linux-kernel - Re: [PATCH] selftests: proc: Make sure wchan works when it exists

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Message-ID: <202110212033.D533BAF@keescook>
Date:   Thu, 21 Oct 2021 20:35:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Shuah Khan <shuah@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        linux-kselftest@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Vito Caputo <vcaputo@...garu.com>,
        Ingo Molnar <mingo@...hat.com>, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
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        yifeifz2@...inois.edu, linux-arch@...r.kernel.org,
        vgupta@...nel.org, "Russell King (Oracle)" <linux@...linux.org.uk>,
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        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] selftests: proc: Make sure wchan works when it exists

On Thu, Oct 21, 2021 at 05:39:38PM -0700, Andy Lutomirski wrote:
> 
> 
> On Thu, Oct 21, 2021, at 1:12 PM, Kees Cook wrote:
> > On Thu, Oct 21, 2021 at 01:03:33PM -0700, Andy Lutomirski wrote:
> >> 
> >> 
> >> On Fri, Oct 8, 2021, at 4:55 PM, Kees Cook wrote:
> >> > This makes sure that wchan contains a sensible symbol when a process is
> >> > blocked. Specifically this calls the sleep() syscall, and expects the
> >> > architecture to have called schedule() from a function that has "sleep"
> >> > somewhere in its name. For example, on the architectures I tested
> >> > (x86_64, arm64, arm, mips, and powerpc) this is "hrtimer_nanosleep":
> >> 
> >> Is this really better than admitting that the whole mechanism is nonsense and disabling it?
> >> 
> >> We could have a fixed string for each task state and call it a day.
> >
> > I consider this to be "future work". In earlier discussions it came up,
> > but there wasn't an obvious clean cost-free way to do this, so instead
> > we're just fixing the broken corner and keeping the mostly working rest
> > of it while cleaning up the weird edges. :)
> 
> True, but we have the caveat that wchan is currently broken, so in some sense we have an easier time killing it now as compared to later.  But if we don't have a fully-fleshed-out idea for how to kill it, then I'm fine with waiting.

It's not actually that broken. Only ORC was fully broken, so all the
other architectures (and non-ORC x86) have been fine. But given the
method of fixing ORC vs wchan, it turns out we could further clean up
the other architectures. But yes, no real plan to remove it, but the
current series fixes things pretty well. :)

-Kees

> 
> >
> > -- 
> > Kees Cook

-- 
Kees Cook