kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 165 insertions, 0 deletions

diff --git a/scripts/Makefile.host b/scripts/Makefile.host
new file mode 100644
index 000000000..c1dedf646
--- /dev/null
+++ b/scripts/Makefile.host
@@ -0,0 +1,165 @@
+# SPDX-License-Identifier: GPL-2.0
+
+# LEX
+# ---------------------------------------------------------------------------
+quiet_cmd_flex = LEX     $@
+      cmd_flex = $(LEX) -o$@ -L $<
+
+$(obj)/%.lex.c: $(src)/%.l FORCE
+	$(call if_changed,flex)
+
+# YACC
+# ---------------------------------------------------------------------------
+quiet_cmd_bison = YACC    $(basename $@).[ch]
+      cmd_bison = $(YACC) -o $(basename $@).c --defines=$(basename $@).h -t -l $<
+
+$(obj)/%.tab.c $(obj)/%.tab.h: $(src)/%.y FORCE
+	$(call if_changed,bison)
+
+# ==========================================================================
+# Building binaries on the host system
+# Binaries are used during the compilation of the kernel, for example
+# to preprocess a data file.
+#
+# Both C and C++ are supported, but preferred language is C for such utilities.
+# Rust is also supported, but it may only be used in scenarios where a Rust
+# toolchain is required to be available (e.g. when  `CONFIG_RUST` is enabled).
+#
+# Sample syntax (see Documentation/kbuild/makefiles.rst for reference)
+# hostprogs := bin2hex
+# Will compile bin2hex.c and create an executable named bin2hex
+#
+# hostprogs     := lxdialog
+# lxdialog-objs := checklist.o lxdialog.o
+# Will compile lxdialog.c and checklist.c, and then link the executable
+# lxdialog, based on checklist.o and lxdialog.o
+#
+# hostprogs       := qconf
+# qconf-cxxobjs   := qconf.o
+# qconf-objs      := menu.o
+# Will compile qconf as a C++ program, and menu as a C program.
+# They are linked as C++ code to the executable qconf
+#
+# hostprogs   := target
+# target-rust := y
+# Will compile `target` as a Rust program, using `target.rs` as the crate root.
+# The crate may consist of several source files.
+
+# C code
+# Executables compiled from a single .c file
+host-csingle	:= $(foreach m,$(hostprogs), \
+			$(if $($(m)-objs)$($(m)-cxxobjs)$($(m)-rust),,$(m)))
+
+# C executables linked based on several .o files
+host-cmulti	:= $(foreach m,$(hostprogs),\
+		   $(if $($(m)-cxxobjs)$($(m)-rust),,$(if $($(m)-objs),$(m))))
+
+# Object (.o) files compiled from .c files
+host-cobjs	:= $(sort $(foreach m,$(hostprogs),$($(m)-objs)))
+
+# C++ code
+# C++ executables compiled from at least one .cc file
+# and zero or more .c files
+host-cxxmulti	:= $(foreach m,$(hostprogs),$(if $($(m)-cxxobjs),$(m)))
+
+# C++ Object (.o) files compiled from .cc files
+host-cxxobjs	:= $(sort $(foreach m,$(host-cxxmulti),$($(m)-cxxobjs)))
+
+# Rust code
+# Executables compiled from a single Rust crate (which may consist of
+# one or more .rs files)
+host-rust	:= $(foreach m,$(hostprogs),$(if $($(m)-rust),$(m)))
+
+host-csingle	:= $(addprefix $(obj)/,$(host-csingle))
+host-cmulti	:= $(addprefix $(obj)/,$(host-cmulti))
+host-cobjs	:= $(addprefix $(obj)/,$(host-cobjs))
+host-cxxmulti	:= $(addprefix $(obj)/,$(host-cxxmulti))
+host-cxxobjs	:= $(addprefix $(obj)/,$(host-cxxobjs))
+host-rust	:= $(addprefix $(obj)/,$(host-rust))
+
+#####
+# Handle options to gcc. Support building with separate output directory
+
+hostc_flags    = -Wp,-MMD,$(depfile) \
+                 $(KBUILD_HOSTCFLAGS) $(HOST_EXTRACFLAGS) \
+                 $(HOSTCFLAGS_$(target-stem).o)
+hostcxx_flags  = -Wp,-MMD,$(depfile) \
+                 $(KBUILD_HOSTCXXFLAGS) $(HOST_EXTRACXXFLAGS) \
+                 $(HOSTCXXFLAGS_$(target-stem).o)
+
+# `--out-dir` is required to avoid temporaries being created by `rustc` in the
+# current working directory, which may be not accessible in the out-of-tree
+# modules case.
+hostrust_flags = --out-dir $(dir $@) --emit=dep-info=$(depfile) \
+		 -Clinker-flavor=gcc -Clinker=$(HOSTCC) \
+		 -Clink-args='$(call escsq,$(KBUILD_HOSTLDFLAGS))' \
+                 $(KBUILD_HOSTRUSTFLAGS) $(HOST_EXTRARUSTFLAGS) \
+                 $(HOSTRUSTFLAGS_$(target-stem))
+
+# $(obj) for including generated headers from checkin source files
+ifdef building_out_of_srctree
+hostc_flags   += -I $(obj)
+hostcxx_flags += -I $(obj)
+endif
+
+#####
+# Compile programs on the host
+
+# Create executable from a single .c file
+# host-csingle -> Executable
+quiet_cmd_host-csingle 	= HOSTCC  $@
+      cmd_host-csingle	= $(HOSTCC) $(hostc_flags) $(KBUILD_HOSTLDFLAGS) -o $@ $< \
+		$(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(target-stem))
+$(host-csingle): $(obj)/%: $(obj)/%.c FORCE
+	$(call if_changed_dep,host-csingle)
+
+# Link an executable based on list of .o files, all plain c
+# host-cmulti -> executable
+quiet_cmd_host-cmulti	= HOSTLD  $@
+      cmd_host-cmulti	= $(HOSTCC) $(KBUILD_HOSTLDFLAGS) -o $@ \
+			  $(addprefix $(obj)/, $($(target-stem)-objs)) \
+			  $(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(target-stem))
+$(host-cmulti): FORCE
+	$(call if_changed,host-cmulti)
+$(call multi_depend, $(host-cmulti), , -objs)
+
+# Create .o file from a single .c file
+# host-cobjs -> .o
+quiet_cmd_host-cobjs	= HOSTCC  $@
+      cmd_host-cobjs	= $(HOSTCC) $(hostc_flags) -c -o $@ $<
+$(host-cobjs): $(obj)/%.o: $(obj)/%.c FORCE
+	$(call if_changed_dep,host-cobjs)
+
+# Link an executable based on list of .o files, a mixture of .c and .cc
+# host-cxxmulti -> executable
+quiet_cmd_host-cxxmulti	= HOSTLD  $@
+      cmd_host-cxxmulti	= $(HOSTCXX) $(KBUILD_HOSTLDFLAGS) -o $@ \
+			  $(foreach o,objs cxxobjs,\
+			  $(addprefix $(obj)/, $($(target-stem)-$(o)))) \
+			  $(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(target-stem))
+$(host-cxxmulti): FORCE
+	$(call if_changed,host-cxxmulti)
+$(call multi_depend, $(host-cxxmulti), , -objs -cxxobjs)
+
+# Create .o file from a single .cc (C++) file
+quiet_cmd_host-cxxobjs	= HOSTCXX $@
+      cmd_host-cxxobjs	= $(HOSTCXX) $(hostcxx_flags) -c -o $@ $<
+$(host-cxxobjs): $(obj)/%.o: $(obj)/%.cc FORCE
+	$(call if_changed_dep,host-cxxobjs)
+
+# Create executable from a single Rust crate (which may consist of
+# one or more `.rs` files)
+# host-rust -> Executable
+quiet_cmd_host-rust	= HOSTRUSTC $@
+      cmd_host-rust	= \
+	$(HOSTRUSTC) $(hostrust_flags) --emit=link=$@ $<
+$(host-rust): $(obj)/%: $(src)/%.rs FORCE
+	+$(call if_changed_dep,host-rust)
+
+targets += $(host-csingle) $(host-cmulti) $(host-cobjs) \
+	   $(host-cxxmulti) $(host-cxxobjs) $(host-rust)
+
+# %.lex.o <- %.lex.c <- %.l
+# %.tab.o <- %.tab.[ch] <- %.y
+targets += $(call intermediate_targets, .lex.o, .lex.c) \
+           $(call intermediate_targets, .tab.o, .tab.c .tab.h)