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Date:   Thu, 23 Sep 2021 19:13:06 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
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        Mark Rutland <mark.rutland@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Stefan Metzmacher <metze@samba.org>,
        Andy Lutomirski <luto@kernel.org>,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
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Subject: Re: [PATCH] proc: Disable /proc/$pid/wchan
Message-Id: <20210923191306.664d39866761778a4a6ea56c@linux-foundation.org>
In-Reply-To: <20210923233105.4045080-1-keescook@chromium.org>
References: <20210923233105.4045080-1-keescook@chromium.org>
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On Thu, 23 Sep 2021 16:31:05 -0700 Kees Cook <keescook@chromium.org> wrote:

> The /proc/$pid/wchan file has been broken by default on x86_64 for 4
> years now[1].

[1] is hard to decrypt.  I think it would be better if this changelog
were to describe the problem directly, completely and succinctly?

> As this remains a potential leak of either kernel
> addresses (when symbolization fails) or limited observation of kernel
> function progress, just remove the contents for good.
> 
> Unconditionally set the contents to "0" and also mark the wchan
> field in /proc/$pid/stat with 0.
> 
> This leaves kernel/sched/fair.c as the only user of get_wchan(). But
> again, since this was broken for 4 years, was this profiling logic
> actually doing anything useful?

Agree that returning a hard-wired "0\n" is the way to go.