linux-kernel - Re: [PATCH v2] rust: bitops: fix missing _find

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Message-ID: <CAH5fLgjtXHH40Pcw=ZoOKPzvJzDA8fohNtC6W6DsfkcE-6vtrQ@mail.gmail.com>
Date: Tue, 6 Jan 2026 10:03:10 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Burak Emir <bqe@...gle.com>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] rust: bitops: fix missing _find_* functions on 32-bit ARM

On Mon, Jan 5, 2026 at 6:03 PM Yury Norov <yury.norov@...il.com> wrote:
>
> On Mon, Jan 05, 2026 at 10:44:06AM +0000, Alice Ryhl wrote:
> > atus: O
> > Content-Length: 4697
> > Lines: 121
> >
> > On 32-bit ARM, you may encounter linker errors such as this one:
> >
> >       ld.lld: error: undefined symbol: _find_next_zero_bit
> >       >>> referenced by rust_binder_main.43196037ba7bcee1-cgu.0
> >       >>>               drivers/android/binder/rust_binder_main.o:(<rust_binder_main::process::Process>::insert_or_update_handle) in archive vmlinux.a
> >       >>> referenced by rust_binder_main.43196037ba7bcee1-cgu.0
> >       >>>               drivers/android/binder/rust_binder_main.o:(<rust_binder_main::process::Process>::insert_or_update_handle) in archive vmlinux.a
> >
> > This error occurs because even though the functions are declared by
> > include/linux/find.h, the definition is #ifdef'd out on 32-bit ARM. This
> > is because arch/arm/include/asm/bitops.h contains:
> >
> >       #define find_first_zero_bit(p,sz)       _find_first_zero_bit_le(p,sz)
> >       #define find_next_zero_bit(p,sz,off)    _find_next_zero_bit_le(p,sz,off)
> >       #define find_first_bit(p,sz)            _find_first_bit_le(p,sz)
> >       #define find_next_bit(p,sz,off)         _find_next_bit_le(p,sz,off)
> >
> > And the underscore-prefixed function is conditional on #ifndef of the
> > non-underscore-prefixed name, but the declaration in find.h is *not*
> > conditional on that #ifndef.
> >
> > To fix the linker error, we ensure that the symbols in question exist
> > when compiling Rust code. We do this by definining them in rust/helpers/
> > whenever the normal definition is #ifndef'd out.
> >
> > Note that these helpers are somewhat unusual in that they do not have
> > the rust_helper_ prefix that most helpers have. Adding the rust_helper_
> > prefix does not compile, as 'bindings::_find_next_zero_bit()' will
> > result in a call to a symbol called _find_next_zero_bit as defined by
> > include/linux/find.h rather than a symbol with the rust_helper_ prefix.
> > This is because when a symbol is present in both include/ and
> > rust/helpers/, the one from include/ wins under the assumption that the
> > current configuration is one where that helper is unnecessary. This
> > heuristic fails for _find_next_zero_bit() because the header file always
> > declares it even if the symbol does not exist.
> >
> > The functions still use the __rust_helper annotation. This lets the
> > wrapper function be inlined into Rust code even if full kernel LTO is
> > not used once the patch series for that feature lands.
> >
> > Cc: stable@...r.kernel.org
> > Fixes: 6cf93a9ed39e ("rust: add bindings for bitops.h")
> > Reported-by: Andreas Hindborg <a.hindborg@...nel.org>
> > Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/x/topic/x/near/561677301
> > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
>
> Which means, you're running active testing, which in turn means that
> Rust is in a good shape indeed. Thanks to you and Andreas for the work.

I've put together this collection of GitHub actions jobs that build
and test a few common configurations, which I used to test this:
https://github.com/Darksonn/linux

> Before I merge it, can you also test m68k build? Arm and m68k are the
> only arches implementing custom API there.

I ran a gcc build for m68k with these patches applied and it built
successfully for me.

Alice