Re: [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute

From: Florian Weimer <fweimer@redhat.com>
To: Marco Elver <elver@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	 Kees Cook <keescook@chromium.org>,
	 Guenter Roeck <linux@roeck-us.net>,
	 Peter Zijlstra <peterz@infradead.org>,
	 Mark Rutland <mark.rutland@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	 Marc Zyngier <maz@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	 James Morse <james.morse@arm.com>,
	 Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	 Catalin Marinas <catalin.marinas@arm.com>,
	 Will Deacon <will@kernel.org>,
	 Nathan Chancellor <nathan@kernel.org>,
	 Nick Desaulniers <ndesaulniers@google.com>,
	 Tom Rix <trix@redhat.com>,  Miguel Ojeda <ojeda@kernel.org>,
	 linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	 linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
	 Dmitry Vyukov <dvyukov@google.com>,
	 Alexander Potapenko <glider@google.com>,
	 kasan-dev@googlegroups.com, linux-toolchains@vger.kernel.org
Subject: Re: [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute
Date: Mon, 07 Aug 2023 14:36:53 +0200	[thread overview]
Message-ID: <87pm3zf2qi.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <CANpmjNN4h2+i3LUG__GHha849PZ3jK=mBoFQWpSz4jffXB4wrw@mail.gmail.com> (Marco Elver's message of "Mon, 7 Aug 2023 14:24:26 +0200")

* Marco Elver:

> Good idea. I had already created
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899, and we need
> better spec to proceed for GCC anyway.

Thanks for the reference.

>> Doesn't this change impact the kernel module ABI?
>>
>> I would really expect a check here
>>
>> > +#if __has_attribute(__preserve_most__)
>> > +# define __preserve_most notrace __attribute__((__preserve_most__))
>> > +#else
>> > +# define __preserve_most
>> > +#endif
>>
>> that this is not a compilation for a module.  Otherwise modules built
>> with a compiler with __preserve_most__ attribute support are
>> incompatible with kernels built with a compiler without that attribute.
>
> That's true, but is it a real problem? Isn't it known that trying to
> make kernel modules built for a kernel with a different config (incl.
> compiler) is not guaranteed to work? See IBT, CFI schemes, kernel
> sanitizers, etc?
>
> If we were to start trying to introduce some kind of minimal kernel to
> module ABI so that modules and kernels built with different toolchains
> keep working together, we'd need a mechanism to guarantee this minimal
> ABI or prohibit incompatible modules and kernels somehow. Is there a
> precedence for this somewhere?

I think the GCC vs Clang thing is expected to work today, isn't it?
Using the Clang-based BPF tools with a GCC-compiled kernel requires a
matching ABI.

The other things you listed result in fairly obvious breakage, sometimes
even module loading failures.  Unconditional crashes are possible as
well.  With __preserve_most__, the issues are much more subtle and may
only appear for some kernel/module compielr combinations and
optimization settings.  The impact of incorrectly clobbered registers
tends to be like that.

Thanks,
Florian

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