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: Sami Tolvanen <samitolvanen@google.com> Reviewed-by: Kees Cook <keescook@chromium.org> Signed-off-by: Kees Cook <keescook@chromium.org> Link: https://lore.kernel.org/r/20201211184633.3213045-3-samitolvanen@google.com
This commit is contained in:
Sami Tolvanen
Kees Cook
parent
3b15cdc159
commit
dc5723b02e
19
Makefile
19
Makefile
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@ -893,6 +893,21 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS)
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export CC_FLAGS_SCS
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endif
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ifdef CONFIG_LTO_CLANG
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ifdef CONFIG_LTO_CLANG_THIN
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CC_FLAGS_LTO += -flto=thin -fsplit-lto-unit
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KBUILD_LDFLAGS += --thinlto-cache-dir=$(extmod-prefix).thinlto-cache
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else
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CC_FLAGS_LTO += -flto
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endif
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CC_FLAGS_LTO += -fvisibility=hidden
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endif
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ifdef CONFIG_LTO
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KBUILD_CFLAGS += $(CC_FLAGS_LTO)
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export CC_FLAGS_LTO
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endif
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ifdef CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_32B
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KBUILD_CFLAGS += -falign-functions=32
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endif
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@ -1479,7 +1494,7 @@ MRPROPER_FILES += include/config include/generated \
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*.spec
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# Directories & files removed with 'make distclean'
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DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS
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DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS .thinlto-cache
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# clean - Delete most, but leave enough to build external modules
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#
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@ -1725,7 +1740,7 @@ PHONY += compile_commands.json
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clean-dirs := $(KBUILD_EXTMOD)
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clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \
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$(KBUILD_EXTMOD)/compile_commands.json
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$(KBUILD_EXTMOD)/compile_commands.json $(KBUILD_EXTMOD)/.thinlto-cache
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PHONY += help
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help:
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91
arch/Kconfig
91
arch/Kconfig
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@ -631,6 +631,97 @@ config SHADOW_CALL_STACK
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reading and writing arbitrary memory may be able to locate them
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and hijack control flow by modifying the stacks.
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config LTO
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bool
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help
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Selected if the kernel will be built using the compiler's LTO feature.
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config LTO_CLANG
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bool
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select LTO
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help
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Selected if the kernel will be built using Clang's LTO feature.
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config ARCH_SUPPORTS_LTO_CLANG
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bool
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help
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An architecture should select this option if it supports:
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- compiling with Clang,
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- compiling inline assembly with Clang's integrated assembler,
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- and linking with LLD.
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config ARCH_SUPPORTS_LTO_CLANG_THIN
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bool
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help
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An architecture should select this option if it can support Clang's
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ThinLTO mode.
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config HAS_LTO_CLANG
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def_bool y
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# Clang >= 11: https://github.com/ClangBuiltLinux/linux/issues/510
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depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD
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depends on $(success,test $(LLVM) -eq 1)
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depends on $(success,test $(LLVM_IAS) -eq 1)
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depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm)
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depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)
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depends on ARCH_SUPPORTS_LTO_CLANG
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depends on !FTRACE_MCOUNT_USE_RECORDMCOUNT
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depends on !KASAN
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depends on !GCOV_KERNEL
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depends on !MODVERSIONS
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help
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The compiler and Kconfig options support building with Clang's
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LTO.
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choice
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prompt "Link Time Optimization (LTO)"
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default LTO_NONE
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help
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This option enables Link Time Optimization (LTO), which allows the
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compiler to optimize binaries globally.
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If unsure, select LTO_NONE. Note that LTO is very resource-intensive
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so it's disabled by default.
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config LTO_NONE
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bool "None"
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help
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Build the kernel normally, without Link Time Optimization (LTO).
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config LTO_CLANG_FULL
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bool "Clang Full LTO (EXPERIMENTAL)"
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depends on HAS_LTO_CLANG
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depends on !COMPILE_TEST
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select LTO_CLANG
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help
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This option enables Clang's full Link Time Optimization (LTO), which
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allows the compiler to optimize the kernel globally. If you enable
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this option, the compiler generates LLVM bitcode instead of ELF
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object files, and the actual compilation from bitcode happens at
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the LTO link step, which may take several minutes depending on the
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kernel configuration. More information can be found from LLVM's
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documentation:
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https://llvm.org/docs/LinkTimeOptimization.html
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During link time, this option can use a large amount of RAM, and
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may take much longer than the ThinLTO option.
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config LTO_CLANG_THIN
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bool "Clang ThinLTO (EXPERIMENTAL)"
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depends on HAS_LTO_CLANG && ARCH_SUPPORTS_LTO_CLANG_THIN
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select LTO_CLANG
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help
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This option enables Clang's ThinLTO, which allows for parallel
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optimization and faster incremental compiles compared to the
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CONFIG_LTO_CLANG_FULL option. More information can be found
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from Clang's documentation:
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https://clang.llvm.org/docs/ThinLTO.html
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If unsure, say Y.
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endchoice
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config HAVE_ARCH_WITHIN_STACK_FRAMES
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bool
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help
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@ -90,15 +90,18 @@
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* .data. We don't want to pull in .data..other sections, which Linux
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* has defined. Same for text and bss.
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*
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* With LTO_CLANG, the linker also splits sections by default, so we need
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* these macros to combine the sections during the final link.
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*
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* RODATA_MAIN is not used because existing code already defines .rodata.x
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* sections to be brought in with rodata.
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*/
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#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
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#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
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#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
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#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX*
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#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral*
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#define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
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#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*
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#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
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#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L*
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#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral*
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#define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
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#else
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#define TEXT_MAIN .text
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@ -111,7 +111,7 @@ endif
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# ---------------------------------------------------------------------------
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quiet_cmd_cc_s_c = CC $(quiet_modtag) $@
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cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS), $(c_flags)) -fverbose-asm -S -o $@ $<
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cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS) $(CC_FLAGS_LTO), $(c_flags)) -fverbose-asm -S -o $@ $<
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$(obj)/%.s: $(src)/%.c FORCE
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$(call if_changed_dep,cc_s_c)
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@ -421,8 +421,15 @@ $(obj)/lib.a: $(lib-y) FORCE
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# Do not replace $(filter %.o,^) with $(real-prereqs). When a single object
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# module is turned into a multi object module, $^ will contain header file
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# dependencies recorded in the .*.cmd file.
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ifdef CONFIG_LTO_CLANG
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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 $@ $(filter %.o,$^)
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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 $@ $(filter %.o,$^)
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endif
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$(multi-used-m): FORCE
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$(call if_changed,link_multi-m)
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@ -30,6 +30,12 @@ quiet_cmd_cc_o_c = CC [M] $@
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ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)
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ifdef CONFIG_LTO_CLANG
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# With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to
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# avoid a second slow LTO link
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prelink-ext := .lto
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endif
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quiet_cmd_ld_ko_o = LD [M] $@
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cmd_ld_ko_o = \
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$(LD) -r $(KBUILD_LDFLAGS) \
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@ -53,8 +59,9 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check), \
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$(cmd); \
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printf '%s\n' 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd, @:)
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# Re-generate module BTFs if either module's .ko or vmlinux changed
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$(modules): %.ko: %.o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
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$(modules): %.ko: %$(prelink-ext).o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
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+$(call if_changed_except,ld_ko_o,vmlinux)
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ifdef CONFIG_DEBUG_INFO_BTF_MODULES
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+$(if $(newer-prereqs),$(call cmd,btf_ko))
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@ -43,6 +43,9 @@ __modpost:
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include include/config/auto.conf
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include scripts/Kbuild.include
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# for ld_flags
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include scripts/Makefile.lib
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MODPOST = scripts/mod/modpost \
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$(if $(CONFIG_MODVERSIONS),-m) \
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$(if $(CONFIG_MODULE_SRCVERSION_ALL),-a) \
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@ -102,12 +105,26 @@ $(input-symdump):
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@echo >&2 'WARNING: Symbol version dump "$@" is missing.'
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@echo >&2 ' Modules may not have dependencies or modversions.'
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ifdef CONFIG_LTO_CLANG
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# With CONFIG_LTO_CLANG, .o files might be LLVM bitcode, so we need to run
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# LTO to compile them into native code before running modpost
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prelink-ext := .lto
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quiet_cmd_cc_lto_link_modules = LTO [M] $@
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cmd_cc_lto_link_modules = $(LD) $(ld_flags) -r -o $@ --whole-archive $^
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%.lto.o: %.o
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$(call if_changed,cc_lto_link_modules)
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endif
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modules := $(sort $(shell cat $(MODORDER)))
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# Read out modules.order to pass in modpost.
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# Otherwise, allmodconfig would fail with "Argument list too long".
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quiet_cmd_modpost = MODPOST $@
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cmd_modpost = sed 's/ko$$/o/' $< | $(MODPOST) -T -
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cmd_modpost = sed 's/\.ko$$/$(prelink-ext)\.o/' $< | $(MODPOST) -T -
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$(output-symdump): $(MODORDER) $(input-symdump) FORCE
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$(output-symdump): $(MODORDER) $(input-symdump) $(modules:.ko=$(prelink-ext).o) FORCE
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$(call if_changed,modpost)
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targets += $(output-symdump)
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@ -56,6 +56,14 @@ modpost_link()
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${KBUILD_VMLINUX_LIBS} \
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--end-group"
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if [ -n "${CONFIG_LTO_CLANG}" ]; then
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# This might take a while, so indicate that we're doing
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# an LTO link
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info LTO ${1}
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else
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info LD ${1}
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fi
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${LD} ${KBUILD_LDFLAGS} -r -o ${1} ${objects}
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}
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@ -103,13 +111,22 @@ vmlinux_link()
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fi
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if [ "${SRCARCH}" != "um" ]; then
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objects="--whole-archive \
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${KBUILD_VMLINUX_OBJS} \
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--no-whole-archive \
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--start-group \
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${KBUILD_VMLINUX_LIBS} \
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--end-group \
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${@}"
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if [ -n "${CONFIG_LTO_CLANG}" ]; then
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# Use vmlinux.o instead of performing the slow LTO
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# link again.
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objects="--whole-archive \
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vmlinux.o \
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--no-whole-archive \
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${@}"
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else
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objects="--whole-archive \
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${KBUILD_VMLINUX_OBJS} \
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--no-whole-archive \
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--start-group \
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${KBUILD_VMLINUX_LIBS} \
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--end-group \
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${@}"
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fi
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${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux} \
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${strip_debug#-Wl,} \
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@ -274,7 +291,6 @@ fi;
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${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init need-builtin=1
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#link vmlinux.o
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info LD vmlinux.o
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modpost_link vmlinux.o
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objtool_link vmlinux.o
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