linux-kernel - Re: [PATCH v2] kasan: don't emit builtin calls when sanitization is off

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Date:   Tue, 23 Jan 2018 17:34:23 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Pirama Arumuga Nainar <pirama@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] kasan: don't emit builtin calls when sanitization is off

On Tue, Jan 23, 2018 at 10:24 AM, Andrey Ryabinin
<aryabinin@...tuozzo.com> wrote:
>
>
> On 01/19/2018 08:58 PM, Andrey Konovalov wrote:
>> With KASAN enabled the kernel has two different memset() functions, one
>> with KASAN checks (memset) and one without (__memset). KASAN uses some
>> macro tricks to use the proper version where required. For example memset()
>> calls in mm/slub.c are without KASAN checks, since they operate on poisoned
>> slab object metadata.
>>
>> The issue is that clang emits memset() calls even when there is no memset()
>> in the source code. They get linked with improper memset() implementation
>> and the kernel fails to boot due to a huge amount of KASAN reports during
>> early boot stages.
>>
>
> So how did you observe this? Why am I not seeing this problem?

I only observed this when I tried to cross-compile the kernel for
arm64 with clang. I suspect this can happen on x86 as well.

>
>> The solution is to add -fno-builtin flag for files with KASAN_SANITIZE := n
>> marker.
>>
>
> I'm not sure I understand how is this solves the problem. And what clang does
> instead of memset()? Does it inlines memset()? But according to GCC's man (clang's man is very vague about this)
> -fno-builtin should do exactly the opposite  - prevent inlining builtin functions
> and always generate a function call.

The issue is that the compiler is allowed to replace plain assignment
with memset() calls. And -fno-builtin suppresses that behavior, but
there are no mentions of this in the documentation that I could find.
I also failed to find the exact place in the source code that leads to
this, perhaps Nick can help with that.