Re: [PATCH 2/7] stacktrace,sched: Make stack_trace_save_tsk() more robust

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Oct 22, 2021 at 09:25:02AM -0700, Kees Cook wrote:
> On Fri, Oct 22, 2021 at 05:09:35PM +0200, Peter Zijlstra wrote:
> >  /**
> >   * stack_trace_save_tsk - Save a task stack trace into a storage array
> >   * @task:	The task to examine
> > @@ -135,7 +142,6 @@ EXPORT_SYMBOL_GPL(stack_trace_save);
> >  unsigned int stack_trace_save_tsk(struct task_struct *tsk, unsigned long *store,
> >  				  unsigned int size, unsigned int skipnr)
> >  {
> > -	stack_trace_consume_fn consume_entry = stack_trace_consume_entry_nosched;
> >  	struct stacktrace_cookie c = {
> >  		.store	= store,
> >  		.size	= size,
> > @@ -143,11 +149,8 @@ unsigned int stack_trace_save_tsk(struct
> >  		.skip	= skipnr + (current == tsk),
> >  	};
> >  
> > -	if (!try_get_task_stack(tsk))
> > -		return 0;
> > +	task_try_func(tsk, try_arch_stack_walk_tsk, &c);
> 
> Pardon my thin understanding of the scheduler, but I assume this change
> doesn't mean stack_trace_save_tsk() stops working for "current", right?
> In trying to answer this for myself, I couldn't convince myself what value
> current->__state have here. Is it one of TASK_(UN)INTERRUPTIBLE ?

current really shouldn't be using stack_trace_save_tsk(), and no you're
quite right, it will not work for current, irrespective of ->__state,
current will always be ->on_rq.

I started auditing stack_trace_save_tsk() users a few days ago, but
didn't look for this particular issue. I suppose I'll have to start over
with that.