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

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Date:   Tue, 23 Jan 2018 12:24:54 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com
Cc:     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 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?

> 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.