linux-kernel - Re: [PATCH 00/19] Enable -Wshadow=local for kernel/sched

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Date: Wed, 17 Apr 2024 01:52:28 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/19] Enable -Wshadow=local for kernel/sched

On Tue, Apr 16, 2024 at 05:29:02PM -0700, Linus Torvalds wrote:
> On Tue, 16 Apr 2024 at 14:15, Kees Cook <keescook@...omium.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; \
>     })

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

#define MAX(a, b)	__MAX(a, UNIQUE_ID(a), b, UNIQUE_ID(b))

At least, I think that was the plan.  This was two years ago and I've
mostly forgotten.

>     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