Re: [RFC PATCH 2/2] arm64: kernel: switch to PIE code generation for relocatable kernels

From: Nick Desaulniers <ndesaulniers@google.com>
To: Fangrui Song <maskray@google.com>
Cc: Ard Biesheuvel <ardb@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	 clang-built-linux <clang-built-linux@googlegroups.com>,
	Will Deacon <will@kernel.org>,
	 Catalin Marinas <catalin.marinas@arm.com>,
	Kees Cook <keescook@chromium.org>,
	 Mark Rutland <mark.rutland@arm.com>,
	Nathan Chancellor <nathan@kernel.org>,
	 Sami Tolvanen <samitolvanen@google.com>
Subject: Re: [RFC PATCH 2/2] arm64: kernel: switch to PIE code generation for relocatable kernels
Date: Thu, 28 Apr 2022 11:53:28 -0700	[thread overview]
Message-ID: <CAKwvOdm_zysTbsMp8GmmrwB3MU9MyZU-jtenH7JpPTX7+ZODKQ@mail.gmail.com> (raw)
In-Reply-To: <20220428065742.rl3w5rz2ni2fhngl@google.com>

On Wed, Apr 27, 2022 at 11:57 PM Fangrui Song <maskray@google.com> wrote:
>
> On 2022-04-28, Ard Biesheuvel wrote:
> >On Thu, 28 Apr 2022 at 04:40, Fangrui Song <maskray@google.com> wrote:
> >>
> >> On 2022-04-27, Ard Biesheuvel wrote:
> >> >Fortunately, we can convince the compiler to handle this in a way that
> >> >is a bit more suitable for freestanding binaries such as the kernel, by
> >> >setting the 'hidden' visibility #pragma, which informs the compiler that
> >> >symbol preemption or CoW footprint are of no concern to us, and so
> >> >PC-relative references that are resolved at link time are perfectly
> >> >fine.
> >>
> >> Agree
> >>
> >
> >The only unfortunate thing is that -fvisibility=hidden does not give
> >us the behavior we want, and we are forced to use the #pragma instead.
>
> Right. For a very long time there had been no option controlling the
> access mode for undefined symbols (-fvisibility= is for defined
> symbols).
>
> I added -fdirect-access-external-data to Clang which supports
> many architectures (x86, aarch64, arm, riscv, ...).
> GCC's x86 port added -mdirect-extern-access in 2022-02 (not available on aarch64).
>
> The use of `#pragma GCC visibility push(hidden)` looks good as a
> portable solution.

Portable, sure, which is fine for now.

But there's just something about injecting a header into ever TU via
-include in order to set a pragma and that there's such pragmas
effecting codegen that makes my skin crawl.

Perhaps we can come up with a formal feature request for toolchain
vendors for an actual command line flag?

Does the pragma have the same effect as
`-fdirect-access-external-data`/`-mdirect-extern-access`, or would
this feature request look like yet another distinct flag?
-- 
Thanks,
~Nick Desaulniers

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