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

[Ard Biesheuvel]

On Fri, 29 Apr 2022 at 09:03, Fangrui Song <maskray at google.com> wrote:
>
> On 2022-04-28, Ard Biesheuvel wrote:
> >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?
>
> `#pragma GCC visibility push(hidden)` is very similar to
> -fvisibility=hidden -fdirect-access-external-data with Clang.
> In Clang there are only two differences:
>
>    // TLS initial-exec model with -fdirect-access-external-data;
>    // TLS local-exec model with `#pragma GCC visibility push(hidden)`
>    extern __thread int var;
>    int foo() { return var; }
>
>    // hidden visibility suppresses -fno-plt.
>    // -fdirect-access-external-data / GCC -mdirect-extern-access doesn't suppress -fno-plt.
>    extern int bar();
>    int foo() { return bar() + 2; }
>
>
> The kernel uses neither TLS nor -fno-plt, so -fvisibility=hidden
> -fdirect-access-external-data can replace `#pragma GCC visibility
> push(hidden)`.
>

OK. But you mentioned that GCC does not implement
-mdirect-extern-access for AArch64, right? So for now, the pragma is
the only portable option we have.

> >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.
>
> -ffreestanding doesn't mean there is no DSO. A libc implementation (e.g.
> musl) may use -ffreestanding to avoid libc dependencies from the host
> environment. It may ship several shared objects and export multiple symbols.
> Implied -fvisibility=hidden will get in the way.
>
> There is a merit to make options orthogonal.

Fair enough.