objects) {
>>> -                       setup_object(s, page, p);
>>> -                       if (likely(idx < page->objects))
>>> -                               set_freepointer(s, p, p + s->size);
>>> -                       else
>>> +                       if (likely(idx < page->objects)) {
>>> +                               next = p + s->size;
>>> +                               next = setup_object(s, page, next);
>>> +                               set_freepointer(s, p, next);
>>> +                       } else
>>>                                 set_freepointer(s, p, NULL);
>>>                 }
>>> -               page->freelist = fixup_red_left(s, start);
>>> +               start = fixup_red_left(s, start);
>>> +               start = setup_object(s, page, start);
>>> +               page->freelist = start;
>>>         }
>>
>> Just want to double-check that this is correct.
>> We now do an additional setup_object call after the loop, but we do 1
>> less in the loop. So total number of calls should be the same, right?
>> However, after the loop we call setup_object for the first object (?),
>> but inside of the loop we skip the call for the last object (?). Am I
>> missing something, or we call ctor twice for the last object and don't
>> call it for the first one?
>
> Inside the loop we call setup_object for the "next" object. So we
> start iterating on the first one, but call setup_object for the
> second. Then the loop moves on to the second one and calls
> setup_object for the third. And so on. So the loop calls setup_object
> for every object (including the last one) except for the first one.
>
> The idea is that we want the freelist pointer that is stored in the
> current object to have a tagged pointer to the next one, so we need to
> assign a tag to the next object before storing the pointer in the
> current one.

Ah, OK, then false alarm.