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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Tue, 15 Jan 2019 18:10:51 +0100
Message-ID: <CACT4Y+ajgTdDyFFWCp0oRSPKZh9ercDpq3pAq2-ZSx7ouAvZYQ@mail.gmail.com>
Subject: Re: [PATCH v3 1/3] powerpc/mm: prepare kernel for KAsan on PPC32
To:     Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nicholas Piggin <npiggin@gmail.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Alexander Potapenko <glider@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linuxppc-dev@lists.ozlabs.org,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux-MM <linux-mm@kvack.org>
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On Tue, Jan 15, 2019 at 6:06 PM Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
>
>
>
> On 1/15/19 2:14 PM, Dmitry Vyukov wrote:
> > On Tue, Jan 15, 2019 at 8:27 AM Christophe Leroy
> > <christophe.leroy@c-s.fr> wrote:
> >> On 01/14/2019 09:34 AM, Dmitry Vyukov wrote:
> >>> On Sat, Jan 12, 2019 at 12:16 PM Christophe Leroy
> >>> <christophe.leroy@c-s.fr> wrote:
> >>> &gt;
> >>> &gt; In kernel/cputable.c, explicitly use memcpy() in order
> >>> &gt; to allow GCC to replace it with __memcpy() when KASAN is
> >>> &gt; selected.
> >>> &gt;
> >>> &gt; Since commit 400c47d81ca38 ("powerpc32: memset: only use dcbz once cache is
> >>> &gt; enabled"), memset() can be used before activation of the cache,
> >>> &gt; so no need to use memset_io() for zeroing the BSS.
> >>> &gt;
> >>> &gt; Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> >>> &gt; ---
> >>> &gt;  arch/powerpc/kernel/cputable.c | 4 ++--
> >>> &gt;  arch/powerpc/kernel/setup_32.c | 6 ++----
> >>> &gt;  2 files changed, 4 insertions(+), 6 deletions(-)
> >>> &gt;
> >>> &gt; diff --git a/arch/powerpc/kernel/cputable.c
> >>> b/arch/powerpc/kernel/cputable.c
> >>> &gt; index 1eab54bc6ee9..84814c8d1bcb 100644
> >>> &gt; --- a/arch/powerpc/kernel/cputable.c
> >>> &gt; +++ b/arch/powerpc/kernel/cputable.c
> >>> &gt; @@ -2147,7 +2147,7 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
> >>> &gt;         struct cpu_spec *t = &amp;the_cpu_spec;
> >>> &gt;
> >>> &gt;         t = PTRRELOC(t);
> >>> &gt; -       *t = *s;
> >>> &gt; +       memcpy(t, s, sizeof(*t));
> >>>
> >>> Hi Christophe,
> >>>
> >>> I understand why you are doing this, but this looks a bit fragile and
> >>> non-scalable. This may not work with the next version of compiler,
> >>> just different than yours version of compiler, clang, etc.
> >>
> >> My felling would be that this change makes it more solid.
> >>
> >> My understanding is that when you do *t = *s, the compiler can use
> >> whatever way it wants to do the copy.
> >> When you do memcpy(), you ensure it will do it that way and not another
> >> way, don't you ?
> >
> > It makes this single line more deterministic wrt code-gen (though,
> > strictly saying compiler can turn memcpy back into inlines
> > instructions, it knows memcpy semantics anyway).
> > But the problem I meant is that the set of places that are subject to
> > this problem is not deterministic. So if we go with this solution,
> > after this change it's in the status "works on your machine" and we
> > either need to commit to not using struct copies and zeroing
> > throughout kernel code or potentially have a long tail of other
> > similar cases, and since they can be triggered by another compiler
> > version, we may need to backport these changes to previous releases
> > too. Whereas if we would go with compiler flags, it would prevent the
> > problem in all current and future places and with other past/future
> > versions of compilers.
> >
>
> The patch will work for any compiler. The point of this patch is to make
> memcpy() visible to the preprocessor which will replace it with __memcpy().

For this single line, yes. But it does not mean that KASAN will work.

> After preprocessor's work, compiler will see just __memcpy() call here.