linux-kernel - Re: [PATCH 1/2] x86/dumpstack: on oops do not rewind stack for kthread

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Date:   Thu, 20 Oct 2016 16:07:28 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Roman Pen <roman.penyaev@...fitbricks.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/dumpstack: on oops do not rewind stack for kthread

On Wed, Sep 21, 2016 at 8:43 AM, Roman Pen
<roman.penyaev@...fitbricks.com> wrote:
> kthread uses stack and keeps completion structure on it to be woken up
> on vfork_done completion.
>
> In commit 2deb4be28 Andy Lutomirski rewinds the stack unconditionally
> and further completion of task->vfork_done for any kthread leads to stack
> corruption (or infinite spin on attempt to spin lock on garbage memory).

This is sort of okay, but it will blow up pretty badly if a kthread
overflows its stack.  Would it make more sense to change
rewind_stack_do_exit() to leave a big enough gap at the top of the
stack to avoid clobbering the completion?

--Andy