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

[Alistair Popple <apopple@nvidia.com>]

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!

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
-- 
2.54.0