[PATCH 13/27] rust: Annotate the intrinsic stubs as returnable

When building the Rust core library, the compiler emits calls to
compiler-rt builtins like __udivti3() for 128-bit arithmetic, which the
kernel doesn't want, so objcopy is used to redirect those calls to stubs
which just panic.

Problem is, their call sites still expect them to return, which confuses
objtool.

Objtool doesn't report any warnings today because the stubs are weak,
which noreturn detection currently ignores.  In preparation for removing
that restriction, annotate them as returnable.

Signed-off-by: Josh Poimboeuf <[email protected]>
---
 rust/.gitignore |  1 +
 rust/Makefile   | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/rust/.gitignore b/rust/.gitignore
index d3829ffab80ba..c198d1a52e60e 100644
--- a/rust/.gitignore
+++ b/rust/.gitignore
@@ -6,5 +6,6 @@ doctests_kernel_generated.rs
 doctests_kernel_generated_kunit.c
 uapi_generated.rs
 exports_*_generated.h
+intrinsics_annotate_generated.c
 doc/
 test/
diff --git a/rust/Makefile b/rust/Makefile
index 627ed79dc6f50..1cbee6f1ca098 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -33,6 +33,9 @@ endif
 
 obj-$(CONFIG_RUST) += exports.o
 
+always-$(CONFIG_RUST) += intrinsics_annotate_generated.c
+obj-$(CONFIG_RUST) += intrinsics_annotate_generated.o
+
 always-$(CONFIG_RUST) += libproc_macro2.rlib libquote.rlib libsyn.rlib
 
 always-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated.rs
@@ -684,6 +687,20 @@ ifneq ($(or $(CONFIG_ARM64),$(and $(CONFIG_RISCV),$(CONFIG_64BIT))),)
 		__ashlti3 __lshrti3
 endif
 
+# For objtool, annotate the redirect-intrinsics as returnable.  Their call
+# sites were compiled against the returnable intrinsics, but the redirected
+# versions are noreturn.
+quiet_cmd_intrinsics_annotate = GEN     $@
+      cmd_intrinsics_annotate = { \
+	echo '/* Generated by rust/Makefile - do not edit */'; \
+	echo '\#include <linux/annotate.h>'; \
+	for s in $(redirect-intrinsics); do \
+		echo "ANNOTATE_IGNORE_NORETURN(__rust$$s);"; \
+	done; } > $@
+
+$(obj)/intrinsics_annotate_generated.c: $(srctree)/rust/Makefile FORCE
+	$(call if_changed,intrinsics_annotate)
+
 ifdef CONFIG_MODVERSIONS
 cmd_gendwarfksyms = $(if $(skip_gendwarfksyms),, \
 	$(call rust_exports,$@,"%s\n") | \
-- 
2.55.0