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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Fri, 8 Jan 2021 21:30:53 +0100
Message-ID: <CAK8P3a3FakV-Y9xkoy_fpYKBNkMvcO7DPOQC8R7ku7yPcgDw3g@mail.gmail.com>
Subject: Re: [PATCH v3] kasan: remove redundant config option
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     Walter Wu <walter-zh.wu@mediatek.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        wsd_upstream <wsd_upstream@mediatek.com>,
        "moderated list:ARM/Mediatek SoC..." 
        <linux-mediatek@lists.infradead.org>
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On Fri, Jan 8, 2021 at 7:56 PM Andrey Konovalov <andreyknvl@google.com> wrote:
> On Fri, Jan 8, 2021 at 5:09 AM Walter Wu <walter-zh.wu@mediatek.com> wrote:

> > @@ -2,6 +2,12 @@
> >  CFLAGS_KASAN_NOSANITIZE := -fno-builtin
> >  KASAN_SHADOW_OFFSET ?= $(CONFIG_KASAN_SHADOW_OFFSET)
> >
> > +ifdef CONFIG_KASAN_STACK
> > +       stack_enable := 1
> > +else
> > +       stack_enable := 0
> > +endif
> > +
>
> AFAIR, Arnd wanted to avoid having KASAN_STACK to be enabled by
> default when compiling with Clang, since Clang instrumentation leads
> to very large kernel stacks, which, in turn, lead to compile-time
> warnings. What I don't remember is why there are two configs.
>
> Arnd, is that correct? What was the reason behind having two configs?

I think I just considered it cleaner than defining the extra variable in the
Makefile at the time, as this was the only place that referenced
CONFIG_KASAN_STACK.

The '#if CONFIG_KASAN_STACK' (rather than #ifdef) that got added
later do make my version more confusing though, so I agree that
Walter's second patch improves it.

Acked-by: Arnd Bergmann <arnd@arndb.de>

On a related note: do you have any hope that clang will ever fix
https://bugs.llvm.org/show_bug.cgi?id=38809 and KASAN_STACK
can be enabled by default on clang without risking stack
overflows?

       Arnd