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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>,
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        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>,
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        Nick Desaulniers <ndesaulniers@google.com>,
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        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
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Subject: Re: [PATCH v2 1/3] compiler_types: Introduce the Clang
 __preserve_most function attribute
References: <20230804090621.400-1-elver@google.com>
        <87il9rgjvw.fsf@oldenburg.str.redhat.com>
        <CANpmjNN4h2+i3LUG__GHha849PZ3jK=mBoFQWpSz4jffXB4wrw@mail.gmail.com>
Date:   Mon, 07 Aug 2023 14:36:53 +0200
In-Reply-To: <CANpmjNN4h2+i3LUG__GHha849PZ3jK=mBoFQWpSz4jffXB4wrw@mail.gmail.com>
        (Marco Elver's message of "Mon, 7 Aug 2023 14:24:26 +0200")
Message-ID: <87pm3zf2qi.fsf@oldenburg.str.redhat.com>
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* 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