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

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Message-ID: <d943a735-742c-49ed-bac0-f5e186fc64b2@de.bosch.com>
Date: Wed, 7 Jan 2026 06:31:14 +0100
From: Dirk Behme <dirk.behme@...bosch.com>
To: Yury Norov <yury.norov@...il.com>, Alice Ryhl <aliceryhl@...gle.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 06/01/2026 18:38, Yury Norov wrote:
> On Tue, Jan 06, 2026 at 10:03:10AM +0100, Alice Ryhl wrote:
>> 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.
> 
> Thanks, Alice! Added in -next for testing. I'm going to send PR with the
> next -rc as it's a real build fix.
> 
> Dirk and everyone, please send your tags before the end of the week, if
> you want.

I see that in -next the mentioned typo is fixed. With that:

Reviewed-by: Dirk Behme <dirk.behme@...bosch.com>

Thanks!

Dirk