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

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

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?

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