Commit c25e1c55 authored by Masahiro Yamada's avatar Masahiro Yamada
Browse files

kbuild: do not create *.prelink.o for Clang LTO or IBT 



When CONFIG_LTO_CLANG=y, additional intermediate *.prelink.o is created
for each module. Also, objtool is postponed until LLVM IR is converted
to ELF.

CONFIG_X86_KERNEL_IBT works in a similar way to postpone objtool until
objects are merged together.

This commit stops generating *.prelink.o, so the build flow will look
similar with/without LTO.

The following figures show how the LTO build currently works, and
how this commit is changing it.

Current build flow
==================

 [1] single-object module

                                      $(LD)
           $(CC)                     +objtool              $(LD)
    foo.c --------------------> foo.o -----> foo.prelink.o -----> foo.ko
                              (LLVM IR)          (ELF)       |    (ELF)
                                                             |
                                                 foo.mod.o --/
                                                 (LLVM IR)

 [2] multi-object module
                                      $(LD)
           $(CC)         $(AR)       +objtool               $(LD)
    foo1.c -----> foo1.o -----> foo.o -----> foo.prelink.o -----> foo.ko
                           |  (archive)          (ELF)       |    (ELF)
    foo2.c -----> foo2.o --/                                 |
                 (LLVM IR)                       foo.mod.o --/
                                                 (LLVM IR)

  One confusion is that foo.o in multi-object module is an archive
  despite of its suffix.

New build flow
==============

 [1] single-object module

  Since there is only one object, there is no need to keep the LLVM IR.
  Use $(CC)+$(LD) to generate an ELF object in one build rule. When LTO
  is disabled, $(LD) is unneeded because $(CC) produces an ELF object.

               $(CC)+$(LD)+objtool              $(LD)
    foo.c ----------------------------> foo.o ---------> foo.ko
                                        (ELF)     |      (ELF)
                                                  |
                                      foo.mod.o --/
                                      (LLVM IR)

 [2] multi-object module

  Previously, $(AR) was used to combine LLVM IR files into an archive,
  but there was no technical reason to do so. Use $(LD) to merge them
  into a single ELF object.

                               $(LD)
             $(CC)            +objtool          $(LD)
    foo1.c ---------> foo1.o ---------> foo.o ---------> foo.ko
                                 |      (ELF)     |      (ELF)
    foo2.c ---------> foo2.o ----/                |
                     (LLVM IR)        foo.mod.o --/
                                      (LLVM IR)

Signed-off-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
Reviewed-by: default avatarNicolas Schier <nicolas@fjasle.eu>
Tested-by: default avatarNathan Chancellor <nathan@kernel.org>
Reviewed-by: default avatarSami Tolvanen <samitolvanen@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM-14 (x86-64)
Acked-by: default avatarJosh Poimboeuf <jpoimboe@kernel.org>
parent 0cfd9006

scripts/Kbuild.include

+4 −0
Original line number Diff line number Diff line
@@ -15,6 +15,10 @@ pound := \# 
# Name of target with a '.' as filename prefix. foo/bar.o => foo/.bar.o

dot-target = $(dir $@).$(notdir $@)



###

# Name of target with a '.tmp_' as filename prefix. foo/bar.o => foo/.tmp_bar.o

tmp-target = $(dir $@).tmp_$(notdir $@)



###

# The temporary file to save gcc -MMD generated dependencies must not

# contain a comma

scripts/Makefile.build

+20 −43
Original line number Diff line number Diff line
@@ -88,10 +88,6 @@ endif 
targets-for-modules := $(foreach x, o mod $(if $(CONFIG_TRIM_UNUSED_KSYMS), usyms), \

				$(patsubst %.o, %.$x, $(filter %.o, $(obj-m))))



ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),)

targets-for-modules += $(patsubst %.o, %.prelink.o, $(filter %.o, $(obj-m)))

endif



ifdef need-modorder

targets-for-modules += $(obj)/modules.order

endif
@@ -152,8 +148,18 @@ $(obj)/%.ll: $(src)/%.c FORCE 
# The C file is compiled and updated dependency information is generated.

# (See cmd_cc_o_c + relevant part of rule_cc_o_c)



is-single-obj-m = $(and $(part-of-module),$(filter $@, $(obj-m)),y)



# When a module consists of a single object, there is no reason to keep LLVM IR.

# Make $(LD) covert LLVM IR to ELF here.

ifdef CONFIG_LTO_CLANG

cmd_ld_single_m = $(if $(is-single-obj-m), ; $(LD) $(ld_flags) -r -o $(tmp-target) $@; mv $(tmp-target) $@)

endif



quiet_cmd_cc_o_c = CC $(quiet_modtag)  $@

      cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $< $(cmd_objtool)

      cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $< \

		$(cmd_ld_single_m) \

		$(cmd_objtool)



ifdef CONFIG_MODVERSIONS

# When module versioning is enabled the following steps are executed:
@@ -219,7 +225,7 @@ objtool_args = \ 
	$(if $(CONFIG_STACK_VALIDATION), --stackval)			\

	$(if $(CONFIG_HAVE_STATIC_CALL_INLINE), --static-call)		\

	--uaccess							\

	$(if $($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT)), --link)	\

	$(if $(delay-objtool), --link)					\

	$(if $(part-of-module), --module)				\

	$(if $(CONFIG_GCOV_KERNEL), --no-unreachable)
@@ -228,21 +234,15 @@ cmd_gen_objtooldep = $(if $(objtool-enabled), { echo ; echo '$@: $$(wildcard $(o 


endif # CONFIG_OBJTOOL



ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),)



# Skip objtool for LLVM bitcode

$(obj)/%.o: objtool-enabled :=



else



# 'OBJECT_FILES_NON_STANDARD := y': skip objtool checking for a directory

# 'OBJECT_FILES_NON_STANDARD_foo.o := 'y': skip objtool checking for a file

# 'OBJECT_FILES_NON_STANDARD_foo.o := 'n': override directory skip for a file



$(obj)/%.o: objtool-enabled = $(if $(filter-out y%, \

	$(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n),y)

is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n),y)



endif

delay-objtool := $(or $(CONFIG_LTO_CLANG),$(CONFIG_X86_KERNEL_IBT))



$(obj)/%.o: objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y))



ifdef CONFIG_TRIM_UNUSED_KSYMS

cmd_gen_ksymdeps = \
@@ -271,24 +271,6 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE 
	$(call if_changed_rule,cc_o_c)

	$(call cmd,force_checksrc)



ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),)

# Module .o files may contain LLVM bitcode, compile them into native code

# before ELF processing

quiet_cmd_cc_prelink_modules = LD [M]  $@

      cmd_cc_prelink_modules =						\

	$(LD) $(ld_flags) -r -o $@					\

		--whole-archive $(filter-out FORCE,$^)			\

		$(cmd_objtool)



# objtool was skipped for LLVM bitcode, run it now that we have compiled

# modules into native code

$(obj)/%.prelink.o: objtool-enabled = y

$(obj)/%.prelink.o: part-of-module := y



$(obj)/%.prelink.o: $(obj)/%.o FORCE

	$(call if_changed,cc_prelink_modules)

endif



cmd_mod = echo $(addprefix $(obj)/, $(call real-search, $*.o, .o, -objs -y -m)) | \

	$(AWK) -v RS='( |\n)' '!x[$$0]++' > $@
@@ -298,7 +280,7 @@ $(obj)/%.mod: FORCE 
# List module undefined symbols

cmd_undefined_syms = $(NM) $< | sed -n 's/^  *U //p' > $@



$(obj)/%.usyms: $(obj)/%$(mod-prelink-ext).o FORCE

$(obj)/%.usyms: $(obj)/%.o FORCE

	$(call if_changed,undefined_syms)



quiet_cmd_cc_lst_c = MKLST   $@
@@ -420,16 +402,11 @@ $(obj)/modules.order: $(obj-m) FORCE 
$(obj)/lib.a: $(lib-y) FORCE

	$(call if_changed,ar)



ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),)

quiet_cmd_link_multi-m = AR [M]  $@

cmd_link_multi-m =						\

	rm -f $@; 						\

	$(AR) cDPrsT $@ @$(patsubst %.o,%.mod,$@)

else

quiet_cmd_link_multi-m = LD [M]  $@

      cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ @$(patsubst %.o,%.mod,$@)

endif

      cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ @$(patsubst %.o,%.mod,$@) $(cmd_objtool)



$(multi-obj-m): objtool-enabled := $(delay-objtool)

$(multi-obj-m): part-of-module := y

$(multi-obj-m): %.o: %.mod FORCE

	$(call if_changed,link_multi-m)

$(call multi_depend, $(multi-obj-m), .o, -objs -y -m)

scripts/Makefile.lib

+0 −7
Original line number Diff line number Diff line
@@ -225,13 +225,6 @@ dtc_cpp_flags = -Wp,-MMD,$(depfile).pre.tmp -nostdinc \ 
		 $(addprefix -I,$(DTC_INCLUDE))                          \

		 -undef -D__DTS__



ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),)

# With CONFIG_LTO_CLANG, .o files in modules might be LLVM bitcode, so we

# need to run LTO to compile them into native code (.lto.o) before further

# processing.

mod-prelink-ext := .prelink

endif



# Useful for describing the dependency of composite objects

# Usage:

#   $(call multi_depend, multi_used_targets, suffix_to_remove, suffix_to_add)

scripts/Makefile.modfinal

+2 −3
Original line number Diff line number Diff line
@@ -9,7 +9,7 @@ __modfinal: 
include include/config/auto.conf

include $(srctree)/scripts/Kbuild.include



# for c_flags and mod-prelink-ext

# for c_flags

include $(srctree)/scripts/Makefile.lib



# find all modules listed in modules.order
@@ -54,9 +54,8 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check), \ 
	$(cmd);                                                              \

	printf '%s\n' 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd, @:)





# Re-generate module BTFs if either module's .ko or vmlinux changed

$(modules): %.ko: %$(mod-prelink-ext).o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE

$(modules): %.ko: %.o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE

	+$(call if_changed_except,ld_ko_o,vmlinux)

ifdef CONFIG_DEBUG_INFO_BTF_MODULES

	+$(if $(newer-prereqs),$(call cmd,btf_ko))

scripts/Makefile.modpost

+2 −7
Original line number Diff line number Diff line
@@ -41,9 +41,6 @@ __modpost: 
include include/config/auto.conf

include $(srctree)/scripts/Kbuild.include



# for mod-prelink-ext

include $(srctree)/scripts/Makefile.lib



MODPOST = scripts/mod/modpost								\

	$(if $(CONFIG_MODVERSIONS),-m)							\

	$(if $(CONFIG_MODULE_SRCVERSION_ALL),-a)					\
@@ -117,8 +114,6 @@ $(input-symdump): 
	@echo >&2 '         Modules may not have dependencies or modversions.'

	@echo >&2 '         You may get many unresolved symbol warnings.'



modules := $(sort $(shell cat $(MODORDER)))



# KBUILD_MODPOST_WARN can be set to avoid error out in case of undefined symbols

ifneq ($(KBUILD_MODPOST_WARN)$(filter-out $(existing-input-symdump), $(input-symdump)),)

MODPOST += -w
@@ -127,9 +122,9 @@ endif 
# Read out modules.order to pass in modpost.

# Otherwise, allmodconfig would fail with "Argument list too long".

quiet_cmd_modpost = MODPOST $@

      cmd_modpost = sed 's/\.ko$$/$(mod-prelink-ext)\.o/' $< | $(MODPOST) -T -

      cmd_modpost = sed 's/ko$$/o/' $< | $(MODPOST) -T -



$(output-symdump): $(MODORDER) $(input-symdump) $(modules:.ko=$(mod-prelink-ext).o) FORCE

$(output-symdump): $(MODORDER) $(input-symdump) FORCE

	$(call if_changed,modpost)



targets += $(output-symdump)