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

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Date:   Fri, 22 Oct 2021 17:57:41 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>, 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,
        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 06:45:14PM +0200, Peter Zijlstra wrote:
> 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.

Heh, we raced to say the same thing. :)

> 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.

FWIW, this shape of thing was one of the reasons I wanted to split
arch_stack_walk() into separate:

* arch_stack_walk_current()
* arch_stack_walk_current_regs()
* arch_stack_walk_blocked_task()

... with similar applying here, since otherwise people won't consider
the distinction between current / !current at the caller level, leading
to junk like this.

Thanks,
Mark.