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Message-ID: <202205162032.5161269A45@keescook>
Date: Mon, 16 May 2022 20:33:25 -0700
From: Kees Cook <keescook@...omium.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Christophe de Dinechin <dinechin@...hat.com>,
Ingo Molnar <mingo@...hat.com>,
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, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] sched/core: Address classes via __begin_sched_classes
On Mon, May 16, 2022 at 08:00:23PM -0700, Kees Cook wrote:
> GCC 12 is very sensitive about array checking, and views all negative
> array accesses as unsafe (a not unreasonable position). Redefine
> sched_class_highest in terms of its location from __begin_sched_classes,
> and redefine sched_class_lowest to the actual lowest sched class instead
> of one lower. This also means the for_class_range() must be redefined to
> be inclusive, which means its 1 caller must be adjusted to have its
> "lowest" argument bumped up one position. Silences this warning:
>
> In file included from kernel/sched/core.c:81:
> kernel/sched/core.c: In function ‘set_rq_online.part.0’:
> kernel/sched/sched.h:2197:52: error: array subscript -1 is outside array bounds of ‘struct sched_class[44343134792571037]’
> [-Werror=array-bounds]
> 2197 | #define sched_class_lowest (__begin_sched_classes - 1)
> | ^
> kernel/sched/sched.h:2200:41: note: in definition of macro ‘for_class_range’
> 2200 | for (class = (_from); class != (_to); class--)
> | ^~~
> kernel/sched/sched.h:2203:53: note: in expansion of macro ‘sched_class_lowest’
> 2203 |for_class_range(class, sched_class_highest, sched_class_lowest)
> | ^~~~~~~~~~~~~~~~~~
> kernel/sched/core.c:9115:17: note: in expansion of macro ‘for_each_class’
> 9115 | for_each_class(class) {
> | ^~~~~~~~~~~~~~
> kernel/sched/sched.h:2193:27: note: at offset -208 into object ‘__begin_sched_classes’ of size [0, 9223372036854775807]
> 2193 | extern struct sched_class __begin_sched_classes[];
> | ^~~~~~~~~~~~~~~~~~~~~
>
> The introduce and use of sched_class_higher() could just be a bare "+ 1",
> but this code's backwards walking and non-inclusive for loop was weird
> enough, it seemed back to explicitly describe the manipulation
> happening.
I just need to start today over. This should read:
The introduction and use of sched_class_higher() could just be a bare
"+ 1", but this code's backwards-walking and non-inclusive for loop was
weird enough, it seemed best to explicitly describe the manipulation
happening.
--
Kees Cook
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