rust: kbuild: auto generate helper exports

This removes the need to explicitly export all symbols.

Generate helper exports similarly to what's currently done for Rust
crates. These helpers are exclusively called from within Rust code and
therefore can be treated similar as other Rust symbols.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Tested-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20240817165302.3852499-1-gary@garyguo.net
[ Fixed dependency path, reworded slightly, edited comment a bit and
  rebased on top of the changes made when applying Andreas' patch
  (e.g. no `README.md` anymore, so moved the edits).  - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: nbdd0121

rust/Makefile

@@ -16,8 +16,8 @@ no-clean-files += libmacros.so
 
 always-$(CONFIG_RUST) += bindings/bindings_generated.rs bindings/bindings_helpers_generated.rs
 obj-$(CONFIG_RUST) += alloc.o bindings.o kernel.o
-always-$(CONFIG_RUST) += exports_alloc_generated.h exports_bindings_generated.h \
-    exports_kernel_generated.h
+always-$(CONFIG_RUST) += exports_alloc_generated.h exports_helpers_generated.h \
+    exports_bindings_generated.h exports_kernel_generated.h
 
 always-$(CONFIG_RUST) += uapi/uapi_generated.rs
 obj-$(CONFIG_RUST) += uapi.o
@@ -313,6 +313,18 @@ $(obj)/exports_core_generated.h: $(obj)/core.o FORCE
 $(obj)/exports_alloc_generated.h: $(obj)/alloc.o FORCE
 	$(call if_changed,exports)
 
+# Even though Rust kernel modules should never use the bindings directly,
+# symbols from the `bindings` crate and the C helpers need to be exported
+# because Rust generics and inlined functions may not get their code generated
+# 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.
+# 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
+	$(call if_changed,exports)
+
 $(obj)/exports_bindings_generated.h: $(obj)/bindings.o FORCE
 	$(call if_changed,exports)
 

rust/exports.c

@@ -17,6 +17,7 @@
 
 #include "exports_core_generated.h"
 #include "exports_alloc_generated.h"
+#include "exports_helpers_generated.h"
 #include "exports_bindings_generated.h"
 #include "exports_kernel_generated.h"
 

rust/helpers/blk.c

@@ -7,10 +7,8 @@ void *rust_helper_blk_mq_rq_to_pdu(struct request *rq)
 {
 	return blk_mq_rq_to_pdu(rq);
 }
-EXPORT_SYMBOL_GPL(rust_helper_blk_mq_rq_to_pdu);
 
 struct request *rust_helper_blk_mq_rq_from_pdu(void *pdu)
 {
 	return blk_mq_rq_from_pdu(pdu);
 }
-EXPORT_SYMBOL_GPL(rust_helper_blk_mq_rq_from_pdu);

rust/helpers/bug.c

@@ -6,4 +6,3 @@ __noreturn void rust_helper_BUG(void)
 {
 	BUG();
 }
-EXPORT_SYMBOL_GPL(rust_helper_BUG);

rust/helpers/build_bug.c

@@ -7,4 +7,3 @@ const char *rust_helper_errname(int err)
 {
 	return errname(err);
 }
-EXPORT_SYMBOL_GPL(rust_helper_errname);

rust/helpers/err.c

@@ -7,16 +7,13 @@ __force void *rust_helper_ERR_PTR(long err)
 {
 	return ERR_PTR(err);
 }
-EXPORT_SYMBOL_GPL(rust_helper_ERR_PTR);
 
 bool rust_helper_IS_ERR(__force const void *ptr)
 {
 	return IS_ERR(ptr);
 }
-EXPORT_SYMBOL_GPL(rust_helper_IS_ERR);
 
 long rust_helper_PTR_ERR(__force const void *ptr)
 {
 	return PTR_ERR(ptr);
 }
-EXPORT_SYMBOL_GPL(rust_helper_PTR_ERR);

rust/helpers/helpers.c

@@ -4,19 +4,6 @@
  * cannot be called either. This file explicitly creates functions ("helpers")
  * that wrap those so that they can be called from Rust.
  *
- * Even though Rust kernel modules should never use the bindings directly, some
- * of these helpers need to be exported because Rust generics and inlined
- * functions may not get their code generated 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, this file exports all the helpers. In the future, this may be
- * revisited to reduce the number of exports after the compiler is informed
- * about the places codegen is required.
- *
- * All symbols are exported as GPL-only to guarantee no GPL-only feature is
- * accidentally exposed.
- *
  * Sorted alphabetically.
  */
 

rust/helpers/kunit.c

@@ -7,4 +7,3 @@ struct kunit *rust_helper_kunit_get_current_test(void)
 {
 	return kunit_get_current_test();
 }
-EXPORT_SYMBOL_GPL(rust_helper_kunit_get_current_test);

rust/helpers/mutex.c

@@ -7,4 +7,3 @@ void rust_helper_mutex_lock(struct mutex *lock)
 {
 	mutex_lock(lock);
 }
-EXPORT_SYMBOL_GPL(rust_helper_mutex_lock);

rust/helpers/page.c

@@ -7,16 +7,13 @@ struct page *rust_helper_alloc_pages(gfp_t gfp_mask, unsigned int order)
 {
 	return alloc_pages(gfp_mask, order);
 }
-EXPORT_SYMBOL_GPL(rust_helper_alloc_pages);
 
 void *rust_helper_kmap_local_page(struct page *page)
 {
 	return kmap_local_page(page);
 }
-EXPORT_SYMBOL_GPL(rust_helper_kmap_local_page);
 
 void rust_helper_kunmap_local(const void *addr)
 {
 	kunmap_local(addr);
 }
-EXPORT_SYMBOL_GPL(rust_helper_kunmap_local);