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

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Date:   Fri, 22 Apr 2022 10:40:08 -0700
From:   Peter Collingbourne <pcc@...gle.com>
To:     Andrey Konovalov <andreyknvl@...il.com>
Cc:     Hyeonggon Yoo <42.hyeyoo@...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 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.

Peter