Remove arch/ blocker by passing CONFIG vars to emulate module_init fully

Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

Makefile

@@ -521,7 +521,7 @@ KBUILD_RUSTCFLAGS_KERNEL :=
 KBUILD_CARGOFLAGS_KERNEL :=
 KBUILD_AFLAGS_MODULE  := -DMODULE
 KBUILD_CFLAGS_MODULE  := -DMODULE
-KBUILD_RUSTCFLAGS_MODULE := --cfg module
+KBUILD_RUSTCFLAGS_MODULE := --cfg MODULE
 KBUILD_CARGOFLAGS_MODULE :=
 KBUILD_LDFLAGS_MODULE :=
 export KBUILD_LDS_MODULE := $(srctree)/scripts/module-common.lds

arch/x86/Kconfig

@@ -148,7 +148,7 @@ config X86
 	select HAVE_ARCH_MMAP_RND_BITS		if MMU
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if MMU && COMPAT
 	select HAVE_ARCH_COMPAT_MMAP_BASES	if MMU && COMPAT
-	#select HAVE_ARCH_PREL32_RELOCATIONS TODO: see the `kernel_module` macro
+	select HAVE_ARCH_PREL32_RELOCATIONS
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
 	select HAVE_ARCH_STACKLEAK

drivers/char/rust_example/src/lib.rs

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 
 #![no_std]
+#![feature(global_asm)]
 
 use kernel::prelude::*;
 
@@ -11,7 +12,7 @@ struct RustExample {
 impl KernelModule for RustExample {
     fn init() -> KernelResult<Self> {
         println!("Rust Example (init)");
-        println!("Am I built-in? {}", !cfg!(module));
+        println!("Am I built-in? {}", !cfg!(MODULE));
         Ok(RustExample {
             message: "on the heap!".to_owned(),
         })

rust/kernel/src/lib.rs

@@ -53,13 +53,23 @@ macro_rules! kernel_module {
 
         // Built-in modules are initialized through an initcall pointer
         //
-        // TODO: find a proper way to emulate the C macro (`module_init`),
-        // including dealing with `HAVE_ARCH_PREL32_RELOCATIONS`
-        #[cfg(not(module))]
+        // TODO: should we compile a C file on the fly to avoid duplication?
+        #[cfg(not(MODULE))]
+        #[cfg(not(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS))]
         #[link_section = ".initcall6.init"]
         #[used]
         pub static __initcall: extern "C" fn() -> $crate::c_types::c_int = init_module;
 
+        #[cfg(not(MODULE))]
+        #[cfg(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS)]
+        global_asm!(
+            r#".section ".initcall6.init", "a"
+            __initcall:
+                .long   init_module - .
+                .previous
+            "#
+        );
+
         // TODO: pass the kernel module name here to generate a unique,
         // helpful symbol name (the name would also useful for the `modinfo`
         // issue below).

scripts/Makefile.lib

@@ -229,7 +229,9 @@ c_flags        = -Wp,-MMD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE)     \
 RUST_BINDGEN_CFLAGS = $(c_flags) $(KBUILD_CFLAGS_MODULE)
 export RUST_BINDGEN_CFLAGS
 
-rustc_flags     = $(_rustc_flags) $(modkern_rustcflags)
+rustc_cfg_flags = $(shell sed -nE 's/^(CONFIG_[^=]+)=(y|m)$$/--cfg \1/p' $(srctree)/include/config/auto.conf | xargs)
+
+rustc_flags     = $(_rustc_flags) $(modkern_rustcflags) $(rustc_cfg_flags)
 
 # Passed by cargo
 RUSTFLAGS = $(rustc_flags)