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

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Message-ID: <CANpmjNOTJsZeH_sMx=3XNZuvCih+A9m3uTeSGcmpNH9YbiF2sQ@mail.gmail.com>
Date:   Thu, 10 Sep 2020 19:41:05 +0200
From:   Marco Elver <elver@...gle.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mark Rutland <mark.rutland@....com>,
        Pekka Enberg <penberg@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>, Qian Cai <cai@....pw>,
        Thomas Gleixner <tglx@...utronix.de>,
        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-MM <linux-mm@...ck.org>
Subject: Re: [PATCH RFC 01/10] mm: add Kernel Electric-Fence infrastructure

On Thu, 10 Sep 2020 at 19:11, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> On Thu, Sep 10, 2020 at 6:19 PM Alexander Potapenko <glider@...gle.com> wrote:
> >
> > On Thu, Sep 10, 2020 at 5:43 PM Dmitry Vyukov <dvyukov@...gle.com> wrote:
> >
> >
> > > > +       /* Calculate address for this allocation. */
> > > > +       if (right)
> > > > +               meta->addr += PAGE_SIZE - size;
> > > > +       meta->addr = ALIGN_DOWN(meta->addr, cache->align);
> > >
> > > I would move this ALIGN_DOWN under the (right) if.
> > > Do I understand it correctly that it will work, but we expect it to do
> > > nothing for !right? If cache align is >PAGE_SIZE, nothing good will
> > > happen anyway, right?
> > > The previous 2 lines look like part of the same calculation -- "figure
> > > out the addr for the right case".
> >
> > Yes, makes sense.
> >
> > > > +
> > > > +       schedule_delayed_work(&kfence_timer, 0);
> > > > +       WRITE_ONCE(kfence_enabled, true);
> > >
> > > Can toggle_allocation_gate run before we set kfence_enabled? If yes,
> > > it can break. If not, it's still somewhat confusing.
> >
> > Correct, it should go after we enable KFENCE. We'll fix that in v2.
> >
> > > > +void __kfence_free(void *addr)
> > > > +{
> > > > +       struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
> > > > +
> > > > +       if (unlikely(meta->cache->flags & SLAB_TYPESAFE_BY_RCU))
> > >
> > > This may deserve a comment as to why we apply rcu on object level
> > > whereas SLAB_TYPESAFE_BY_RCU means slab level only.
> >
> > Sorry, what do you mean by "slab level"?
> > SLAB_TYPESAFE_BY_RCU means we have to wait for possible RCU accesses
> > in flight before freeing objects from that slab - that's basically
> > what we are doing here below:
>
> Exactly! You see it is confusing :)
> SLAB_TYPESAFE_BY_RCU does not mean that. rcu-freeing only applies to
> whole pages, that's what I mean by "slab level" (whole slabs are freed
> by rcu).

In the case here, we have to defer freeing the object, because unlike
real SLAB_TYPESAFE_BY_RCU slabs, our page here may get recycled for
other-typed objects. We can update the comment to be clearer.