Re: [PATCH v2] kbuild: Check for unknown options with cc-option usage in Kconfig and clang

[Nathan Chancellor <natechancellor@gmail.com>]

On Thu, Jul 25, 2019 at 08:47:30AM -0700, Stephen Boyd wrote:
> If the particular version of clang a user has doesn't enable
> -Werror=unknown-warning-option by default, even though it is the
> default[1], then make sure to pass the option to the Kconfig cc-option
> command so that testing options from Kconfig files works properly.
> Otherwise, depending on the default values setup in the clang toolchain
> we will silently assume options such as -Wmaybe-uninitialized are
> supported by clang, when they really aren't.
> 
> A compilation issue only started happening for me once commit
> 589834b3a009 ("kbuild: Add -Werror=unknown-warning-option to
> CLANG_FLAGS") was applied on top of commit b303c6df80c9 ("kbuild:
> compute false-positive -Wmaybe-uninitialized cases in Kconfig"). This
> leads kbuild to try and test for the existence of the
> -Wmaybe-uninitialized flag with the cc-option command in
> scripts/Kconfig.include, and it doesn't see an error returned from the
> option test so it sets the config value to Y. Then the Makefile tries to
> pass the unknown option on the command line and
> -Werror=unknown-warning-option catches the invalid option and breaks the
> build. Before commit 589834b3a009 ("kbuild: Add
> -Werror=unknown-warning-option to CLANG_FLAGS") the build works fine,
> but any cc-option test of a warning option in Kconfig files silently
> evaluates to true, even if the warning option flag isn't supported on
> clang.
> 
> Note: This doesn't change cc-option usages in Makefiles because those
> use a different rule that includes KBUILD_CFLAGS by default (see the
> __cc-option command in scripts/Kbuild.incluide). The KBUILD_CFLAGS
> variable already has the -Werror=unknown-warning-option flag set. Thanks
> to Doug for pointing out the different rule.
> 
> [1] https://clang.llvm.org/docs/DiagnosticsReference.html#wunknown-warning-option
> Cc: Peter Smith <peter.smith@linaro.org>
> Cc: Nathan Chancellor <natechancellor@gmail.com>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
> Signed-off-by: Stephen Boyd <swboyd@chromium.org>

Commit message wording looks better to me, thanks for the change!

I think we might also want to add:

Cc: stable@vger.kernel.org # 4.19+

in addition to Doug's suggested fixes tag because my patch has been
AUTOSEL'd by Sasha:

https://lore.kernel.org/lkml/20190719040732.17285-44-sashal@kernel.org/

https://git.kernel.org/sashal/linux-stable/c/a28859fa4fea5a834a53d86d51e502012ce09c57

Alternatively, I can just mention that this patch needs to be picked up
in addition to that one when it is formally sent out in a stable review.

Cheers,
Nathan