Date:   Fri, 22 Oct 2021 09:25:02 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.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 05:09:35PM +0200, Peter Zijlstra wrote:
> Recent patches to get_wchan() made it more robust by only doing the
> unwind when the task was blocked and serialized against wakeups.
> 
> Extract this functionality as a simpler companion to task_call_func()
> named task_try_func() that really only cares about blocked tasks. Then
> employ this new function to implement the same robustness for
> ARCH_STACKWALK based stack_trace_save_tsk().
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  include/linux/wait.h |    1 
>  kernel/sched/core.c  |   62 ++++++++++++++++++++++++++++++++++++++++++++-------
>  kernel/stacktrace.c  |   13 ++++++----
>  3 files changed, 63 insertions(+), 13 deletions(-)
> 
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -1162,5 +1162,6 @@ int autoremove_wake_function(struct wait
>  
>  typedef int (*task_call_f)(struct task_struct *p, void *arg);
>  extern int task_call_func(struct task_struct *p, task_call_f func, void *arg);
> +extern int task_try_func(struct task_struct *p, task_call_f func, void *arg);
>  
>  #endif /* _LINUX_WAIT_H */
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1966,21 +1966,21 @@ bool sched_task_on_rq(struct task_struct
>  	return task_on_rq_queued(p);
>  }
>  
> +static int try_get_wchan(struct task_struct *p, void *arg)
> +{
> +	unsigned long *wchan = arg;
> +	*wchan = __get_wchan(p);
> +	return 0;
> +}
> +
>  unsigned long get_wchan(struct task_struct *p)
>  {
>  	unsigned long ip = 0;
> -	unsigned int state;
>  
>  	if (!p || p == current)
>  		return 0;
>  
> -	/* Only get wchan if task is blocked and we can keep it that way. */
> -	raw_spin_lock_irq(&p->pi_lock);
> -	state = READ_ONCE(p->__state);
> -	smp_rmb(); /* see try_to_wake_up() */
> -	if (state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq)
> -		ip = __get_wchan(p);
> -	raw_spin_unlock_irq(&p->pi_lock);
> +	task_try_func(p, try_get_wchan, &ip);
>  
>  	return ip;
>  }
> @@ -4184,6 +4184,52 @@ int task_call_func(struct task_struct *p
>  	return ret;
>  }
>  
> +/*
> + * task_try_func - Invoke a function on task in blocked state
> + * @p: Process for which the function is to be invoked
> + * @func: Function to invoke
> + * @arg: Argument to function
> + *
> + * Fix the task in a blocked state, when possible. And if so, invoke @func on it.
> + *
> + * Returns:
> + *  -EBUSY or whatever @func returns
> + */
> +int task_try_func(struct task_struct *p, task_call_f func, void *arg)
> +{
> +	unsigned long flags;
> +	unsigned int state;
> +	int ret = -EBUSY;
> +
> +	raw_spin_lock_irqsave(&p->pi_lock, flags);
> +
> +	state = READ_ONCE(p->__state);
> +
> +	/*
> +	 * Ensure we load p->on_rq after p->__state, otherwise it would be
> +	 * possible to, falsely, observe p->on_rq == 0.
> +	 *
> +	 * See try_to_wake_up() for a longer comment.
> +	 */
> +	smp_rmb();
> +
> +	/*
> +	 * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
> +	 * the task is blocked. Make sure to check @state since ttwu() can drop
> +	 * locks at the end, see ttwu_queue_wakelist().
> +	 */
> +	if (state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq) {
> +		/*
> +		 * The task is blocked and we're holding off wakeupsr. For any
> +		 * of the other task states, see task_call_func().
> +		 */
> +		ret = func(p, arg);
> +	}
> +
> +	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +	return ret;
> +}
> +
>  /**
>   * wake_up_process - Wake up a specific process
>   * @p: The process to be woken up.
> --- a/kernel/stacktrace.c
> +++ b/kernel/stacktrace.c
> @@ -123,6 +123,13 @@ unsigned int stack_trace_save(unsigned l
>  }
>  EXPORT_SYMBOL_GPL(stack_trace_save);
>  
> +static int try_arch_stack_walk_tsk(struct task_struct *tsk, void *arg)
> +{
> +	stack_trace_consume_fn consume_entry = stack_trace_consume_entry_nosched;
> +	arch_stack_walk(consume_entry, arg, tsk, NULL);
> +	return 0;
> +}
> +
>  /**
>   * 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 ?

Assuming this does actually remain callable for current:

Reviewed-by: Kees Cook <keescook@...omium.org>


>  
> -	arch_stack_walk(consume_entry, &c, tsk, NULL);
> -	put_task_stack(tsk);
>  	return c.len;
>  }
>  
> 
> 

-- 
Kees Cook

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