Re: [PATCH v2] rust: bitops: fix missing _find_* functions on 32-bit ARM

[Yury Norov <yury.norov@gmail.com>]

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@vger.kernel.org
> Fixes: 6cf93a9ed39e ("rust: add bindings for bitops.h")
> Reported-by: Andreas Hindborg <a.hindborg@kernel.org>
> Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/x/topic/x/near/561677301
> Signed-off-by: Alice Ryhl <aliceryhl@google.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.

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

Thanks,
Yury

> ---
> Changes in v2:
> - Remove rust_helper_ prefix from helpers.
> - Improve commit message.
> - The set of functions for which a helper is added is changed so that it
>   matches arch/arm/include/asm/bitops.h
> - Link to v1: https://lore.kernel.org/r/20251203-bitops-find-helper-v1-1-5193deb57766@google.com
> ---
>  rust/helpers/bitops.c | 42 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/rust/helpers/bitops.c b/rust/helpers/bitops.c
> index 5d0861d29d3f0d705a014ae4601685828405f33b..e79ef9e6d98f969e2a0a2a6f62d9fcec3ef0fd72 100644
> --- a/rust/helpers/bitops.c
> +++ b/rust/helpers/bitops.c
> @@ -1,6 +1,7 @@
>  // SPDX-License-Identifier: GPL-2.0
>  
>  #include <linux/bitops.h>
> +#include <linux/find.h>
>  
>  void rust_helper___set_bit(unsigned long nr, unsigned long *addr)
>  {
> @@ -21,3 +22,44 @@ void rust_helper_clear_bit(unsigned long nr, volatile unsigned long *addr)
>  {
>  	clear_bit(nr, addr);
>  }
> +
> +/*
> + * The rust_helper_ prefix is intentionally omitted below so that the
> + * declarations in include/linux/find.h are compatible with these helpers.
> + *
> + * Note that the below #ifdefs mean that the helper is only created if C does
> + * not provide a definition.
> + */
> +#ifdef find_first_zero_bit
> +__rust_helper
> +unsigned long _find_first_zero_bit(const unsigned long *p, unsigned long size)
> +{
> +	return find_first_zero_bit(p, size);
> +}
> +#endif /* find_first_zero_bit */
> +
> +#ifdef find_next_zero_bit
> +__rust_helper
> +unsigned long _find_next_zero_bit(const unsigned long *addr,
> +				  unsigned long size, unsigned long offset)
> +{
> +	return find_next_zero_bit(addr, size, offset);
> +}
> +#endif /* find_next_zero_bit */
> +
> +#ifdef find_first_bit
> +__rust_helper
> +unsigned long _find_first_bit(const unsigned long *addr, unsigned long size)
> +{
> +	return find_first_bit(addr, size);
> +}
> +#endif /* find_first_bit */
> +
> +#ifdef find_next_bit
> +__rust_helper
> +unsigned long _find_next_bit(const unsigned long *addr, unsigned long size,
> +			     unsigned long offset)
> +{
> +	return find_next_bit(addr, size, offset);
> +}
> +#endif /* find_next_bit */
> 
> ---
> base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
> change-id: 20251203-bitops-find-helper-25ed1bbae700
> 
> Best regard