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

From: Mark Brown <broonie@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Andrey Konovalov <andreyknvl@google.com>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Michal Marek <michal.lkml@markovi.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dmitry Vyukov <dvyukov@google.com>, Qian Cai <cai@lca.pw>,
	Alexander Potapenko <glider@google.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Christoph Lameter <cl@linux.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Evgenii Stepanov <eugenis@google.com>
Subject: Re: [PATCH] kasan: turn off asan-stack for clang-8 and earlier
Date: Wed, 20 Feb 2019 18:44:08 +0000	[thread overview]
Message-ID: <20190220184408.GG9878@sirena.org.uk> (raw)
In-Reply-To: <CAKwvOd=XrNNb=r3qgsT8O6JNzL4hg4pqfJJFcY2DzA0Ooc=E3w@mail.gmail.com>

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On Wed, Feb 20, 2019 at 10:07:36AM -0800, Nick Desaulniers wrote:

> I like Evgenii's idea:
> https://bugs.llvm.org/show_bug.cgi?id=38809#c10

That's a suggestion to tune the inlining heuristics.

> While I myself share Arnd's goal of driving compiler warnings to zero,
> in general I'd prefer not to disable warning-producing-features or
> disable warnings outright for cases where we have some ideas of
> changes we can make to the compiler.  There's probably a list now of
> false warnings produced by old versions of Clang from bugs in Clang
> that we fixed.  I'm not interested in additionally trying to work
> around those somehow in kernel sources.

We do have infrastructure in the kernel for managing warnings based on
compiler version (Arnd was looking at some improvements to that IIRC),
if we've got a kernel that builds with a given compiler it's worth
looking at tuning what we do with that compiler.  If newer versions of
the compiler work better or have new options we can turn things on for
them.

> Qian previously pointed out that most drivers don't produce this
> warning under KASAN+Clang.  While 114 is a lot, what are the chances
> that someone NEEDS a KASAN+Clang build to compile warning free and
> happen to include one of these problematic drivers?  And if there is a
> chance they do observe the warning, are we doing a disservice by
> disabling the feature (-asan-stack=1) outright for the whole kernel,
> or disabling the warning (`-Wstack-frame-larger-than=`) which can flag
> issues unrelated to KASAN?

People doing treewide work and subsystem maintainers are a reasonably
important target for this sort of thing - for example people looking at
the kernelci output.  It's a lot easier to pay attention to problems if
you don't have to wade through large numbers of false positives.

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