linux-kernel - Re: [PATCH 1/3] sched/headers: Fix compilation error with GCC 12

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Date:   Thu, 21 Apr 2022 09:32:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christophe de Dinechin <dinechin@...hat.com>,
        trivial@...nel.org, Ben Segall <bsegall@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH 1/3] sched/headers: Fix compilation error with GCC 12

On Wed, Apr 20, 2022 at 11:45:05AM -0700, Kees Cook wrote:

> > -Wno-array-bounds
> 
> Please no; we just spent two years fixing all the old non-flexible array
> definitions and so many other things fixed for this to be enable because
> it finds actual flaws (but we turned it off when it was introduced
> because of how much sloppy old code we had).
> 
> > Is the obvious fix-all cure. The thing is, I want to hear if this new
> > warning has any actual use or is just crack induced madness like many of
> > the warnings we turn off.
> 
> Yes, it finds real flaws. And also yes, it is rather opinionated about
> some "tricks" that have worked in C, but frankly, most of those tricks
> end up being weird/accidentally-correct and aren't great for long-term
> readability or robustness. Though I'm not speaking specifically to this
> proposed patch; I haven't looked closely at it yet.

So the whole access outside object is UB thing in C is complete rubbish
from an OS perspective. The memory is there and there are geniune uses
for it.

And so far, the patches I've seen for it make the code actively worse.
So we need a sane annotation to tell the compiler to shut up already
without making the code an unreadable mess.