From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Dmitry Vyukov <dvyukov@google.com>,
Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Nicholas Piggin <npiggin@gmail.com>,
Linux-MM <linux-mm@kvack.org>,
Alexander Potapenko <glider@google.com>,
"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
Paul Mackerras <paulus@samba.org>,
kasan-dev <kasan-dev@googlegroups.com>,
linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v3 1/3] powerpc/mm: prepare kernel for KAsan on PPC32
Date: Tue, 15 Jan 2019 18:25:35 +0100 [thread overview]
Message-ID: <301f5826-64ab-1cf4-7e7e-cd026de77bca@c-s.fr> (raw)
In-Reply-To: <CACT4Y+ajgTdDyFFWCp0oRSPKZh9ercDpq3pAq2-ZSx7ouAvZYQ@mail.gmail.com>
Le 15/01/2019 à 18:10, Dmitry Vyukov a écrit :
> 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:
>>>>> >
>>>>> > In kernel/cputable.c, explicitly use memcpy() in order
>>>>> > to allow GCC to replace it with __memcpy() when KASAN is
>>>>> > selected.
>>>>> >
>>>>> > Since commit 400c47d81ca38 ("powerpc32: memset: only use dcbz once cache is
>>>>> > enabled"), memset() can be used before activation of the cache,
>>>>> > so no need to use memset_io() for zeroing the BSS.
>>>>> >
>>>>> > Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>>>>> > ---
>>>>> > arch/powerpc/kernel/cputable.c | 4 ++--
>>>>> > arch/powerpc/kernel/setup_32.c | 6 ++----
>>>>> > 2 files changed, 4 insertions(+), 6 deletions(-)
>>>>> >
>>>>> > diff --git a/arch/powerpc/kernel/cputable.c
>>>>> b/arch/powerpc/kernel/cputable.c
>>>>> > index 1eab54bc6ee9..84814c8d1bcb 100644
>>>>> > --- a/arch/powerpc/kernel/cputable.c
>>>>> > +++ b/arch/powerpc/kernel/cputable.c
>>>>> > @@ -2147,7 +2147,7 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
>>>>> > struct cpu_spec *t = &the_cpu_spec;
>>>>> >
>>>>> > t = PTRRELOC(t);
>>>>> > - *t = *s;
>>>>> > + 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.
This problem can affect any arch I believe. Maybe the 'solution' would
be to run a generic script similar to
arch/powerpc/kernel/prom_init_check.sh on all objects compiled with
KASAN_SANITIZE_object.o := n don't include any reference to memcpy()
memset() or memmove() ?
Christophe
next prev parent reply other threads:[~2019-01-15 17:27 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-12 11:16 [PATCH v3 0/3] KASAN for powerpc/32 Christophe Leroy
2019-01-12 11:16 ` [PATCH v3 1/3] powerpc/mm: prepare kernel for KAsan on PPC32 Christophe Leroy
2019-01-14 9:34 ` Dmitry Vyukov
2019-01-15 7:27 ` Christophe Leroy
2019-01-15 11:14 ` Dmitry Vyukov
2019-01-15 17:07 ` Andrey Ryabinin
2019-01-15 17:10 ` Dmitry Vyukov
2019-01-15 17:25 ` Christophe Leroy [this message]
2019-01-16 10:03 ` Dmitry Vyukov
2019-01-12 11:16 ` [PATCH v3 2/3] powerpc/32: Move early_init() in a separate file Christophe Leroy
2019-01-12 11:16 ` [PATCH v3 3/3] powerpc/32: Add KASAN support Christophe Leroy
2019-01-15 17:23 ` Andrey Ryabinin
2019-01-21 7:17 ` Christophe Leroy
2019-01-21 8:30 ` Dmitry Vyukov
2019-01-21 8:37 ` Christophe Leroy
2019-01-21 9:24 ` Dmitry Vyukov
2019-01-21 9:30 ` Christophe Leroy
2019-01-21 10:36 ` Christophe Leroy
2019-01-21 12:33 ` Dmitry Vyukov
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