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

[Dirk Behme]

On 06/01/2026 18:38, Yury Norov wrote:
> 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@xxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > > > Fixes: 6cf93a9ed39e ("rust: add bindings for bitops.h")
> > > > Reported-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
> > > > Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/x/topic/x/near/561677301
> > > > Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> > >
> > > 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.

I see that in -next the mentioned typo is fixed. With that:

Reviewed-by: Dirk Behme <dirk.behme@xxxxxxxxxxxx>

Thanks!

Dirk