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

[Alistair Popple <apopple@nvidia.com>]

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:

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?

---

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)


> Best,
> Gary
> 
> >
> > Fix this by exporting the `uapi` symbols in the same way as `bindings`.
> >
> > Fixes: 4e1746656839 ("rust: uapi: Add UAPI crate")
> > Signed-off-by: Alistair Popple <apopple@nvidia.com>
> > Assisted-by: LLM
> > ---
> >  rust/Makefile  | 7 ++++++-
> >  rust/exports.c | 1 +
> >  2 files changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/rust/Makefile b/rust/Makefile
> > index da1a7409d984..083992a5c1c9 100644
> > --- a/rust/Makefile
> > +++ b/rust/Makefile
> > @@ -27,6 +27,7 @@ always-$(CONFIG_RUST) += exports_bindings_generated.h exports_kernel_generated.h
> >  
> >  always-$(CONFIG_RUST) += uapi/uapi_generated.rs
> >  obj-$(CONFIG_RUST) += uapi.o
> > +always-$(CONFIG_RUST) += exports_uapi_generated.h
> >  
> >  ifdef CONFIG_RUST_BUILD_ASSERT_ALLOW
> >  obj-$(CONFIG_RUST) += build_error.o
> > @@ -575,7 +576,8 @@ $(obj)/exports_core_generated.h: $(obj)/core.o FORCE
> >  # in the crate where they are defined. Other helpers, called from non-inline
> >  # functions, may not be exported, in principle. However, in general, the Rust
> >  # compiler does not guarantee codegen will be performed for a non-inline
> > -# function either. Therefore, we export all symbols from helpers and bindings.
> > +# function either. Therefore, we export all symbols from helpers, bindings and
> > +# uapi.
> >  # In the future, this may be revisited to reduce the number of exports after
> >  # the compiler is informed about the places codegen is required.
> >  $(obj)/exports_helpers_generated.h: $(obj)/helpers/helpers.o FORCE
> > @@ -587,6 +589,9 @@ $(obj)/exports_bindings_generated.h: $(obj)/bindings.o FORCE
> >  $(obj)/exports_kernel_generated.h: $(obj)/kernel.o FORCE
> >  	$(call if_changed,exports)
> >  
> > +$(obj)/exports_uapi_generated.h: $(obj)/uapi.o FORCE
> > +	$(call if_changed,exports)
> > +
> >  quiet_cmd_rustc_procmacrolibrary = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) PL $@
> >        cmd_rustc_procmacrolibrary = \
> >  	$(rustc_target_envs) \
> > diff --git a/rust/exports.c b/rust/exports.c
> > index 1b52460b0f4e..434f0b73151f 100644
> > --- a/rust/exports.c
> > +++ b/rust/exports.c
> > @@ -17,6 +17,7 @@
> >  
> >  #include "exports_core_generated.h"
> >  #include "exports_bindings_generated.h"
> > +#include "exports_uapi_generated.h"
> >  #include "exports_kernel_generated.h"
> >  
> >  #ifndef CONFIG_RUST_INLINE_HELPERS
> >
> > base-commit: b705c185105762676aa6ec16cf976101df87cc35
> 
>