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kbuild: lto: force rebuilds when switching CONFIG_LTO

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When doing non-clean builds and switching between CONFIG_LTO=n and
CONFIG_LTO=y, the build system (correctly) didn't notice that assembly
and LTO-excluded C object files were rewritten in place by objtool (to
add the .orc_unwind* sections), since their build command lines were the
same between CONFIG_LTO=y and CONFIG_LTO=n. The objtool step would fail:

vmlinux.o: warning: objtool: file already has .orc_unwind section, skipping
make: *** [Makefile:1194: vmlinux] Error 255

Avoid this by making sure the build will see a difference between an LTO
and non-LTO build (by including "-fno-lto" in KBUILD_*FLAGS). This will
get ignored when CC_FLAGS_LTO is present, and will not be included at
all when CONFIG_LTO=n.

Change-Id: I7b6a3221d0972ad13db0d138c66f3789eec60062
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
This commit is contained in:
Sami Tolvanen 2021-02-23 13:59:52 -08:00 • committed by Alexander Winkowski
commit 99876597c2
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1 changed files with 2 additions and 1 deletions
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Makefile
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@ -964,7 +964,8 @@ endif
ifdef CONFIG_LTO
CC_FLAGS_LTO := $(CC_FLAGS_LTO_CLANG)
KBUILD_CFLAGS += $(CC_FLAGS_LTO)
KBUILD_CFLAGS += -fno-lto $(CC_FLAGS_LTO)
KBUILD_AFLAGS += -fno-lto
export CC_FLAGS_LTO
endif