Re: [PATCH] [RESEND v2] structleak: disable STRUCTLEAK_BYREF in combination with KASAN_STACK

From: Arnd Bergmann <arnd@arndb.de>
To: Alexander Potapenko <glider@google.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>,
	Marco Elver <elver@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	James Morris <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Kees Cook <keescook@chromium.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	linux-security-module <linux-security-module@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] [RESEND v2] structleak: disable STRUCTLEAK_BYREF in combination with KASAN_STACK
Date: Mon, 22 Jul 2019 16:26:40 +0200	[thread overview]
Message-ID: <CAK8P3a28c5V5VnsFrttgtCC5+i87yuAT-G5xx50KOsKUJ6-VKg@mail.gmail.com> (raw)
In-Reply-To: <CAG_fn=UxowACw5w+erKaAPRr4SWk3WbLTfAgJj=cOL4HgZHK=Q@mail.gmail.com>

On Mon, Jul 22, 2019 at 3:43 PM Alexander Potapenko <glider@google.com> wrote:
> On Mon, Jul 22, 2019 at 1:41 PM Arnd Bergmann <arnd@arndb.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