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Date:   Fri, 24 Sep 2021 14:54:24 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Vito Caputo <vcaputo@...garu.com>, Jann Horn <jannh@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Jens Axboe <axboe@...nel.dk>,
        Peter Zijlstra <peterz@...radead.org>,
        Stefan Metzmacher <metze@...ba.org>,
        Andy Lutomirski <luto@...nel.org>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kenta.Tada@...y.com" <Kenta.Tada@...y.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Michael WeiƟ <michael.weiss@...ec.fraunhofer.de>,
        Anand K Mistry <amistry@...gle.com>,
        Alexey Gladkov <legion@...nel.org>,
        Michal Hocko <mhocko@...e.com>, Helge Deller <deller@....de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrea Righi <andrea.righi@...onical.com>,
        Ohhoon Kwon <ohoono.kwon@...sung.com>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] proc: Disable /proc/$pid/wchan

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.

Thanks,
Mark.

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