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

[Fangrui Song <maskray@google.com>]

On 2022-04-28, Ard Biesheuvel wrote:
>On Thu, 28 Apr 2022 at 20:53, Nick Desaulniers <ndesaulniers@google.com> wrote:
>>
>> 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 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)`.

>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.

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