Subject: Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty

From: Ingo Molnar <[email-masked]>
Date: Wed, 29 Mar 2017 08:30:05 +0200

* Masami Hiramatsu <[email-masked]> wrote:

> @@ -1824,6 +1823,30 @@ void unregister_jprobes(struct jprobe **jps, int num)
>  EXPORT_SYMBOL_GPL(unregister_jprobes);
>  
>  #ifdef CONFIG_KRETPROBES
> +
> +/* Try to use free instance first, if failed, try to allocate new instance */
> +struct kretprobe_instance *kretprobe_alloc_instance(struct kretprobe *rp)
> +{
> +	struct kretprobe_instance *ri = NULL;
> +	unsigned long flags = 0;
> +
> +	raw_spin_lock_irqsave(&rp->lock, flags);
> +	if (!hlist_empty(&rp->free_instances)) {
> +		ri = hlist_entry(rp->free_instances.first,
> +				struct kretprobe_instance, hlist);
> +		hlist_del(&ri->hlist);
> +	}
> +	raw_spin_unlock_irqrestore(&rp->lock, flags);
> +
> +	/* Populate max active instance if possible */
> +	if (!ri && rp->maxactive < KRETPROBE_MAXACTIVE_ALLOC) {
> +		ri = kmalloc(sizeof(*ri) + rp->data_size, GFP_ATOMIC);
> +		if (ri)
> +			rp->maxactive++;
> +	}
> +
> +	return ri;
> +}
>  /*
>   * This kprobe pre_handler is registered with every kretprobe. When probe
>   * hits it will set up the return probe.
> @@ -1846,14 +1869,8 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  	}
>  
>  	/* TODO: consider to only swap the RA after the last pre_handler fired */
> -	hash = hash_ptr(current, KPROBE_HASH_BITS);
> -	raw_spin_lock_irqsave(&rp->lock, flags);
> -	if (!hlist_empty(&rp->free_instances)) {
> -		ri = hlist_entry(rp->free_instances.first,
> -				struct kretprobe_instance, hlist);
> -		hlist_del(&ri->hlist);
> -		raw_spin_unlock_irqrestore(&rp->lock, flags);
> -
> +	ri = kretprobe_alloc_instance(rp);
> +	if (ri) {
>  		ri->rp = rp;
>  		ri->task = current;
>  
> @@ -1868,13 +1885,13 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  
>  		/* XXX(hch): why is there no hlist_move_head? */
>  		INIT_HLIST_NODE(&ri->hlist);
> +		hash = hash_ptr(current, KPROBE_HASH_BITS);
>  		kretprobe_table_lock(hash, &flags);
>  		hlist_add_head(&ri->hlist, &kretprobe_inst_table[hash]);
>  		kretprobe_table_unlock(hash, &flags);
> -	} else {
> +	} else
>  		rp->nmissed++;
> -		raw_spin_unlock_irqrestore(&rp->lock, flags);
> -	}
> +
>  	return 0;
>  }
>  NOKPROBE_SYMBOL(pre_handler_kretprobe);

So this is something I missed while the original code was merged, but the concept 
looks a bit weird: why do we do any "allocation" while a handler is executing?

That's fundamentally fragile. What's the maximum number of parallel 
'kretprobe_instance' required per kretprobe - one per CPU?

If so then we should preallocate all of them when they are installed and not do 
any alloc/free dance when executing them.

This will also speed them up, and increase robustness all around.

Thanks,

	Ingo