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

[Ard Biesheuvel]

On Thu, 28 Apr 2022 at 20:53, Nick Desaulniers <ndesaulniers at google.com> wrote:
>
> On Wed, Apr 27, 2022 at 11:57 PM Fangrui Song <maskray at google.com> wrote:
> >
> > On 2022-04-28, Ard Biesheuvel wrote:
> > >On Thu, 28 Apr 2022 at 04:40, Fangrui Song <maskray at 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 wvisould
> this feature request look like yet another distinct flag?

I agree that this is rather nasty. What I don't understand is why
-fvisibility=hidden gives different behavior to begin with, or why
-ffreestanding -fpie builds don't default to hidden visibility for
symbol declarations as well as definitions.