rust: bitops: fix missing _find_* functions on 32-bit ARM

2026-01-14T03:16:39+00:00

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:(::insert_or_update_handle) in archive vmlinux.a >>> referenced by rust_binder_main.43196037ba7bcee1-cgu.0 >>> drivers/android/binder/rust_binder_main.o:(::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 defining 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. Yury: arches are free to implement they own find_bit() functions. Most rely on generic implementation, but arm32 and m86k - not; so they require custom handling. Alice confirmed it fixes the build for both. Cc: stable@vger.kernel.org Fixes: 6cf93a9ed39e ("rust: add bindings for bitops.h") Reported-by: Andreas Hindborg Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/x/topic/x/near/561677301 Tested-by: Andreas Hindborg Reviewed-by: Dirk Behme Signed-off-by: Alice Ryhl Signed-off-by: Yury Norov (NVIDIA)

rust: add bindings for bitops.h

2025-09-22T19:52:44+00:00

Makes atomic set_bit and clear_bit inline functions as well as the non-atomic variants __set_bit and __clear_bit available to Rust. Adds a new MAINTAINERS section BITOPS API BINDINGS [RUST]. Suggested-by: Alice Ryhl Suggested-by: Yury Norov Signed-off-by: Burak Emir Reviewed-by: Alice Ryhl Acked-by: Yury Norov (NVIDIA) Signed-off-by: Yury Norov (NVIDIA)

kernel/linux.git/rust/helpers/bitops.c, branch v6.19.11

rust: bitops: fix missing _find_* functions on 32-bit ARM

rust: add bindings for bitops.h