linux-kernel - Re: [PATCH] [RESEND v2] structleak: disable STRUCTLEAK_BYREF in combination with KASAN

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Date:   Mon, 22 Jul 2019 16:26:40 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Dmitriy Vyukov <dvyukov@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [RESEND v2] structleak: disable STRUCTLEAK_BYREF in
 combination with KASAN_STACK

On Mon, Jul 22, 2019 at 3:43 PM Alexander Potapenko <glider@...gle.com> wrote:
> On Mon, Jul 22, 2019 at 1:41 PM Arnd Bergmann <arnd@...db.de> wrote:
> >
> > KASAN_STACK is currently implied by KASAN on gcc, but could be made a
> > user selectable option if we want to allow combining (non-stack) KASAN
> > with GCC_PLUGIN_STRUCTLEAK_BYREF.
> >
> > Note that it would be possible to specifically address the files that
> > print the warning, but presumably the overall stack usage is still
> > significantly higher than in other configurations, so this would not
> > address the full problem.
> >
> > I could not test this with CONFIG_INIT_STACK_ALL, which may or may not
> > suffer from a similar problem.
> We would love to be able to run KASAN together with
> CONFIG_INIT_STACK_ALL on syzbot, as this will potentially reduce the
> number of flaky errors.

Doesn't that just limit the usefulness of KASAN, as you no longer catch
actual accesses to unintialized variables that KASAN is designed to find?

> Given that we already increase the stack size in KASAN builds, how big
> of a problem are these warnings?
> Perhaps it's better to disable them in this configuration, or push the limit up?

I'm really hoping to lower the per-function limit for 'allmodconfig' builds,
since both a high limit and lots of bogus warnings prevent us from
noticing any newly introduced functions that use a lot of kernel stack
without KASAN.

An allmodconfig build (and ideally also any randconfig build) should always
complete without warnings to be useful for compile testing.

       Arnd