[PATCH v2 2/2] rust: exports: emit ___EXPORT_SYMBOL without C declarations

[Giuliano Procida <gprocida@google.com>]

Define `EXPORT_SYMBOL_RUST_GPL(sym)` using `___EXPORT_SYMBOL` directly:

	asm(__stringify(___EXPORT_SYMBOL(sym, "GPL", "")))

matching the `__ASSEMBLY__` path.

Previously, `EXPORT_SYMBOL_RUST_GPL(sym)` declared `extern int sym;` so
that it could invoke `EXPORT_SYMBOL_GPL(sym)`, which required `sym` to
be a declared C identifier for `typeof(sym)`, `__ADDRESSABLE(sym)`, and
(when `CONFIG_GENDWARFKSYMS` is enabled) `__GENDWARFKSYMS_EXPORT(sym)`.

This resulted in the compiler emitting DWARF debug information in
`rust/exports.o` and `drivers/gpu/nova-core/nova_core_exports.o`
describing every exported Rust symbol, functions and data alike, as a C
`int` variable.

Although `rust/Makefile` sets `skip_gendwarfksyms = 1` on `exports.o`
and `drivers/gpu/Makefile` runs `gendwarfksyms` on `nova_core.o` rather
than `nova_core_exports.o`, the bogus `int` DWARF entries were still
present in `vmlinux` and `nova-core.ko`, where they conflicted with the
true DWARF types emitted by `rustc` and confused DWARF consumers such as
debuggers and ABI analysis tools.

Omitting the C-level wrappers from `__EXPORT_SYMBOL` is safe because:
- `__GENDWARFKSYMS_EXPORT(sym)` is unneeded because `gendwarfksyms` runs
  directly on the defining Rust and helper object files rather than on
  `rust/exports.o` or `nova_core_exports.o`.
- `extern int sym;`, `extern typeof(sym) sym;`, and `__ADDRESSABLE(sym)`
  have no effect in `rust/exports.c` or `nova_core_exports.c`: the
  symbols are already emitted in separate translation units (scraped via
  `nm --defined-only`), the fake `extern int` declaration never produced
  kCFI function type IDs, and `___EXPORT_SYMBOL` already emits a
  `.quad`/`.long` reference to `sym` in `.export_symbol` that satisfies
  both ELF linking and LTO module-asm symbol resolution.
- `DEFAULT_SYMBOL_NAMESPACE`, `__DISABLE_EXPORTS`, and `__GENKSYMS__`
  are not used by `rust/exports.c` or `nova_core_exports.c`.

Signed-off-by: Giuliano Procida <gprocida@google.com>
---
 drivers/gpu/Makefile   | 4 ++--
 include/linux/export.h | 9 +++++----
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/Makefile b/drivers/gpu/Makefile
index e372fc02139f..1269ce862ad2 100644
--- a/drivers/gpu/Makefile
+++ b/drivers/gpu/Makefile
@@ -47,8 +47,8 @@ $(obj)/nova-core/nova_core_exports.o: $(obj)/nova-core/exports_nova_core_generat
 CFLAGS_nova-core/nova_core_exports.o := -I $(objtree)/$(obj)/nova-core
 
 ifdef CONFIG_MODVERSIONS
-# The C export shim declares Rust symbols as `extern int`, so reuse its export
-# list but generate symbol CRCs from the Rust object instead of the shim's DWARF.
+# The C export shim has no DWARF type info for Rust symbols, so reuse
+# its export list but generate symbol CRCs from the Rust object.
 $(obj)/nova-core/nova_core_exports.o: private cmd_gensymtypes_c = \
 	$(call getexportsymbols,\1) | \
 	$(objtree)/scripts/gendwarfksyms/gendwarfksyms \
diff --git a/include/linux/export.h b/include/linux/export.h
index 4aeb422d86af..2a0362cc54e1 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -94,16 +94,17 @@
 #define EXPORT_SYMBOL_FOR_MODULES(sym, mods) __EXPORT_SYMBOL(sym, "GPL", "module:" mods)
 
 /*
- * A hack to export Rust symbols for loadable modules without having to redo
- * the entire `EXPORT_SYMBOL_GPL` logic in Rust.
+ * Export Rust symbols for loadable modules without having to redo the entire
+ * `EXPORT_SYMBOL_GPL` 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).
  *
  * All symbols are exported as GPL-only to guarantee no GPL-only feature is
- * accidentally exposed.
+ * accidentally exposed. There is no support for symbol namespaces.
  */
-#define EXPORT_SYMBOL_RUST_GPL(sym) extern int sym; EXPORT_SYMBOL_GPL(sym)
+#define EXPORT_SYMBOL_RUST_GPL(sym)				\
+	asm(__stringify(___EXPORT_SYMBOL(sym, "GPL", "")))
 
 #endif /* _LINUX_EXPORT_H */
-- 
2.56.0.218.g1ec904b173-goog