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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 20, 2016 at 04:07:28PM -0700, Andy Lutomirski wrote:
> On Wed, Sep 21, 2016 at 8:43 AM, Roman Pen
> <roman.penyaev@profitbricks.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?

We need to preserve the entire struct kthread on the stack, kthread just
abuses that pointer to stash an on-stack kthread descriptor. See
kthread():

  current->vfork_done = &self.exited;

Its a horrible horrible thing kthread does. I suppose there might have
been some intent by keeping that exited completion last in the
structure, but *shudder*.

But yes, leaving enough stack to not clobber that might keep this horror
show working.

ISTR talk about alternative schemes for this a long time ago, but I
cannot recall :-(