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

[Andreas Hindborg <a.hindborg@kernel.org>]

Alice Ryhl <aliceryhl@google.com> writes:

> 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>

Built tested for arm32 with bindgen 0.72.1 with rustc-1.78.0 and
rustc-1.88.0.

Tested-by: Andreas Hindborg <a.hindborg@kernel.org>


Best regards,
Andreas Hindborg