linux-kernel - Re: [PATCH 2/7] stacktrace,sched: Make stack_trace_save

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Date:   Fri, 22 Oct 2021 18:45:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, akpm@...ux-foundation.org,
        mark.rutland@....com, zhengqi.arch@...edance.com,
        linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org,
        mpe@...erman.id.au, paul.walmsley@...ive.com, palmer@...belt.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com, borntraeger@...ibm.com,
        linux-arch@...r.kernel.org, ardb@...nel.org
Subject: Re: [PATCH 2/7] stacktrace,sched: Make stack_trace_save_tsk() more
 robust

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.