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[PATCH] kbuild: Fail the build early when no lz4 present

From: Borislav Petkov <bp@xxxxxxx>

When building randconfigs, the build fails at kernel compression stage
due to missing lz4 on the system but CONFIG_KERNEL_LZ4 has been selected
by randconfig. The result looks somethins like this:

    (cat arch/x86/boot/compressed/vmlinux.bin arch/x86/boot/compressed/vmlinux.relocs | lz4c -l -c1 stdin stdout && printf \334\141\301\001) > arch/x86/boot/compressed/vmlinux.bin.lz4 || (rm -f arch/x86/boot/compressed/vmlinux.bin.lz4 ; false)
  /bin/sh: 1: lz4c: not found
  make[2]: *** [arch/x86/boot/compressed/Makefile:143: arch/x86/boot/compressed/vmlinux.bin.lz4] Error 1
  make[1]: *** [arch/x86/boot/Makefile:112: arch/x86/boot/compressed/vmlinux] Error 2
  make: *** [arch/x86/Makefile:290: bzImage] Error 2

Fail the build much earlier by checking for lz4c presence before doing
anything else.

Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: Michal Marek <michal.lkml@xxxxxxxxxxx>
Cc: linux-kbuild@xxxxxxxxxxxxxxx
 Makefile | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Makefile b/Makefile
index a0c32de80482..f91de649234b 100644
--- a/Makefile
+++ b/Makefile
@@ -788,6 +788,12 @@ KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
 LDFLAGS_vmlinux += --gc-sections
+ifeq ($(CONFIG_SHELL which lz4c),)
+$(error "lz4 tool not found on this system but CONFIG_KERNEL_LZ4 enabled")
 # arch Makefile may override CC so keep this after arch Makefile is included
 NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)