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.peny...@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 :-(
