Commit 5e9e95cc authored by Masahiro Yamada's avatar Masahiro Yamada
Browse files

kbuild: implement CONFIG_TRIM_UNUSED_KSYMS without recursion 

When CONFIG_TRIM_UNUSED_KSYMS is enabled, Kbuild recursively traverses
the directory tree to determine which EXPORT_SYMBOL to trim. If an
EXPORT_SYMBOL turns out to be unused by anyone, Kbuild begins the
second traverse, where some source files are recompiled with their
EXPORT_SYMBOL() tuned into a no-op.

Linus stated negative opinions about this slowness in commits:

 - 5cf0fd59 ("Kbuild: disable TRIM_UNUSED_KSYMS option")
 - a555bdd0 ("Kbuild: enable TRIM_UNUSED_KSYMS again, with some guarding")

We can do this better now. The final data structures of EXPORT_SYMBOL
are generated by the modpost stage, so modpost can selectively emit
KSYMTAB entries that are really used by modules.

Commit f73edc89 ("kbuild: unify two modpost invocations") is another
ground-work to do this in a one-pass algorithm. With the list of modules,
modpost sets sym->used if it is used by a module. modpost emits KSYMTAB
only for symbols with sym->used==true.

BTW, Nicolas explained why the trimming was implemented with recursion:

  https://lore.kernel.org/all/2o2rpn97-79nq-p7s2-nq5-8p83391473r@syhkavp.arg/



Actually, we never achieved that level of optimization where the chain
reaction of trimming comes into play because:

 - CONFIG_LTO_CLANG cannot remove any unused symbols
 - CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled only for vmlinux,
   but not modules

If deeper trimming is required, we need to revisit this, but I guess
that is unlikely to happen.

Signed-off-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
parent 700c48b4

.gitignore

+0 −2
Original line number Diff line number Diff line
@@ -51,7 +51,6 @@ 
*.symversions

*.tab.[ch]

*.tar

*.usyms

*.xz

*.zst

Module.symvers
@@ -112,7 +111,6 @@ modules.order 
#

/include/config/

/include/generated/

/include/ksym/

/arch/*/include/generated/



# stgit generated dirs

Makefile

+3 −19
Original line number Diff line number Diff line
@@ -1193,28 +1193,12 @@ endif 
export KBUILD_VMLINUX_LIBS

export KBUILD_LDS          := arch/$(SRCARCH)/kernel/vmlinux.lds



# Recurse until adjust_autoksyms.sh is satisfied

PHONY += autoksyms_recursive

ifdef CONFIG_TRIM_UNUSED_KSYMS

# For the kernel to actually contain only the needed exported symbols,

# we have to build modules as well to determine what those symbols are.

# (this can be evaluated only once include/config/auto.conf has been included)

KBUILD_MODULES := 1



autoksyms_recursive: $(build-dir) modules.order

	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/adjust_autoksyms.sh \

	  "$(MAKE) -f $(srctree)/Makefile autoksyms_recursive"

endif



autoksyms_h := $(if $(CONFIG_TRIM_UNUSED_KSYMS), include/generated/autoksyms.h)



quiet_cmd_autoksyms_h = GEN     $@

      cmd_autoksyms_h = mkdir -p $(dir $@); \

			$(CONFIG_SHELL) $(srctree)/scripts/gen_autoksyms.sh $@



$(autoksyms_h):

	$(call cmd,autoksyms_h)



# '$(AR) mPi' needs 'T' to workaround the bug of llvm-ar <= 14

quiet_cmd_ar_vmlinux.a = AR      $@

      cmd_ar_vmlinux.a = \
@@ -1223,7 +1207,7 @@ quiet_cmd_ar_vmlinux.a = AR $@ 
	$(AR) mPiT $$($(AR) t $@ | sed -n 1p) $@ $$($(AR) t $@ | grep -F -f $(srctree)/scripts/head-object-list.txt)



targets += vmlinux.a

vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt autoksyms_recursive FORCE

vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt FORCE

	$(call if_changed,ar_vmlinux.a)



PHONY += vmlinux_o
@@ -1279,7 +1263,7 @@ scripts: scripts_basic scripts_dtc 
PHONY += prepare archprepare



archprepare: outputmakefile archheaders archscripts scripts include/config/kernel.release \

	asm-generic $(version_h) $(autoksyms_h) include/generated/utsrelease.h \

	asm-generic $(version_h) include/generated/utsrelease.h \

	include/generated/compile.h include/generated/autoconf.h remove-stale-files



prepare0: archprepare
@@ -2039,7 +2023,7 @@ clean: $(clean-dirs) 
		-o -name '*.dtb.S' -o -name '*.dtbo.S' \

		-o -name '*.dt.yaml' \

		-o -name '*.dwo' -o -name '*.lst' \

		-o -name '*.su' -o -name '*.mod' -o -name '*.usyms' \

		-o -name '*.su' -o -name '*.mod' \

		-o -name '.*.d' -o -name '.*.tmp' -o -name '*.mod.c' \

		-o -name '*.lex.c' -o -name '*.tab.[ch]' \

		-o -name '*.asn1.[ch]' \

include/linux/export.h

+10 −57
Original line number Diff line number Diff line
@@ -42,7 +42,7 @@ extern struct module __this_module; 
	.long sym

#endif



#define ____EXPORT_SYMBOL(sym, license, ns)		\

#define ___EXPORT_SYMBOL(sym, license, ns)		\

	.section ".export_symbol","a"		ASM_NL	\

	__export_symbol_##sym:			ASM_NL	\

		.asciz license			ASM_NL	\
@@ -50,24 +50,6 @@ extern struct module __this_module; 
		__EXPORT_SYMBOL_REF(sym)	ASM_NL	\

	.previous



#ifdef __GENKSYMS__



#define ___EXPORT_SYMBOL(sym, sec, ns)	__GENKSYMS_EXPORT_SYMBOL(sym)



#elif defined(__ASSEMBLY__)



#define ___EXPORT_SYMBOL(sym, license, ns) \

	____EXPORT_SYMBOL(sym, license, ns)



#else



#define ___EXPORT_SYMBOL(sym, license, ns)			\

	extern typeof(sym) sym;					\

	__ADDRESSABLE(sym)					\

	asm(__stringify(____EXPORT_SYMBOL(sym, license, ns)))



#endif



#if !defined(CONFIG_MODULES) || defined(__DISABLE_EXPORTS)



/*
@@ -77,50 +59,21 @@ extern struct module __this_module; 
 */

#define __EXPORT_SYMBOL(sym, sec, ns)



#elif defined(CONFIG_TRIM_UNUSED_KSYMS)

#elif defined(__GENKSYMS__)



#include <generated/autoksyms.h>

#define __EXPORT_SYMBOL(sym, sec, ns)	__GENKSYMS_EXPORT_SYMBOL(sym)



/*

 * For fine grained build dependencies, we want to tell the build system

 * about each possible exported symbol even if they're not actually exported.

 * We use a symbol pattern __ksym_marker_<symbol> that the build system filters

 * from the $(NM) output (see scripts/gen_ksymdeps.sh). These symbols are

 * discarded in the final link stage.

 */



#ifdef __ASSEMBLY__



#define __ksym_marker(sym)					\

	.section ".discard.ksym","a" ;				\

__ksym_marker_##sym: ;						\

	.previous



#else



#define __ksym_marker(sym)	\

	static int __ksym_marker_##sym[0] __section(".discard.ksym") __used

#elif defined(__ASSEMBLY__)



#endif

#define __EXPORT_SYMBOL(sym, license, ns) \

	___EXPORT_SYMBOL(sym, license, ns)



#define __EXPORT_SYMBOL(sym, sec, ns)					\

	__ksym_marker(sym);						\

	__cond_export_sym(sym, sec, ns, __is_defined(__KSYM_##sym))

#define __cond_export_sym(sym, sec, ns, conf)				\

	___cond_export_sym(sym, sec, ns, conf)

#define ___cond_export_sym(sym, sec, ns, enabled)			\

	__cond_export_sym_##enabled(sym, sec, ns)

#define __cond_export_sym_1(sym, sec, ns) ___EXPORT_SYMBOL(sym, sec, ns)



#ifdef __GENKSYMS__

#define __cond_export_sym_0(sym, sec, ns) __GENKSYMS_EXPORT_SYMBOL(sym)

#else

#define __cond_export_sym_0(sym, sec, ns) /* nothing */

#endif



#else



#define __EXPORT_SYMBOL(sym, sec, ns)	___EXPORT_SYMBOL(sym, sec, ns)

#define __EXPORT_SYMBOL(sym, license, ns)			\

	extern typeof(sym) sym;					\

	__ADDRESSABLE(sym)					\

	asm(__stringify(___EXPORT_SYMBOL(sym, license, ns)))



#endif /* CONFIG_MODULES */

scripts/Makefile.build

+1 −14
Original line number Diff line number Diff line
@@ -82,7 +82,7 @@ ifdef need-builtin 
targets-for-builtin += $(obj)/built-in.a

endif



targets-for-modules := $(foreach x, o mod $(if $(CONFIG_TRIM_UNUSED_KSYMS), usyms), \

targets-for-modules := $(foreach x, o mod, \

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



ifdef need-modorder
@@ -217,18 +217,12 @@ is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(basetar 


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



ifdef CONFIG_TRIM_UNUSED_KSYMS

cmd_gen_ksymdeps = \

	$(CONFIG_SHELL) $(srctree)/scripts/gen_ksymdeps.sh $@ >> $(dot-target).cmd

endif



ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),)

cmd_warn_shared_object = $(if $(word 2, $(modname-multi)),$(warning $(kbuild-file): $*.o is added to multiple modules: $(modname-multi)))

endif



define rule_cc_o_c

	$(call cmd_and_fixdep,cc_o_c)

	$(call cmd,gen_ksymdeps)

	$(call cmd,checksrc)

	$(call cmd,checkdoc)

	$(call cmd,gen_objtooldep)
@@ -239,7 +233,6 @@ endef 


define rule_as_o_S

	$(call cmd_and_fixdep,as_o_S)

	$(call cmd,gen_ksymdeps)

	$(call cmd,gen_objtooldep)

	$(call cmd,gen_symversions_S)

	$(call cmd,warn_shared_object)
@@ -258,12 +251,6 @@ cmd_mod = printf '%s\n' $(call real-search, $*.o, .o, -objs -y -m) | \ 
$(obj)/%.mod: FORCE

	$(call if_changed,mod)



# List module undefined symbols

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



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

	$(call if_changed,undefined_syms)



quiet_cmd_cc_lst_c = MKLST   $@

      cmd_cc_lst_c = $(CC) $(c_flags) -g -c -o $*.o $< && \

		     $(CONFIG_SHELL) $(srctree)/scripts/makelst $*.o \

scripts/Makefile.modpost

+7 −0
Original line number Diff line number Diff line
@@ -91,6 +91,13 @@ targets += .vmlinux.objs 
.vmlinux.objs: vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE

	$(call if_changed,vmlinux_objs)



ifdef CONFIG_TRIM_UNUSED_KSYMS

ksym-wl := $(CONFIG_UNUSED_KSYMS_WHITELIST)

ksym-wl := $(if $(filter-out /%, $(ksym-wl)),$(srctree)/)$(ksym-wl)

modpost-args += -t $(addprefix -u , $(ksym-wl))

modpost-deps += $(ksym-wl)

endif



ifeq ($(wildcard vmlinux.o),)

missing-input := vmlinux.o

output-symdump := modules-only.symvers