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

["Gary Guo" <gary@garyguo.net>]

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? 

>
> 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.

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

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)
>