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

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Message-ID: <aAKrq2InExQk7f_k@dell-precision-5540>
Date: Fri, 18 Apr 2025 15:44:43 -0400
From: Ben Wolsieffer <ben.wolsieffer@...ring.com>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: Naresh Kamboju <naresh.kamboju@...aro.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Christian Schrrefl <chrisi.schrefl@...il.com>,
	Russell King <rmk+kernel@...linux.org.uk>,
	Rudraksha Gupta <guptarud@...il.com>,
	Alice Ryhl <aliceryhl@...gle.com>, Ard Biesheuvel <ardb@...nel.org>,
	anders.roxell@...aro.org, arnd@...db.de, dan.carpenter@...aro.org,
	laura.nao@...labora.com, linux-kernel@...r.kernel.org,
	lkft-triage@...ts.linaro.org, regressions@...ts.linux.dev,
	rust-for-linux@...r.kernel.org, torvalds@...ux-foundation.org,
	Nick Clifton <nickc@...hat.com>,
	Richard Earnshaw <richard.earnshaw@....com>,
	Ramana Radhakrishnan <ramanara@...dia.com>
Subject: Re: Build: arm rustgcc unknown argument '-mno-fdpic'

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@...aro.org> wrote:
> >
> > On Tue, 8 Apr 2025 at 00:07, Miguel Ojeda <ojeda@...nel.org> wrote:
> > >
> > > On Mon, 07 Apr 2025 22:58:02 +0530 Naresh Kamboju <naresh.kamboju@...aro.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