Received-SPF: pass (google.com: domain of linux-kernel+bounces-147801-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Precedence: bulk
MIME-Version: 1.0
References: <20220302043451.2441320-1-willy@infradead.org> <202404161413.8B4810C5@keescook>
In-Reply-To: <202404161413.8B4810C5@keescook>
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 16 Apr 2024 17:29:02 -0700
Message-ID: <CAHk-=wg7+dfpEvDnTnB-eR9QRZ2VySbxOEHbYqL3Ai5wwkon5Q@mail.gmail.com>
Subject: Re: [PATCH 00/19] Enable -Wshadow=local for kernel/sched
To: Kees Cook <keescook@chromium.org>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>, Ingo Molnar <mingo@redhat.com>, 
	Peter Zijlstra <peterz@infradead.org>, Juri Lelli <juri.lelli@redhat.com>, 
	Vincent Guittot <vincent.guittot@linaro.org>, Dietmar Eggemann <dietmar.eggemann@arm.com>, 
	Steven Rostedt <rostedt@goodmis.org>, Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>, 
	Daniel Bristot de Oliveira <bristot@redhat.com>, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"

On Tue, 16 Apr 2024 at 14:15, Kees Cook <keescook@chromium.org> wrote:
>
> I was looking at -Wshadow=local again, and remembered this series. It
> sounded like things were close, but a tweak was needed. What would be
> next to get this working?

So what is the solution to

    #define MAX(a,b) ({ \
        typeof(a) __a = (a); \
        typeof(b) __b = (b); \
        __a > __b ? __a : __b; \
    })

    int test(int a, int b, int c)
    {
        return MAX(a, MAX(b,c));
    }

where -Wshadow=all causes insane warnings that are bogus garbage?

Honestly, Willy's patch-series is a hack to avoid this kind of very
natural nested macro pattern.

But it's a horrible hack, and it does it by making the code actively worse.

Here's the deal: if we can't handle somethng like the above without
warning, -Wshadow isn't getting enabled.

Because we don't write worse code because of bad warnings.

IOW, what is the sane way to just say "this variable can shadow the
use site, and it's fine"?

Without that kind of out, I don't think -Wshadow=local is workable.

              Linus