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

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

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@gmail.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@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.
> 
> 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.

Thanks,
Yury