Re: Build: arm rustgcc unknown argument '-mno-fdpic'

[Ben Wolsieffer <ben.wolsieffer@hefring.com>]

On Tue, Apr 15, 2025 at 05:38:54PM +0200, Miguel Ojeda wrote:
> On Tue, Apr 15, 2025 at 1:40 PM Naresh Kamboju
> <naresh.kamboju@linaro.org> wrote:
> >
> > On Tue, 8 Apr 2025 at 00:07, Miguel Ojeda <ojeda@kernel.org> wrote:
> > >
> > > On Mon, 07 Apr 2025 22:58:02 +0530 Naresh Kamboju <naresh.kamboju@linaro.org> wrote:
> > > >
> > > > Regressions on arm build with config rustgcc-lkftconfig-kselftest on the
> > > > Linux mainline and next failed with CONFIG_RUST=y enabled.
> > >
> > > > Bad: next-20250327
> > > > Good: next-20250326
> > >
> > > > Unable to generate bindings: clang diagnosed error: error: unknown
> > > > argument: '-mno-fdpic'
> > >
> > > I assume this is the arm support, i.e. commit ccb8ce526807 ("ARM: 9441/1:
> > > rust: Enable Rust support for ARMv7").
> > >
> > > Clang does not seem to support `-mno-fdpic`, thus you probably need to add it to
> > > `bindgen_skip_c_flags` in `rust/Makefile` so that it gets skipped when the C
> > > compiler is GCC.
> > >
> > > If you do so, please double-check if the flag could potentially alter the ABI in
> > > a way that `bindgen` would generate the wrong bindings.
> >
> > I tested this idea and it works but I don't know enough about
> > rust to double-check if the flag could potentially alter the
> > ABI in a way that `bindgen` would generate the wrong bindings.
> 
> Yeah, it would be nice to have someone knowledgeable about the
> arch/GCC/flag confirm or not -- I don't think Rust knowledge is
> needed, i.e. it is mostly about how GCC behaves with/without the flag
> and vs. Clang (since `bindgen` uses libclang to parse code).
> 
> The original commit adding it to arm (and similar ones for sh and xtensa) says:
> 
>     When building with an arm-*-uclinuxfdpiceabi toolchain, the FDPIC ABI is
>     enabled by default but should not be used to build the kernel.
> 
> So it sounds like it is only an issue for particular toolchains/targets anyway?
> 
> Cc'ing Ben who introduced it, and the arm port GCC maintainers in case
> they may be able to tell us more -- thanks in advance, and sorry for
> any potential noise!

FDPIC is only relevant with no-MMU targets, and then only for userspace.
When configured for the arm-*-uclinuxfdpiceabi target, GCC enables FDPIC
by default to facilitate compiling userspace programs. FDPIC is never
used for the kernel, and we pass -mno-fdpic when building the kernel to
override the default and make sure FDPIC is disabled.

Therefore, it is perfectly safe to omit that flag for kernel builds with
Clang.

Ben