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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Mark Rutland' <mark.rutland@arm.com>,
        Kees Cook <keescook@chromium.org>
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Subject: RE: [PATCH] proc: Disable /proc/$pid/wchan
Thread-Topic: [PATCH] proc: Disable /proc/$pid/wchan
Thread-Index: AQHXsUu6q24nweQ620C+j1L7pwAX2Ku3nV0w
Date:   Mon, 27 Sep 2021 09:16:17 +0000
Message-ID: <19928a006302450a89b17e7f9b9e4848@AcuMS.aculab.com>
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From: Mark Rutland
> Sent: 24 September 2021 14:54
> 
> On Thu, Sep 23, 2021 at 06:16:16PM -0700, Kees Cook wrote:
> > On Thu, Sep 23, 2021 at 05:22:30PM -0700, Vito Caputo wrote:
> > > Instead of unwinding stacks maybe the kernel should be sticking an
> > > entrypoint address in the current task struct for get_wchan() to
> > > access, whenever userspace enters the kernel?
> >
> > wchan is supposed to show where the kernel is at the instant the
> > get_wchan() happens. (i.e. recording it at syscall entry would just
> > always show syscall entry.)
> 
> It's supposed to show where a blocked task is blocked; the "wait
> channel".
> 
> I'd wanted to remove get_wchan since it requires cross-task stack
> walking, which is generally painful.
> 
> We could instead have the scheduler entrypoints snapshot their caller
> into a field in task_struct. If there are sufficiently few callers, that
> could be an inline wrapper that passes a __func__ string. Otherwise, we
> still need to symbolize.

It ought to be something stashed in the 'wait_queue_head'.
Perhaps defaulting to the address/name of the function that
initialised it.

That would be much nearer the original (pre Linux) semantics.

	David

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