[PATCH v5 2/4] rust: convert exports.c to assembly

[Giuliano Procida <gprocida@google.com>]

Previously, `rust/exports.c` defined `EXPORT_SYMBOL_RUST_GPL(sym)` with
`extern int sym;` so that it could invoke the C `EXPORT_SYMBOL_GPL(sym)`
macro, which requires `sym` to be a declared C identifier in scope for
`typeof(sym)`, `__ADDRESSABLE(sym)` and `__GENDWARFKSYMS_EXPORT(sym)`.

This caused the C compiler to emit DWARF debug information in
`rust/exports.o` describing every exported Rust and helper symbol,
functions and data alike, as a C `int` variable. Those bogus `int` DWARF
entries ended up in `vmlinux`, conflicting with the true DWARF types
emitted by `rustc` and confusing debuggers and ABI analysis tools.

Convert `rust/exports.c` to `rust/exports.S` and emit
`EXPORT_SYMBOL_GPL(sym)` directly. Under `__ASSEMBLY__`,
`<linux/export.h>` expands `EXPORT_SYMBOL_GPL(sym)` straight to
`___EXPORT_SYMBOL(sym, "GPL", ...)` without requiring symbol
declarations or emitting bogus DWARF entries, while also avoiding a C
compilation step.

Signed-off-by: Giuliano Procida <gprocida@google.com>
---
 rust/Makefile                 |  2 +-
 rust/{exports.c => exports.S} | 12 +++++-------
 2 files changed, 6 insertions(+), 8 deletions(-)
 rename rust/{exports.c => exports.S} (72%)

diff --git a/rust/Makefile b/rust/Makefile
index 0ac9ab28cc2b..2e5f5a84bb24 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -564,7 +564,7 @@ rust_exports = $(NM) -p --defined-only $(1) | awk '$$2~/(T|R|D|B)/ && $$3!~/__(p
 
 quiet_cmd_exports = EXPORTS $@
       cmd_exports = \
-	$(call rust_exports,$<,"EXPORT_SYMBOL_RUST_GPL(%s);\n") > $@
+	$(call rust_exports,$<,"EXPORT_SYMBOL_GPL(%s)\n") > $@
 
 $(obj)/exports_core_generated.h: $(obj)/core.o FORCE
 	$(call if_changed,exports)
diff --git a/rust/exports.c b/rust/exports.S
similarity index 72%
rename from rust/exports.c
rename to rust/exports.S
index 1b52460b0f4e..e1a6923081fa 100644
--- a/rust/exports.c
+++ b/rust/exports.S
@@ -1,11 +1,11 @@
-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * A hack to export Rust symbols for loadable modules without having to redo
  * the entire `include/linux/export.h` logic in Rust.
  *
  * This requires Rust's new/future `v0` mangling scheme because the default one
- * ("legacy") uses invalid characters for C identifiers (thus we cannot use the
- * `EXPORT_SYMBOL_*` macros).
+ * ("legacy") uses invalid characters for C identifiers (thus we could not use
+ * the `EXPORT_SYMBOL_*` macros).
  *
  * All symbols are exported as GPL-only to guarantee no GPL-only feature is
  * accidentally exposed.
@@ -13,8 +13,6 @@
 
 #include <linux/export.h>
 
-#define EXPORT_SYMBOL_RUST_GPL(sym) extern int sym; EXPORT_SYMBOL_GPL(sym)
-
 #include "exports_core_generated.h"
 #include "exports_bindings_generated.h"
 #include "exports_kernel_generated.h"
@@ -23,7 +21,7 @@
 #include "exports_helpers_generated.h"
 #endif
 
-// For modules using `rust/build_error.rs`.
+/* For modules using `rust/build_error.rs`. */
 #ifdef CONFIG_RUST_BUILD_ASSERT_ALLOW
-EXPORT_SYMBOL_RUST_GPL(rust_build_error);
+EXPORT_SYMBOL_GPL(rust_build_error)
 #endif
-- 
2.56.0.567.gdb80ae8cd5-goog