Re: [PATCH] rust: bitops: add helpers for _find_* functions

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Dec 03, 2025 at 11:35:54PM +0100, Andreas Hindborg wrote:
> Alice Ryhl <aliceryhl@google.com> writes:
> 
> > On 32-bit ARM, _find_next_bit does not exist because ARM provides that
> > function through a #define instead. This means that Rust, which calls
> > the underscored version, fails to find it. This triggers errors:
> >
> > 	ld.lld: error: undefined symbol: _find_next_bit
> > 	>>> referenced by bitmap.rs:459 (rust/kernel/bitmap.rs:459)
> > 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_bitmap_borrow) in archive vmlinux.a
> > 	>>> referenced by bitmap.rs:459 (rust/kernel/bitmap.rs:459)
> > 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_bitmap_set_clear_find) in archive vmlinux.a
> > 	>>> referenced by bitmap.rs:459 (rust/kernel/bitmap.rs:459)
> > 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_bitmap_set_clear_find) in archive vmlinux.a
> > 	>>> referenced 10 more times
> >
> > 	ld.lld: error: undefined symbol: _find_next_zero_bit
> > 	>>> referenced by bitmap.rs:479 (rust/kernel/bitmap.rs:479)
> > 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_bitmap_copy) in archive vmlinux.a
> > 	>>> referenced by bitmap.rs:479 (rust/kernel/bitmap.rs:479)
> > 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_bitmap_set_clear_find) in archive vmlinux.a
> > 	>>> referenced by bitmap.rs:479 (rust/kernel/bitmap.rs:479)
> > 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_owned_bitmap_out_of_bounds) in archive vmlinux.a
> > 	>>> referenced 6 more times
> >
> > To fix this, add Rust helpers in this particular case.
> >
> > 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
> > Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > ---
> >  rust/helpers/bitops.c | 32 ++++++++++++++++++++++++++++++++
> >  1 file changed, 32 insertions(+)
> >
> > diff --git a/rust/helpers/bitops.c b/rust/helpers/bitops.c
> > index 5d0861d29d3f0d705a014ae4601685828405f33b..84061af591a261f7268ffe6535282bf3c7608e2d 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 +22,34 @@ void rust_helper_clear_bit(unsigned long nr, volatile unsigned long *addr)
> >  {
> >  	clear_bit(nr, addr);
> >  }
> > +
> > +/*
> > + * Rust normally calls the single-underscore-prefixed version of these
> > + * functions, which are not inlined. However, on some platforms, they do not
> > + * exist. In those cases, provide a rust helper for the underscored version.
> > + */
> > +#ifdef find_next_zero_bit
> > +__rust_helper unsigned long
> > +rust_helper__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_next_bit
> > +__rust_helper unsigned long
> > +rust_helper__find_next_bit(const unsigned long *addr, unsigned long size,
> > +			   unsigned long offset)
> > +{
> > +	return find_next_bit(addr, size, offset);
> > +}
> > +#endif /* find_next_bit */
> > +
> > +#ifdef find_last_bit
> > +__rust_helper unsigned long
> > +rust_helper__find_last_bit(const unsigned long *addr, unsigned long size)
> > +{
> > +	return find_last_bit(addr, size);
> > +}
> > +#endif /* find_last_bit */
> >
> > ---
> > base-commit: 54e3eae855629702c566bd2e130d9f40e7f35bde
> > change-id: 20251203-bitops-find-helper-25ed1bbae700
> >
> > Best regards,
> > -- 
> > Alice Ryhl <aliceryhl@google.com>
> 
> I messed up when testing this patch. It actually does not solve the
> issue.
> 
> It appears that bindgen will emit an extern "C" declaration for the
> missing C functions, even though they are not in the symbol table. When
> we add the rust helpers, the declarations emitted based on the
> non-existent C functions will mask the rust helper functions.

Yeah, when Rust sees both a helper and real function, we configured it
to pick the real one.

> We can circumvent this by using another name for our helper. While it
> works, it is not fixing the root cause. I am not sure why bindgen emits
> these functions even though they are not in the symbol table at the end.

I think a much better workaround is to adjust the header file:

#ifndef find_next_zero_bit
unsigned long _find_next_zero_bit(const unsigned long *addr, unsigned long nbits,
					 unsigned long start);
#endif

Alice