linux-kernel - Re: [PATCH] Makefile.extrawarn: re-enable -Wformat for clang

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Date:   Thu, 21 Jul 2022 08:10:27 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>, Tom Rix <trix@...hat.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH] Makefile.extrawarn: re-enable -Wformat for clang

On Thu, Jul 21, 2022 at 07:27:34AM -0700, Nick Desaulniers wrote:
> On Wed, Jul 20, 2022 at 4:23 PM Justin Stitt <justinstitt@...gle.com> wrote:
> >
> > There's been an ongoing mission to re-enable the -Wformat warning for
> > Clang. A previous attempt at enabling the warning showed that there were
> > many instances of this warning throughout the codebase. The sheer amount
> > of these warnings really polluted builds and thus -Wno-format was added
> > to _temporarily_ toggle them off.
> >
> > After many patches the warning has largely been eradicated for x86,
> > x86_64, arm, and arm64 on a variety of configs. The time to enable the
> > warning has never been better as it seems for the first time we are
> > ahead of them and can now solve them as they appear rather than tackling
> > from a backlog.
> >
> > As to the root cause of this large backlog of warnings, Clang seems to
> > pickup on some more nuanced cases of format warnings caused by implicit
> > integer conversion as well as default argument promotions from
> > printf-like functions.
> >
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/378
> > Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> > ---
> > Previous attempt: (https://patchwork.kernel.org/project/linux-kbuild/patch/20190201210853.244043-1-jflat@chromium.org/)
> >
> > Note:
> > For this patch to land on its feet, the plethora of supporting patches that
> > fixed various -Wformat warnings need to be picked up. Thanfully, a lot
> > of them have!
> >
> > Here are the patches still waiting to be picked up:
> > * https://lore.kernel.org/all/20220718230626.1029318-1-justinstitt@google.com/
> > * https://lore.kernel.org/all/20220711222919.2043613-1-justinstitt@google.com/
> 
> Hi Masahiro, Nathan, and Tom,
> What are your thoughts for _when_ in the release cycle this should be
> picked up?  I worry that if we don't remove this soon, we will
> backslide, and more -Wformat issues will crop up making removing this
> in the future like digging in sand.  Justin has chased down many
> instances of this warning, and I'm happy to help clean up fallout from
> landing this.

Let me do a series of builds with the two patches above against
next-20220721 to see if there are any instances of this warning across
the less frequently tested architectures then I will review/ack this.

I don't think we need to worry much about backslide, as -Wformat is
enabled with W=1, which the 0day folks already test with, so new
instances of this warning should get reported to the authors when they
are introduced so they can be fixed immediately. However, I would still
like to see this applied sooner rather than later, although I would also
like us to be completely warning clean before doing so, especially with
-Werror now being selected with all{mod,yes}config. -rc8 is this Sunday
and final should be July 31st so I think this could be applied at some
point between those two dates then maybe sent to Linus for a late pull
request once all other trees have been merged but that is ultimately up
to Masahiro.

Cheers,
Nathan

> >
> >  scripts/Makefile.extrawarn | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> > index f5f0d6f09053..9bbaf7112a9b 100644
> > --- a/scripts/Makefile.extrawarn
> > +++ b/scripts/Makefile.extrawarn
> > @@ -47,7 +47,6 @@ else
> >
> >  ifdef CONFIG_CC_IS_CLANG
> >  KBUILD_CFLAGS += -Wno-initializer-overrides
> > -KBUILD_CFLAGS += -Wno-format
> >  KBUILD_CFLAGS += -Wno-sign-compare
> >  KBUILD_CFLAGS += -Wno-format-zero-length
> >  KBUILD_CFLAGS += $(call cc-disable-warning, pointer-to-enum-cast)
> > --
> > 2.37.0.170.g444d1eabd0-goog
> >
> 
> 
> -- 
> Thanks,
> ~Nick Desaulniers