[PATCH] kbuild: mark prepare0 as PHONY to fix external module build

[Masahiro Yamada]

Commit c3ff2a5193fa ("powerpc/32: add stack protector support")
caused kernel panic on PowerPC if an external module is used with
CONFIG_STACKPROTECTOR because the 'prepare' target was not executed
for the external module build.

Commit e07db28eea38 ("kbuild: fix single target build for external
module") turned it into a build error because the 'prepare' target is
now executed but the 'prepare0' target is missing for the external
module build.

External module on arm/arm64 with CONFIG_STACKPROTECTOR_PER_TASK is
also broken in the same way.

Move 'PHONY += prepare0' to the common place. Make is fine with missing
rule for phony targets.

I minimize the change so it can be easily backported to 4.20.x

To fix v4.20 for external modules of PowerPC, please backport
e07db28eea38 ("kbuild: fix single target build for external module"),
and then this commit.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=201891
Fixes: e07db28eea38 ("kbuild: fix single target build for external module")
Fixes: c3ff2a5193fa ("powerpc/32: add stack protector support")
Fixes: 189af4657186 ("ARM: smp: add support for per-task stack canaries")
Fixes: 0a1213fa7432 ("arm64: enable per-task stack canaries")
Cc: linux-stable <stable at vger.kernel.org> # v4.20
Reported-by: Samuel Holland <samuel at sholland.org>
Reported-by: Alexey Kardashevskiy <aik at ozlabs.ru>
Signed-off-by: Masahiro Yamada <yamada.masahiro at socionext.com>
---

 Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 499b968..789b332 100644
--- a/Makefile
+++ b/Makefile
@@ -955,6 +955,7 @@ ifdef CONFIG_STACK_VALIDATION
   endif
 endif
 
+PHONY += prepare0
 
 ifeq ($(KBUILD_EXTMOD),)
 core-y		+= kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/
@@ -1061,8 +1062,7 @@ scripts: scripts_basic scripts_dtc
 # archprepare is used in arch Makefiles and when processed asm symlink,
 # version.h and scripts_basic is processed / created.
 
-# Listed in dependency order
-PHONY += prepare archprepare prepare0 prepare1 prepare2 prepare3
+PHONY += prepare archprepare prepare1 prepare2 prepare3
 
 # prepare3 is used to check if we are building in a separate output directory,
 # and if so do:
-- 
2.7.4