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

[Masahiro Yamada]

On Tue, Jan 15, 2019 at 7:45 PM Alexey Kardashevskiy <aik@xxxxxxxxx> wrote:
>
>
>
> On 15/01/2019 18:19, Masahiro Yamada wrote:
> > 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@xxxxxxxxxxxxxxx> # v4.20
> > Reported-by: Samuel Holland <samuel@xxxxxxxxxxxx>
> > Reported-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
>
>
> Tested-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
>
> Thanks for fixing this! Out of curiosity - how does this work?
> arch/powerpc/Makefile's stack_protector_prepare still depends on
> prepare0 which is still defined only if KBUILD_EXTMOD=="" but somehow it
> does not bother ./Makefile? Thanks,


Missing phony targets are ignored.


I do not know if it is officially documented,
but GNU Make works like that.

Is the following test code helpful?


[Test Makefile]

prepare: prepare0
        @echo hello world

.PHONY: prepare prepare0


[Result]
$ make
hello world



-- 
Best Regards
Masahiro Yamada