[PATCH 3/3] rust: Require RUST_MODULES for module support

[Matthew Maurer <mmaurer@google.com>]

Currently, we don't support MODVERSIONS for Rust symbols. For users that
want to use Rust in the kernel but for whom MODVERSIONS is required,
this allows the use of built-in Rust code even with MODVERSIONS enabled.
It may additionally allow code-size reduction by avoiding exporting
symbols that won't be used without Rust modules.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 init/Kconfig          | 21 +++++++++++++++++----
 kernel/module/Kconfig |  1 +
 rust/exports.c        |  4 ++++
 3 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index 9ffb103fc927..6912dbbee3f1 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1885,7 +1885,6 @@ config RUST
 	bool "Rust support"
 	depends on HAVE_RUST
 	depends on RUST_IS_AVAILABLE
-	depends on !MODVERSIONS
 	depends on !GCC_PLUGINS
 	depends on !RANDSTRUCT
 	depends on !DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE
@@ -1896,13 +1895,27 @@ config RUST
 	  This allows other Rust-related options, like drivers written in Rust,
 	  to be selected.
 
-	  It is also required to be able to load external kernel modules
-	  written in Rust.
-
 	  See Documentation/rust/ for more information.
 
 	  If unsure, say N.
 
+config RUST_MODULES
+	bool "Rust Module Support"
+	depends on RUST
+	depends on MODULES
+	rust_modules
+	help
+	  Enables support for Rust Modules in the kernel.
+
+	  This is required to load external kernel modules written in Rust.
+
+	  The two primary reasons to consider disabling this are:
+	  * Allow MODVERSIONS support
+	  * Allow additional code to be optimized out by the compiler if you
+	    know that you'll only be using built-in Rust code.
+
+	  If unsure, say Y.
+
 config RUSTC_VERSION_TEXT
 	string
 	depends on RUST
diff --git a/kernel/module/Kconfig b/kernel/module/Kconfig
index 0ea1b2970a23..a76128887618 100644
--- a/kernel/module/Kconfig
+++ b/kernel/module/Kconfig
@@ -158,6 +158,7 @@ config MODULE_UNLOAD_TAINT_TRACKING
 	  shown. If unsure, say N.
 
 config MODVERSIONS
+	depends on !RUST_MODULES
 	bool "Module versioning support"
 	help
 	  Usually, you have to use modules compiled with your kernel.
diff --git a/rust/exports.c b/rust/exports.c
index 83e2a7070cae..96b706048ed4 100644
--- a/rust/exports.c
+++ b/rust/exports.c
@@ -13,7 +13,11 @@
 
 #include <linux/module.h>
 
+#ifdef CONFIG_RUST_MODULES
 #define EXPORT_SYMBOL_RUST_GPL(sym) extern int sym; EXPORT_SYMBOL_GPL(sym)
+#else
+#define EXPORT_SYMBOL_RUST_GPL(sym)
+#endif
 
 #include "exports_core_generated.h"
 #include "exports_alloc_generated.h"
-- 
2.42.0.869.gea05f2083d-goog