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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Thu, 7 Feb 2019 09:27:27 +0900
Message-ID: <CAK7LNATAdXh4d39VfNqmLX-V-46sLwVXK+YhE1AiMCDutvUr0w@mail.gmail.com>
Subject: Re: [PATCH] kbuild, LLVMLinux: Don't suppress format warnings
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Jon Flatley <jflat@chromium.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Michal Marek <michal.lkml@markovi.net>
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On Thu, Feb 7, 2019 at 9:24 AM Masahiro Yamada
<yamada.masahiro@socionext.com> wrote:
>
> On Thu, Feb 7, 2019 at 3:03 AM Nathan Chancellor
> <natechancellor@gmail.com> wrote:
> >
> > On Wed, Feb 06, 2019 at 09:36:55AM -0800, Nick Desaulniers wrote:
> > > On Wed, Feb 6, 2019 at 9:32 AM Jon Flatley <jflat@chromium.org> wrote:
> > > >
> > > > On Wed, Feb 6, 2019 at 8:45 AM Nathan Chancellor
> > > > <natechancellor@gmail.com> wrote:
> > > > >
> > > > > On Tue, Feb 05, 2019 at 05:26:05PM +0900, Masahiro Yamada wrote:
> > > > > > On Sat, Feb 2, 2019 at 6:10 AM <jflat@chromium.org> wrote:
> > > > > > >
> > > > > > > From: Jon Flatley <jflat@chromium.org>
> > > > > > >
> > > > > > > gcc produces format warnings that clang suppresses. To keep behavior
> > > > > > > consistent between gcc and clang, don't suppress format warnings in
> > > > > > > clang.
> > > > > > >
> > > > > > > Signed-off-by: Jon Flatley <jflat@chromium.org>
> > > > > > > ---
> > > > > >
> > > > > > Applied to linux-kbuild.
> > > > > > Thanks.
> > > > > >
> > > > > >
> > > > >
> > > > > Hi Jon and Masahiro,
> > > > >
> > > > > Just as a heads up, this introduces a ton of warnings (duh). Isn't the
> > > > > typical plan behind turning on warnings that were disabled to build with
> > > > > 'W=', fix them all, then turn them on so as not to pollute the build?
> > > > >
> > > > > Log file: https://gist.github.com/443db156e56cd3c0f6b21d9d77728d80
> > >
> > > Oh boy, that's a lot.  Too many to fix quickly IMO.
> > >
> > > > >
> > > > > Note a big chunk of them come from one scnprintf call in
> > > > > include/linux/usb/wusb.h but still, there are many other warnings that
> > > > > make quite a bit of noise. Some seem relatively easy to fix, which I
> > > > > suppose I will try to tackle soon.
> > > > >
> > > > > Thanks,
> > > > > Nathan
> > > > >
> > > >
> > > > Hi Nathan,
> > > >
> > > > This was definitely not my intention.
> > > > I noticed the added warnings this morning and was considering calling
> > > > for a revert on this patch.
> > > >
> > > > The intent was to match the behavior of gcc, as it has -Wformat enabled.
> > > > It was rather naive of me to assume the behavior of -Wformat would be
> > > > the same in both gcc and clang.
> > > > Indeed, it seems gcc is more permissive about what format
> > > > substitutions it allows.
> > > >
> >
> > My guess is that it has something to do with how the compilers
> > internally handle certain specifier promotions (GCC probably just
> > silently ignores the 'h' part of the specifier whereas Clang warns) but
> > I didn't do any actual research into the matter. Probably should before
> > looking into all of the warnings :)
> >
> > > > For example passing int to the "%hu" format specifier is fine in gcc
> > > > under -Wformat but produces a warning in clang.
> > > > Maybe this was the motivation for adding -Wno-format to clang in the
> > > > first place.
> > >
> > > Sorry, I'm late to this thread.  What is it reverting; who authored
> > > the original patch? Was it mka@chromium.org?
> > >
> >
> > This patch is turning on '-Wformat' for Clang, which was disabled in
> > commit 3d3d6b847420 ("kbuild: LLVMLinux: Adapt warnings for compilation
> > with clang").
> >
> > > > This difference is puzzling to me, and I wonder if it's by design.
> > >
> > > Probably; internally let's sync up with the Clang devs to understand
> > > this difference more.
> > >
> >
> > Yes, I do think it would be a good idea to turn this on eventually but
> > it'd be wise to understand why Clang emits a warning but GCC doesn't.
> >
> > > >
> > > > Considering the whole point of this patch was to sync up this behavior
> > > > between gcc and clang I am OK with reverting this.
> > >
> > > Is this patch in -next, or has it already hit mainline?  I think it's
> > > better to revert, then start upstreaming fixes, then re-land it once
> > > we're warning free.
> > >
> >
> > It's in linux-kbuild/kbuild, it hasn't hit -next yet.
>
>
> Right, it is just staying in my branch.
> I will drop it.
>
>
> I should have tested this carefully. Sorry.


BTW, it would be nice if kbuild test robot
could test building with clang ...


-- 
Best Regards
Masahiro Yamada