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Date:   Sat, 29 Aug 2020 10:23:24 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Eddy_Wu@...ndmicro.com" <Eddy_Wu@...ndmicro.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 19/23] kprobes: Remove kretprobe hash

On Fri, 28 Aug 2020 22:29:12 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, Aug 28, 2020 at 07:32:11PM +0000, Eddy_Wu@...ndmicro.com wrote:
> > 
> > > -----Original Message-----
> > > From: Masami Hiramatsu <mhiramat@...nel.org>
> > >
> > > @@ -1311,24 +1257,23 @@ void kprobe_busy_end(void)
> > >  void kprobe_flush_task(struct task_struct *tk)
> > >  {
> > >         struct kretprobe_instance *ri;
> > > +       struct llist_node *node;
> > >
> > > +       /* Early boot, not yet initialized. */
> > >         if (unlikely(!kprobes_initialized))
> > >                 return;
> > >
> > >         kprobe_busy_begin();
> > >
> > > +       node = current->kretprobe_instances.first;
> > > +       current->kretprobe_instances.first = NULL;
> > 
> > I think we are flushing tk instead of current here.
> > After fixing this to tk, the NULL pointer deference is gone!
> 

Cool!!

> Ah, very good! I can indeed confirm that that cures things.

OK, I'll merge this too.

Thank you,

> 
> ---
> Index: linux-2.6/kernel/kprobes.c
> ===================================================================
> --- linux-2.6.orig/kernel/kprobes.c
> +++ linux-2.6/kernel/kprobes.c
> @@ -1275,9 +1275,7 @@ void kprobe_flush_task(struct task_struc
> 
>  	kprobe_busy_begin();
> 
> -	node = current->kretprobe_instances.first;
> -	current->kretprobe_instances.first = NULL;
> -
> +	node == __llist_del_all(&tk->kretprobe_instances);
>  	while (node) {
>  		ri = container_of(node, struct kretprobe_instance, llist);
>  		node = node->next;
> @@ -1871,6 +1869,7 @@ unsigned long __kretprobe_trampoline_han
>  	struct llist_node *first, *node;
>  	struct kretprobe *rp;
> 
> +	/* Find all nodes for this frame. */
>  	first = node = current->kretprobe_instances.first;
>  	while (node) {
>  		ri = container_of(node, struct kretprobe_instance, llist);
> @@ -1900,7 +1899,7 @@ unsigned long __kretprobe_trampoline_han
>  	while (first) {
>  		ri = container_of(first, struct kretprobe_instance, llist);
>  		rp = get_kretprobe(ri);
> -		node = first->next;
> +		first = first->next;
> 
>  		if (rp && rp->handler) {
>  			__this_cpu_write(current_kprobe, &rp->kp);
> @@ -1910,8 +1909,6 @@ unsigned long __kretprobe_trampoline_han
>  		}
> 
>  		recycle_rp_inst(ri);
> -
> -		first = node;
>  	}
> 
>  	return (unsigned long)correct_ret_addr;
> Index: linux-2.6/include/linux/llist.h
> ===================================================================
> --- linux-2.6.orig/include/linux/llist.h
> +++ linux-2.6/include/linux/llist.h
> @@ -237,6 +237,14 @@ static inline struct llist_node *llist_d
>  	return xchg(&head->first, NULL);
>  }
> 
> +static inline struct llist_node *__llist_del_all(struct llist_head *head)
> +{
> +	struct llist_node *first = head->first;
> +
> +	head->first = NULL;
> +	return first;
> +}
> +
>  extern struct llist_node *llist_del_first(struct llist_head *head);
> 
>  struct llist_node *llist_reverse_order(struct llist_node *head);
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>

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