linux-kernel - Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure

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Message-ID: <CAAeHK+y0aPAZ8zheD5vWFDR-9YCTR251i0F1pZ9QfXuiaW0r8w@mail.gmail.com>
Date:   Tue, 29 Sep 2020 15:48:27 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jonathan.Cameron@...wei.com, Jonathan Corbet <corbet@....net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, sjpark@...zon.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure

On Tue, Sep 29, 2020 at 3:11 PM Marco Elver <elver@...gle.com> wrote:
>
> On Tue, Sep 29, 2020 at 02:42PM +0200, Andrey Konovalov wrote:
> [...]
> > > +        */
> > > +       index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;
> >
> > Why do we subtract 1 here? We do have the metadata entry reserved for something?
>
> Above the declaration of __kfence_pool it says:
>
>         * We allocate an even number of pages, as it simplifies calculations to map
>         * address to metadata indices; effectively, the very first page serves as an
>         * extended guard page, but otherwise has no special purpose.
>
> Hopefully that clarifies the `- 1` here.

So there are two guard pages at the beginning and only then a page
that holds an object?