kbuild: rust: split up helpers.c

This patch splits up the rust helpers C file. When rebasing patch sets on
upstream linux, merge conflicts in helpers.c is common and time consuming [1].
Thus, split the file so that each kernel component can live in a separate file.

This patch lists helper files explicitly and thus conflicts in the file list is
still likely. However, they should be more simple to resolve than the conflicts
usually seen in helpers.c.

Link:
https://rust-for-linux.zulipchat.com/#narrow/stream/288089-General/topic/Splitting.20up.20helpers.2Ec/near/426694012 [1]
Signed-off-by: Andreas Hindborg <[email protected]>

Author: metaspace

rust/Makefile

@@ -6,10 +6,22 @@ rustdoc_output := $(objtree)/Documentation/output/rust/rustdoc
 obj-$(CONFIG_RUST) += core.o compiler_builtins.o
 always-$(CONFIG_RUST) += exports_core_generated.h
 
-# Missing prototypes are expected in the helpers since these are exported
-# for Rust only, thus there is no header nor prototypes.
-obj-$(CONFIG_RUST) += helpers.o
-CFLAGS_REMOVE_helpers.o = -Wmissing-prototypes -Wmissing-declarations
+RUST_HELPERS := \
+	bug \
+	build_assert \
+	build_bug \
+	err \
+	kunit \
+	mutex \
+	refcount \
+	signal \
+	spinlock\
+	task \
+	wait \
+	workqueue
+
+export RUST_HELPERS
+obj-y += helpers/
 
 always-$(CONFIG_RUST) += libmacros.so
 no-clean-files += libmacros.so
@@ -339,6 +351,10 @@ quiet_cmd_bindgen = BINDGEN $@
 		-o $@ -- $(bindgen_c_flags_final) -DMODULE \
 		$(bindgen_target_cflags) $(bindgen_target_extra)
 
+quiet_cmd_cat = CAT     $@
+      cmd_cat = \
+	cat $(patsubst %,$(srctree)/$(src)/helpers/%.c, $(RUST_HELPERS)) > $@
+
 $(obj)/bindings/bindings_generated.rs: private bindgen_target_flags = \
     $(shell grep -Ev '^#|^$$' $(srctree)/$(src)/bindgen_parameters)
 $(obj)/bindings/bindings_generated.rs: private bindgen_target_extra = ; \
@@ -353,9 +369,9 @@ $(obj)/uapi/uapi_generated.rs: $(src)/uapi/uapi_helper.h \
     $(src)/bindgen_parameters FORCE
 	$(call if_changed_dep,bindgen)
 
-# See `CFLAGS_REMOVE_helpers.o` above. In addition, Clang on C does not warn
-# with `-Wmissing-declarations` (unlike GCC), so it is not strictly needed here
-# given it is `libclang`; but for consistency, future Clang changes and/or
+# See `ccflags-remove-y` in helpers/Makefile . In addition, Clang on C does not
+# warn with `-Wmissing-declarations` (unlike GCC), so it is not strictly needed
+# here given it is `libclang`; but for consistency, future Clang changes and/or
 # a potential future GCC backend for `bindgen`, we disable it too.
 $(obj)/bindings/bindings_helpers_generated.rs: private bindgen_target_flags = \
     --blocklist-type '.*' --allowlist-var '' \
@@ -364,9 +380,13 @@ $(obj)/bindings/bindings_helpers_generated.rs: private bindgen_target_cflags = \
     -I$(objtree)/$(obj) -Wno-missing-prototypes -Wno-missing-declarations
 $(obj)/bindings/bindings_helpers_generated.rs: private bindgen_target_extra = ; \
     sed -Ei 's/pub fn rust_helper_([a-zA-Z0-9_]*)/#[link_name="rust_helper_\1"]\n    pub fn \1/g' $@
-$(obj)/bindings/bindings_helpers_generated.rs: $(src)/helpers.c FORCE
+$(obj)/bindings/bindings_helpers_generated.rs: $(obj)/helpers/helpers_combined.c FORCE
 	$(call if_changed_dep,bindgen)
 
+targets += helpers/helpers_combined.c
+$(obj)/helpers/helpers_combined.c: $(patsubst %,$(src)/helpers/%.c, $(RUST_HELPERS)) FORCE
+	$(call if_changed,cat)
+
 quiet_cmd_exports = EXPORTS $@
       cmd_exports = \
 	$(NM) -p --defined-only $< \

rust/helpers.c

@@ -0,0 +1,7 @@
+
+# Note: `RUST_HELPERS` is defined in `../Makefile`
+
+# Missing prototypes are expected in the helpers since these are exported
+# for Rust only, thus there is no header nor prototypes.
+obj-$(CONFIG_RUST) += $(patsubst %,%.o, $(RUST_HELPERS))
+ccflags-remove-y += -Wmissing-prototypes -Wmissing-declarations

rust/helpers/Makefile

@@ -0,0 +1,18 @@
+# Rust Helpers
+
+Non-trivial C macros cannot be used in Rust. Similarly, inlined C functions
+cannot be called either. The files in this directory explicitly create functions
+("helpers") that wrap those so that they can be called from Rust.
+
+Even though Rust kernel modules should never use directly the bindings, 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.

rust/helpers/README.md

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bug.h>
+
+__noreturn void rust_helper_BUG(void)
+{
+	BUG();
+}
+EXPORT_SYMBOL_GPL(rust_helper_BUG);

rust/helpers/bug.c

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/build_bug.h>
+
+/*
+ * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
+ * use it in contexts where Rust expects a `usize` like slice (array) indices.
+ * `usize` is defined to be the same as C's `uintptr_t` type (can hold any
+ * pointer) but not necessarily the same as `size_t` (can hold the size of any
+ * single object). Most modern platforms use the same concrete integer type for
+ * both of them, but in case we find ourselves on a platform where
+ * that's not true, fail early instead of risking ABI or
+ * integer-overflow issues.
+ *
+ * If your platform fails this assertion, it means that you are in
+ * danger of integer-overflow bugs (even if you attempt to add
+ * `--no-size_t-is-usize`). It may be easiest to change the kernel ABI on
+ * your platform such that `size_t` matches `uintptr_t` (i.e., to increase
+ * `size_t`, because `uintptr_t` has to be at least as big as `size_t`).
+ */
+static_assert(
+	sizeof(size_t) == sizeof(uintptr_t) &&
+	__alignof__(size_t) == __alignof__(uintptr_t),
+	"Rust code expects C `size_t` to match Rust `usize`"
+);

rust/helpers/build_assert.c

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/export.h>
+#include <linux/errname.h>
+
+const char *rust_helper_errname(int err)
+{
+	return errname(err);
+}
+EXPORT_SYMBOL_GPL(rust_helper_errname);

rust/helpers/build_bug.c

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/err.h>
+#include <linux/export.h>
+
+__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/err.c

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <kunit/test-bug.h>
+#include <linux/export.h>
+
+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/kunit.c

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/export.h>
+#include <linux/mutex.h>
+
+void rust_helper_mutex_lock(struct mutex *lock)
+{
+	mutex_lock(lock);
+}
+EXPORT_SYMBOL_GPL(rust_helper_mutex_lock);