Re: [PATCH] rust: kbuild: export symbols from the `uapi` crate

[Alistair Popple <apopple@nvidia.com>]

On 2026-09-10 at 21:36 +1000, Gary Guo <gary@garyguo.net> wrote...
> On Thu Sep 10, 2026 at 3:21 AM BST, Alistair Popple wrote:
> > On 2026-09-10 at 00:25 +1000, Gary Guo <gary@garyguo.net> wrote...
> >> On Wed Sep 9, 2026 at 7:06 AM BST, Alistair Popple wrote:
> >> > Symbols from the `bindings` crate and the C helpers are exported so
> >> > that loadable modules can link against code the compiler chose not to
> >> > instantiate in the module itself. The `uapi` crate has the same problem
> >> > but nothing from it is exported.
> >> >
> >> > In practice this goes unnoticed because the only functions in `uapi`
> >> > are the trivial `Default` implementations generated by `bindgen`,
> >> > which rustc inlines across crates. With CONFIG_RUST_DEBUG_ASSERTIONS=y
> >> > however, the `Default` implementation for structs can grow past the
> >> > automatic cross-crate inlining threshold. The module then references the
> >> > copy in `uapi.o`, which is not exported, and the build fails:
> >> >
> >> >   ERROR: modpost: drivers/gpu/nova-drm.ko: symbol '_RNvXsH_Csk9v2ZIpWbWt_4uapiNtB5_17drm_nova_info_gpuNtNtCsfr3MPOfBGpN_4core7default7Default7default' undefined!
> >> 
> >> Hmm, uapi' ideally should just be type exports and produce no code.
> >> 
> >> Default implementation should carry `#[inline]` annotation so they're
> >> only codegenned in actual user crate. Rust's `#[derive(Default)]` would always
> >> add that. Can you check if that's bindgen-produced manual Default impl that
> >> somehow misses the attribute?
> >
> > Right, bindgen-produced Default impl do not have that attribute. For example
> > here is the bindgen generated implementation I see:
> 
> Perhaps you can open an issue upstream? 

For sure - see https://github.com/rust-lang/rust-bindgen/issues/3494

I wasn't quite sure of the justification for why we don't want to just export
these, so please feel free to comment there.

> >
> > impl Default for drm_nova_info_gpu {
> >     fn default() -> Self {
> >         let mut s = ::core::mem::MaybeUninit::<Self>::uninit();
> >         unsafe {
> >             ::core::ptr::write_bytes(s.as_mut_ptr(), 0, 1);
> >             s.assume_init()
> >         }
> >     }
> > }
> >
> > Manually adding #[inline] fixes the problem of course. I guess we could add some
> > sed Makefile hackery to manually add it. For example something like the below,
> > not sure if that would be preferred?
> 
> Yeah, textual hacks are a bit too fragile (especially that it'll break when
> upstream bindgen adds these annotations). So if you need these impls, I think
> exporting these symbols are fine.

Fun though :) But yes, I agree this is pretty hacky so would rather export the
symbols, at least until bindgen is fixed.

> That said, you can achieve what you need with `drm_nova_info_gpu::zeroed()`
> instead?

We could, but I'd rather fix this here as people are inevitably going to try and
use default::Default() and the linker errors that come from it are pretty obtuse
and difficult to diagnose what the problem really is.

 - Alistair

> Best,
> Gary
> 
> >
> > ---
> >
> > diff --cc rust/Makefile
> > index da1a7409d984,da1a7409d984..535a3bf181bb
> > --- a/rust/Makefile
> > +++ b/rust/Makefile
> > @@@ -525,13 -525,13 +525,16 @@@ quiet_cmd_bindgen = BINDGEN $
> >   $(obj)/bindings/bindings_generated.rs: private bindgen_target_flags = \
> >       $(shell grep -Ev '^#|^$$' $(src)/bindgen_parameters)
> >   $(obj)/bindings/bindings_generated.rs: private bindgen_target_extra = ; \
> > --    sed -Ei 's/pub const RUST_CONST_HELPER_([a-zA-Z0-9_]*)/pub const \1/g' $@
> > ++    sed -Ei 's/pub const RUST_CONST_HELPER_([a-zA-Z0-9_]*)/pub const \1/g; \
> > ++        s/^([[:space:]]*)fn default\(\) -> Self \{/\1#[inline]\n\1fn default() -> Self {/' $@
> >   $(obj)/bindings/bindings_generated.rs: $(src)/bindings/bindings_helper.h \
> >       $(src)/bindgen_parameters FORCE
> >         $(call if_changed_dep,bindgen)
> >   
> >   $(obj)/uapi/uapi_generated.rs: private bindgen_target_flags = \
> >       $(shell grep -Ev '^#|^$$' $(src)/bindgen_parameters)
> > ++$(obj)/uapi/uapi_generated.rs: private bindgen_target_extra = ; \
> > ++    sed -Ei 's/^([[:space:]]*)fn default\(\) -> Self \{/\1#[inline]\n\1fn default() -> Self {/' $@
> >   $(obj)/uapi/uapi_generated.rs: $(src)/uapi/uapi_helper.h \
> >       $(src)/bindgen_parameters FORCE
> >         $(call if_changed_dep,bindgen)
> >
>