Commit dc5723b0 authored by Sami Tolvanen's avatar Sami Tolvanen Committed by Kees Cook
Browse files

kbuild: add support for Clang LTO 

This change adds build system support for Clang's Link Time
Optimization (LTO). With -flto, instead of ELF object files, Clang
produces LLVM bitcode, which is compiled into native code at link
time, allowing the final binary to be optimized globally. For more
details, see:

  https://llvm.org/docs/LinkTimeOptimization.html

The Kconfig option CONFIG_LTO_CLANG is implemented as a choice,
which defaults to LTO being disabled. To use LTO, the architecture
must select ARCH_SUPPORTS_LTO_CLANG and support:

  - compiling with Clang,
  - compiling all assembly code with Clang's integrated assembler,
  - and linking with LLD.

While using CONFIG_LTO_CLANG_FULL results in the best runtime
performance, the compilation is not scalable in time or
memory. CONFIG_LTO_CLANG_THIN enables ThinLTO, which allows
parallel optimization and faster incremental builds. ThinLTO is
used by default if the architecture also selects
ARCH_SUPPORTS_LTO_CLANG_THIN:

  https://clang.llvm.org/docs/ThinLTO.html



To enable LTO, LLVM tools must be used to handle bitcode files, by
passing LLVM=1 and LLVM_IAS=1 options to make:

  $ make LLVM=1 LLVM_IAS=1 defconfig
  $ scripts/config -e LTO_CLANG_THIN
  $ make LLVM=1 LLVM_IAS=1

To prepare for LTO support with other compilers, common parts are
gated behind the CONFIG_LTO option, and LTO can be disabled for
specific files by filtering out CC_FLAGS_LTO.

Signed-off-by: default avatarSami Tolvanen <samitolvanen@google.com>
Reviewed-by: default avatarKees Cook <keescook@chromium.org>
Signed-off-by: default avatarKees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201211184633.3213045-3-samitolvanen@google.com
parent 3b15cdc1

Makefile

+17 −2
Original line number Diff line number Diff line
@@ -893,6 +893,21 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS) 
export CC_FLAGS_SCS

endif



ifdef CONFIG_LTO_CLANG

ifdef CONFIG_LTO_CLANG_THIN

CC_FLAGS_LTO	+= -flto=thin -fsplit-lto-unit

KBUILD_LDFLAGS	+= --thinlto-cache-dir=$(extmod-prefix).thinlto-cache

else

CC_FLAGS_LTO	+= -flto

endif

CC_FLAGS_LTO	+= -fvisibility=hidden

endif



ifdef CONFIG_LTO

KBUILD_CFLAGS	+= $(CC_FLAGS_LTO)

export CC_FLAGS_LTO

endif



ifdef CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_32B

KBUILD_CFLAGS += -falign-functions=32

endif
@@ -1479,7 +1494,7 @@ MRPROPER_FILES += include/config include/generated \ 
		  *.spec



# Directories & files removed with 'make distclean'

DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS

DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS .thinlto-cache



# clean - Delete most, but leave enough to build external modules

#
@@ -1725,7 +1740,7 @@ PHONY += compile_commands.json 


clean-dirs := $(KBUILD_EXTMOD)

clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \

	$(KBUILD_EXTMOD)/compile_commands.json

	$(KBUILD_EXTMOD)/compile_commands.json $(KBUILD_EXTMOD)/.thinlto-cache



PHONY += help

help:

arch/Kconfig

+91 −0
Original line number Diff line number Diff line
@@ -631,6 +631,97 @@ config SHADOW_CALL_STACK 
	  reading and writing arbitrary memory may be able to locate them

	  and hijack control flow by modifying the stacks.



config LTO

	bool

	help

	  Selected if the kernel will be built using the compiler's LTO feature.



config LTO_CLANG

	bool

	select LTO

	help

	  Selected if the kernel will be built using Clang's LTO feature.



config ARCH_SUPPORTS_LTO_CLANG

	bool

	help

	  An architecture should select this option if it supports:

	  - compiling with Clang,

	  - compiling inline assembly with Clang's integrated assembler,

	  - and linking with LLD.



config ARCH_SUPPORTS_LTO_CLANG_THIN

	bool

	help

	  An architecture should select this option if it can support Clang's

	  ThinLTO mode.



config HAS_LTO_CLANG

	def_bool y

	# Clang >= 11: https://github.com/ClangBuiltLinux/linux/issues/510

	depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD

	depends on $(success,test $(LLVM) -eq 1)

	depends on $(success,test $(LLVM_IAS) -eq 1)

	depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm)

	depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)

	depends on ARCH_SUPPORTS_LTO_CLANG

	depends on !FTRACE_MCOUNT_USE_RECORDMCOUNT

	depends on !KASAN

	depends on !GCOV_KERNEL

	depends on !MODVERSIONS

	help

	  The compiler and Kconfig options support building with Clang's

	  LTO.



choice

	prompt "Link Time Optimization (LTO)"

	default LTO_NONE

	help

	  This option enables Link Time Optimization (LTO), which allows the

	  compiler to optimize binaries globally.



	  If unsure, select LTO_NONE. Note that LTO is very resource-intensive

	  so it's disabled by default.



config LTO_NONE

	bool "None"

	help

	  Build the kernel normally, without Link Time Optimization (LTO).



config LTO_CLANG_FULL

	bool "Clang Full LTO (EXPERIMENTAL)"

	depends on HAS_LTO_CLANG

	depends on !COMPILE_TEST

	select LTO_CLANG

	help

          This option enables Clang's full Link Time Optimization (LTO), which

          allows the compiler to optimize the kernel globally. If you enable

          this option, the compiler generates LLVM bitcode instead of ELF

          object files, and the actual compilation from bitcode happens at

          the LTO link step, which may take several minutes depending on the

          kernel configuration. More information can be found from LLVM's

          documentation:



	    https://llvm.org/docs/LinkTimeOptimization.html



	  During link time, this option can use a large amount of RAM, and

	  may take much longer than the ThinLTO option.



config LTO_CLANG_THIN

	bool "Clang ThinLTO (EXPERIMENTAL)"

	depends on HAS_LTO_CLANG && ARCH_SUPPORTS_LTO_CLANG_THIN

	select LTO_CLANG

	help

	  This option enables Clang's ThinLTO, which allows for parallel

	  optimization and faster incremental compiles compared to the

	  CONFIG_LTO_CLANG_FULL option. More information can be found

	  from Clang's documentation:



	    https://clang.llvm.org/docs/ThinLTO.html



	  If unsure, say Y.

endchoice



config HAVE_ARCH_WITHIN_STACK_FRAMES

	bool

	help

include/asm-generic/vmlinux.lds.h

+7 −4
Original line number Diff line number Diff line
@@ -90,15 +90,18 @@ 
 * .data. We don't want to pull in .data..other sections, which Linux

 * has defined. Same for text and bss.

 *

 * With LTO_CLANG, the linker also splits sections by default, so we need

 * these macros to combine the sections during the final link.

 *

 * RODATA_MAIN is not used because existing code already defines .rodata.x

 * sections to be brought in with rodata.

 */

#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION

#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)

#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*

#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX*

#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral*

#define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*

#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*

#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*

#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L*

#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral*

#define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*

#else

#define TEXT_MAIN .text

scripts/Makefile.build

+8 −1
Original line number Diff line number Diff line
@@ -111,7 +111,7 @@ endif 
# ---------------------------------------------------------------------------



quiet_cmd_cc_s_c = CC $(quiet_modtag)  $@

      cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS), $(c_flags)) -fverbose-asm -S -o $@ $<

      cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS) $(CC_FLAGS_LTO), $(c_flags)) -fverbose-asm -S -o $@ $<



$(obj)/%.s: $(src)/%.c FORCE

	$(call if_changed_dep,cc_s_c)
@@ -421,8 +421,15 @@ $(obj)/lib.a: $(lib-y) FORCE 
# Do not replace $(filter %.o,^) with $(real-prereqs). When a single object

# module is turned into a multi object module, $^ will contain header file

# dependencies recorded in the .*.cmd file.

ifdef CONFIG_LTO_CLANG

quiet_cmd_link_multi-m = AR [M]  $@

cmd_link_multi-m =						\

	rm -f $@; 						\

	$(AR) cDPrsT $@ $(filter %.o,$^)

else

quiet_cmd_link_multi-m = LD [M]  $@

      cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ $(filter %.o,$^)

endif



$(multi-used-m): FORCE

	$(call if_changed,link_multi-m)

scripts/Makefile.modfinal

+8 −1
Original line number Diff line number Diff line
@@ -30,6 +30,12 @@ quiet_cmd_cc_o_c = CC [M] $@ 


ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)



ifdef CONFIG_LTO_CLANG

# With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to

# avoid a second slow LTO link

prelink-ext := .lto

endif



quiet_cmd_ld_ko_o = LD [M]  $@

      cmd_ld_ko_o =                                                     \

	$(LD) -r $(KBUILD_LDFLAGS)					\
@@ -53,8 +59,9 @@ 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: %.o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE

$(modules): %.ko: %$(prelink-ext).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))