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: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM-14 (x86-64)
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
This commit is contained in:
Masahiro Yamada
parent
0cfd90060d
commit
c25e1c5582
6 changed files with 28 additions and 67 deletions
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@ -88,10 +88,6 @@ endif
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targets-for-modules := $(foreach x, o mod $(if $(CONFIG_TRIM_UNUSED_KSYMS), usyms), \
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$(patsubst %.o, %.$x, $(filter %.o, $(obj-m))))
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ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),)
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targets-for-modules += $(patsubst %.o, %.prelink.o, $(filter %.o, $(obj-m)))
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endif
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ifdef need-modorder
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targets-for-modules += $(obj)/modules.order
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endif
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@ -152,8 +148,18 @@ $(obj)/%.ll: $(src)/%.c FORCE
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# The C file is compiled and updated dependency information is generated.
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# (See cmd_cc_o_c + relevant part of rule_cc_o_c)
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is-single-obj-m = $(and $(part-of-module),$(filter $@, $(obj-m)),y)
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# When a module consists of a single object, there is no reason to keep LLVM IR.
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# Make $(LD) covert LLVM IR to ELF here.
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ifdef CONFIG_LTO_CLANG
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cmd_ld_single_m = $(if $(is-single-obj-m), ; $(LD) $(ld_flags) -r -o $(tmp-target) $@; mv $(tmp-target) $@)
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endif
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quiet_cmd_cc_o_c = CC $(quiet_modtag) $@
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cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $< $(cmd_objtool)
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cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $< \
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$(cmd_ld_single_m) \
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$(cmd_objtool)
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ifdef CONFIG_MODVERSIONS
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# When module versioning is enabled the following steps are executed:
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@ -219,7 +225,7 @@ objtool_args = \
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$(if $(CONFIG_STACK_VALIDATION), --stackval) \
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$(if $(CONFIG_HAVE_STATIC_CALL_INLINE), --static-call) \
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--uaccess \
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$(if $($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT)), --link) \
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$(if $(delay-objtool), --link) \
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$(if $(part-of-module), --module) \
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$(if $(CONFIG_GCOV_KERNEL), --no-unreachable)
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@ -228,21 +234,15 @@ cmd_gen_objtooldep = $(if $(objtool-enabled), { echo ; echo '$@: $$(wildcard $(o
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endif # CONFIG_OBJTOOL
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ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),)
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# Skip objtool for LLVM bitcode
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$(obj)/%.o: objtool-enabled :=
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else
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# 'OBJECT_FILES_NON_STANDARD := y': skip objtool checking for a directory
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# 'OBJECT_FILES_NON_STANDARD_foo.o := 'y': skip objtool checking for a file
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# 'OBJECT_FILES_NON_STANDARD_foo.o := 'n': override directory skip for a file
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$(obj)/%.o: objtool-enabled = $(if $(filter-out y%, \
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$(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n),y)
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is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n),y)
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endif
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delay-objtool := $(or $(CONFIG_LTO_CLANG),$(CONFIG_X86_KERNEL_IBT))
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$(obj)/%.o: objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y))
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ifdef CONFIG_TRIM_UNUSED_KSYMS
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cmd_gen_ksymdeps = \
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@ -271,24 +271,6 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
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$(call if_changed_rule,cc_o_c)
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$(call cmd,force_checksrc)
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ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),)
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# Module .o files may contain LLVM bitcode, compile them into native code
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# before ELF processing
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quiet_cmd_cc_prelink_modules = LD [M] $@
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cmd_cc_prelink_modules = \
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$(LD) $(ld_flags) -r -o $@ \
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--whole-archive $(filter-out FORCE,$^) \
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$(cmd_objtool)
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# objtool was skipped for LLVM bitcode, run it now that we have compiled
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# modules into native code
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$(obj)/%.prelink.o: objtool-enabled = y
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$(obj)/%.prelink.o: part-of-module := y
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$(obj)/%.prelink.o: $(obj)/%.o FORCE
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$(call if_changed,cc_prelink_modules)
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endif
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cmd_mod = echo $(addprefix $(obj)/, $(call real-search, $*.o, .o, -objs -y -m)) | \
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$(AWK) -v RS='( |\n)' '!x[$$0]++' > $@
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@ -298,7 +280,7 @@ $(obj)/%.mod: FORCE
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# List module undefined symbols
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cmd_undefined_syms = $(NM) $< | sed -n 's/^ *U //p' > $@
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$(obj)/%.usyms: $(obj)/%$(mod-prelink-ext).o FORCE
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$(obj)/%.usyms: $(obj)/%.o FORCE
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$(call if_changed,undefined_syms)
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quiet_cmd_cc_lst_c = MKLST $@
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@ -420,16 +402,11 @@ $(obj)/modules.order: $(obj-m) FORCE
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$(obj)/lib.a: $(lib-y) FORCE
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$(call if_changed,ar)
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ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),)
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quiet_cmd_link_multi-m = AR [M] $@
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cmd_link_multi-m = \
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rm -f $@; \
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$(AR) cDPrsT $@ @$(patsubst %.o,%.mod,$@)
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else
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quiet_cmd_link_multi-m = LD [M] $@
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cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ @$(patsubst %.o,%.mod,$@)
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endif
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cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ @$(patsubst %.o,%.mod,$@) $(cmd_objtool)
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$(multi-obj-m): objtool-enabled := $(delay-objtool)
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$(multi-obj-m): part-of-module := y
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$(multi-obj-m): %.o: %.mod FORCE
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$(call if_changed,link_multi-m)
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$(call multi_depend, $(multi-obj-m), .o, -objs -y -m)
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