linux-kernel - Re: [PATCH] linux/bits.h: fix -Wtype-limits warnings in GENMASK_INPUT

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Date:   Mon, 7 Mar 2022 13:15:41 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        Rikard Falkeborn <rikard.falkeborn@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] linux/bits.h: fix -Wtype-limits warnings in GENMASK_INPUT_CHECK()

On Mon, Mar 7, 2022 at 11:58 AM Alexander Lobakin
<alexandr.lobakin@...el.com> wrote:
> From: Andy Shevchenko <andy.shevchenko@...il.com>
> > Have you fixed W=1 warnings?
> > Without fixing W=1 (which makes much more sense, when used with
> > WERROR=y && COMPILE_TEST=y) this has no value.
>
> How is this connected?
> When I do `make W=2 path/to/my/code`, I want to see the actual code
> problems, not something that comes from the include files.
> When I do `make W=2 path/to/new/code/from/lkml`, I want to see the
> actual new warnings, not something coming from the includes.
> It's much easier to overlook or miss some real warnings when the
> stderr is being flooded by the warnings from the include files.
> I'm aware there are some scripts to compare before/after, but I
> don't want to use them just because "this has to value".
> I don't want to do `make W=2 KCFLAGS='-Wno-shadow -Wno-type-limits'`
> because then I'm not able to spot the actual shadow or type limit
> problems in my/new code.
> I fixed several `-Wshadow` warnings previously in the include files
> related to networking, and *nobody* said "this has no value" or
> NAKed it. And `-Wshadow` has always been in W=2.

I agree: if we decide that W=2 warnings are completely useless, we should
either remove the option to build a W=2 kernel or remove some of the warning
flags that go into it. My feeling is that both W=2 in general, and the
Wtype-limits have some value, and that reducing the number of W=2 by
30% as this patch does is a useful goal by itself.

A different question is whether this particular patch is the best
workaround for the warnings, or if a nicer alternative can be found,
such as moving -Wtype-limits to W=3, or using an open-coded variant
of __is_constexpr() that includes the comparison in a way that avoids the
warning.

       Arnd