objtool/kbuild/rust: enable objtool for Rust

Now that we should be `objtool`-warning free, enable `objtool` for
Rust too.

Before this patch series, we were already getting warnings under e.g. IBT
builds, since those would see Rust code via `vmlinux.o`.
Tested-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240725183325.122827-7-ojeda@kernel.org
[ Solved trivial conflict. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/Makefile

@@ -344,7 +344,8 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
 		--crate-type rlib -L$(objtree)/$(obj) \
 		--crate-name $(patsubst %.o,%,$(notdir $@)) $< \
 		--sysroot=/dev/null \
-	$(if $(rustc_objcopy),;$(OBJCOPY) $(rustc_objcopy) $@)
+	$(if $(rustc_objcopy),;$(OBJCOPY) $(rustc_objcopy) $@) \
+	$(cmd_objtool)
 
 rust-analyzer:
 	$(Q)$(srctree)/scripts/generate_rust_analyzer.py \
@@ -366,44 +367,49 @@ ifneq ($(or $(CONFIG_ARM64),$(and $(CONFIG_RISCV),$(CONFIG_64BIT))),)
 		__ashlti3 __lshrti3
 endif
 
+define rule_rustc_library
+	$(call cmd_and_fixdep,rustc_library)
+	$(call cmd,gen_objtooldep)
+endef
+
 $(obj)/core.o: private skip_clippy = 1
 $(obj)/core.o: private skip_flags = -Wunreachable_pub
 $(obj)/core.o: private rustc_objcopy = $(foreach sym,$(redirect-intrinsics),--redefine-sym $(sym)=__rust$(sym))
 $(obj)/core.o: private rustc_target_flags = $(core-cfgs)
 $(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs FORCE
-	+$(call if_changed_dep,rustc_library)
+	+$(call if_changed_rule,rustc_library)
 ifneq ($(or $(CONFIG_X86_64),$(CONFIG_X86_32)),)
 $(obj)/core.o: scripts/target.json
 endif
 
 $(obj)/compiler_builtins.o: private rustc_objcopy = -w -W '__*'
 $(obj)/compiler_builtins.o: $(src)/compiler_builtins.rs $(obj)/core.o FORCE
-	+$(call if_changed_dep,rustc_library)
+	+$(call if_changed_rule,rustc_library)
 
 $(obj)/alloc.o: private skip_clippy = 1
 $(obj)/alloc.o: private skip_flags = -Wunreachable_pub
 $(obj)/alloc.o: private rustc_target_flags = $(alloc-cfgs)
 $(obj)/alloc.o: $(RUST_LIB_SRC)/alloc/src/lib.rs $(obj)/compiler_builtins.o FORCE
-	+$(call if_changed_dep,rustc_library)
+	+$(call if_changed_rule,rustc_library)
 
 $(obj)/build_error.o: $(src)/build_error.rs $(obj)/compiler_builtins.o FORCE
-	+$(call if_changed_dep,rustc_library)
+	+$(call if_changed_rule,rustc_library)
 
 $(obj)/bindings.o: $(src)/bindings/lib.rs \
     $(obj)/compiler_builtins.o \
     $(obj)/bindings/bindings_generated.rs \
     $(obj)/bindings/bindings_helpers_generated.rs FORCE
-	+$(call if_changed_dep,rustc_library)
+	+$(call if_changed_rule,rustc_library)
 
 $(obj)/uapi.o: $(src)/uapi/lib.rs \
     $(obj)/compiler_builtins.o \
     $(obj)/uapi/uapi_generated.rs FORCE
-	+$(call if_changed_dep,rustc_library)
+	+$(call if_changed_rule,rustc_library)
 
 $(obj)/kernel.o: private rustc_target_flags = --extern alloc \
     --extern build_error --extern macros --extern bindings --extern uapi
 $(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/alloc.o $(obj)/build_error.o \
     $(obj)/libmacros.so $(obj)/bindings.o $(obj)/uapi.o FORCE
-	+$(call if_changed_dep,rustc_library)
+	+$(call if_changed_rule,rustc_library)
 
 endif # CONFIG_RUST

scripts/Makefile.build

@@ -288,10 +288,15 @@ rust_common_cmd = \
 # would not match each other.
 
 quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
-      cmd_rustc_o_rs = $(rust_common_cmd) --emit=obj=$@ $<
+      cmd_rustc_o_rs = $(rust_common_cmd) --emit=obj=$@ $< $(cmd_objtool)
+
+define rule_rustc_o_rs
+	$(call cmd_and_fixdep,rustc_o_rs)
+	$(call cmd,gen_objtooldep)
+endef
 
 $(obj)/%.o: $(obj)/%.rs FORCE
-	+$(call if_changed_dep,rustc_o_rs)
+	+$(call if_changed_rule,rustc_o_rs)
 
 quiet_cmd_rustc_rsi_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
       cmd_rustc_rsi_rs = \