linux-kernel - Re: [PATCH] kasan: turn off asan-stack for clang-8 and earlier

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Message-ID: <CAK8P3a0z7Y-uLgc8pWrdt712ic4R2nLbceg_zTCAC=ctG9LK9Q@mail.gmail.com>
Date:   Wed, 20 Feb 2019 22:40:29 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>, Qian Cai <cai@....pw>,
        Alexander Potapenko <glider@...gle.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Christoph Lameter <cl@...ux.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH] kasan: turn off asan-stack for clang-8 and earlier

On Wed, Feb 20, 2019 at 10:13 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> In the example in https://bugs.llvm.org/show_bug.cgi?id=38809#c12
> (https://godbolt.org/z/ylsGSQ) there is no inlining, yet clang uses
> over ten times as much stack space as gcc, for reasons I still
> can't explain. My assumption right now is that the underlying bug
> causes most of the problems with excessive stack usage in
> allmodconfig kernels.

Here is an even more minimal example:

struct s { int i[5]; } f(void);
void g(void) { f(); f();}

https://godbolt.org/z/d_KWkh

It's clear that clang does /something/ here when asan-stack=1 is
set, but I fail to see what it is, or why that is necessary.

The output of clang with asan-stack=0 is the expected
code, and basically identical to what gcc produces with or
without asan-stack.

      Arnd