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

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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 2022-05-27 19:01:49 +09:00
parent 0cfd90060d
commit c25e1c5582
6 changed files with 28 additions and 67 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
scripts/Kbuild.include
View File

@ -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

63
scripts/Makefile.build
View File

@ -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)

7
scripts/Makefile.lib
View File

@ -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)

5
scripts/Makefile.modfinal
View File

@ -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))

9
scripts/Makefile.modpost
View File

@ -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)

7
scripts/mod/modpost.c
View File

@ -1903,10 +1903,6 @@ static char *remove_dot(char *s)
size_t m = strspn(s + n + 1, "0123456789");
if (m && (s[n + m + 1] == '.' || s[n + m + 1] == 0))
s[n] = 0;
/* strip trailing .prelink */
if (strends(s, ".prelink"))
s[strlen(s) - 8] = '\0';
}
return s;
}
@ -2028,9 +2024,6 @@ static void read_symbols(const char *modname)
/* strip trailing .o */
tmp = NOFAIL(strdup(modname));
tmp[strlen(tmp) - 2] = '\0';
/* strip trailing .prelink */
if (strends(tmp, ".prelink"))
tmp[strlen(tmp) - 8] = '\0';
mod = new_module(tmp);
free(tmp);
}