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Subject[PATCH v2] x86/boot: Fix if_changed build flip/flop
Dirk Gouders reported that two consecutive "make" invocations on an
already compiled tree will show alternating behaviors:

$ make
CALL scripts/checksyscalls.sh
DESCEND objtool
CHK include/generated/compile.h
DATAREL arch/x86/boot/compressed/vmlinux
Kernel: arch/x86/boot/bzImage is ready (#48)
Building modules, stage 2.
MODPOST 165 modules

$ make
CALL scripts/checksyscalls.sh
DESCEND objtool
CHK include/generated/compile.h
LD arch/x86/boot/compressed/vmlinux
ZOFFSET arch/x86/boot/zoffset.h
AS arch/x86/boot/header.o
LD arch/x86/boot/setup.elf
OBJCOPY arch/x86/boot/setup.bin
OBJCOPY arch/x86/boot/vmlinux.bin
BUILD arch/x86/boot/bzImage
Setup is 15644 bytes (padded to 15872 bytes).
System is 6663 kB
CRC 3eb90f40
Kernel: arch/x86/boot/bzImage is ready (#48)
Building modules, stage 2.
MODPOST 165 modules

He bisected it back to:

commit 98f78525371b ("x86/boot: Refuse to build with data relocations")

The root cause was the use of the "if_changed" kbuild function multiple
times for the same target. It was designed to only be used once per
target, otherwise it will effectively always trigger, flipping back and
forth between the two commands getting recorded by "if_changed". Instead,
this patch merges the two commands into a single function to get stable
build artifacts (i.e. .vmlinux.cmd), and a single build behavior.

Reported-by: Dirk Gouders <dirk@gouders.net>
Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
---
v2: improved changelog, added Reviewed-by
---
arch/x86/boot/compressed/Makefile | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index fa42f895fdde..169c2feda14a 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -106,9 +106,13 @@ define cmd_check_data_rel
done
endef

+# We need to run two commands under "if_changed", so merge them into a
+# single invocation.
+quiet_cmd_check-and-link-vmlinux = LD $@
+ cmd_check-and-link-vmlinux = $(cmd_check_data_rel); $(cmd_ld)
+
$(obj)/vmlinux: $(vmlinux-objs-y) FORCE
- $(call if_changed,check_data_rel)
- $(call if_changed,ld)
+ $(call if_changed,check-and-link-vmlinux)

OBJCOPYFLAGS_vmlinux.bin := -R .comment -S
$(obj)/vmlinux.bin: vmlinux FORCE
--
2.17.1

--
Kees Cook
Pixel Security

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