linux-kernel - Re: [PATCH] mm: make minimum slab alignment a runtime property

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Date:   Sun, 24 Apr 2022 14:16:58 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Peter Collingbourne <pcc@...gle.com>
Cc:     Andrey Konovalov <andreyknvl@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pekka Enberg <penberg@...nel.org>, cl@...ux.org,
        roman.gushchin@...ux.dev, Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] mm: make minimum slab alignment a runtime property

On Fri, Apr 22, 2022 at 10:40:08AM -0700, Peter Collingbourne wrote:
> On Fri, Apr 22, 2022 at 9:09 AM Andrey Konovalov <andreyknvl@...il.com> wrote:
> >
> > On Fri, Apr 22, 2022 at 2:39 PM Hyeonggon Yoo <42.hyeyoo@...il.com> wrote:
> > >
> > > > > kasan_hw_tags_enabled() is also false when kasan is just not initialized yet.
> > > > > What about writing a new helper something like kasan_is_disabled()
> > > > > instead?
> > > >
> > > > The decision of whether to enable KASAN is made early, before the slab
> > > > allocator is initialized (start_kernel -> smp_prepare_boot_cpu ->
> > > > kasan_init_hw_tags vs start_kernel -> mm_init -> kmem_cache_init). If
> > > > you think about it, this needs to be the case for KASAN to operate
> > > > correctly because it influences the behavior of the slab allocator via
> > > > the kasan_*poison* hooks. So I don't think we can end up calling this
> > > > function before then.
> > >
> > > Sounds not bad. I wanted to make sure the value of arch_slab_minaligned()
> > > is not changed during its execution.
> > >
> > > Just some part of me thought something like this would be more
> > > intuitive/robust.
> > >
> > > if (systems_supports_mte() && kasan_arg != KASAN_ARG_OFF)
> > >         return MTE_GRANULE_SIZE;
> > > else
> > >         return __alignof__(unsigned long long);
> >
> > Hi Hyeonggon,
> >
> > We could add and use kasan_hw_rags_requested(), which would return
> > (systems_supports_mte() && kasan_arg != KASAN_ARG_OFF).
> >
> > However, I'm not sure we will get a fully static behavior:
> > systems_supports_mte() also only starts returning proper result at
> > some point during CPU bring-up if I'm not mistaken.
> >
> > Thanks!
> 
> Yes, either way we are going to rely on something that hasn't
> obviously been initialized yet, so I think we should stick with what I
> have since it's used by the rest of the KASAN code as well.
>

Okay then we should anyway rely on something not initialized at early
stage of boot process.

And I don't expect much problem on current version.

Thanks!

> Peter

-- 
Thanks,
Hyeonggon