linux-kernel - Makefile: CC_IMPLICIT_FALLTHROUGH passed quoted as argument to gcc

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Message-ID: <YatpectAYsWnmPy2@eldamar.lan>
Date:   Sat, 4 Dec 2021 14:13:29 +0100
From:   Salvatore Bonaccorso <carnil@...ian.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Makefile: CC_IMPLICIT_FALLTHROUGH passed quoted as argument to gcc

Hi Gustavo,

Since dee2b702bcf0 ("kconfig: Add support for -Wimplicit-fallthrough")
CONFIG_CC_IMPLICIT_FALLTHROUGH value is passed quoted to the gcc
invocation.

This appears to cause issues for (external) module builds. It was
reported in Debian for the nvidia module, cf.
https://bugs.debian.org/1001083 but might happen as well in other
cases.

Andreas suggested to replace the

KBUILD_CFLAGS += $(KBUILD_CFLAGS-y) $(CONFIG_CC_IMPLICIT_FALLTHROUGH)

with

KBUILD_CFLAGS += $(KBUILD_CFLAGS-y) $(patsubst "%",%,$(CONFIG_CC_IMPLICIT_FALLTHROUGH))

Is this something you would consider doing or should the issue be
handled exclusively in the particular OOT module build case?

Regards,
Salvatore