Re: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features

[Naveen N. Rao]

Masami Hiramatsu wrote:
> On Fri, 4 May 2018 12:06:42 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > On Sat, 5 May 2018 00:48:28 +0900
> > Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> >
> > > > Also, when looking at the kprobe code, I was looking at this 
> > > > function:
> > > >   
> > > > > /* Ftrace callback handler for kprobes -- called under preepmt disabed */
> > > > > void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> > > > > 			   struct ftrace_ops *ops, struct pt_regs *regs)
> > > > > {
> > > > > 	struct kprobe *p;
> > > > > 	struct kprobe_ctlblk *kcb;
> > > > > 
> > > > > 	/* Preempt is disabled by ftrace */
> > > > > 	p = get_kprobe((kprobe_opcode_t *)ip);
> > > > > 	if (unlikely(!p) || kprobe_disabled(p))
> > > > > 		return;
> > > > > 
> > > > > 	kcb = get_kprobe_ctlblk();
> > > > > 	if (kprobe_running()) {
> > > > > 		kprobes_inc_nmissed_count(p);
> > > > > 	} else {
> > > > > 		unsigned long orig_ip = regs->ip;
> > > > > 		/* Kprobe handler expects regs->ip = ip + 1 as breakpoint hit */
> > > > > 		regs->ip = ip + sizeof(kprobe_opcode_t);
> > > > > 
> > > > > 		/* To emulate trap based kprobes, preempt_disable here */
> > > > > 		preempt_disable();
> > > > > 		__this_cpu_write(current_kprobe, p);
> > > > > 		kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> > > > > 		if (!p->pre_handler || !p->pre_handler(p, regs)) {
> > > > > 			__skip_singlestep(p, regs, kcb, orig_ip);
> > > > > 			preempt_enable_no_resched();  
> > > > 
> > > > This preemption disabling and enabling looks rather strange. Looking at
> > > > git blame, it appears this was added for jprobes. Can we remove it now
> > > > that jprobes is going away?  
> > > 
> > > No, that is not for jprobes but for compatibility with kprobe's user
> > > handler. Since this transformation is done silently, user can not
> > > change their handler for ftrace case. So we need to keep this condition
> > > same as original kprobes.
> > > 
> > > And anyway, for using smp_processor_id() for accessing per-cpu,
> > > we should disable preemption, correct?
> >
> > But as stated at the start of the function:
> >
> >  /* Preempt is disabled by ftrace */
>
> Ah, yes. So this is only for the jprobes.
>
> > The reason I ask, is that we have for this function:
> >
> > 		/* To emulate trap based kprobes, preempt_disable here */
> > 		preempt_disable();
> > 		__this_cpu_write(current_kprobe, p);
> > 		kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> > 		if (!p->pre_handler || !p->pre_handler(p, regs)) {
> > 			__skip_singlestep(p, regs, kcb, orig_ip);
> > 			preempt_enable_no_resched();
> > 		}
> >
> > And in arch/x86/kernel/kprobes/core.c we have:
> >
> > 	preempt_disable();
> >
> > 	kcb = get_kprobe_ctlblk();
> > 	p = get_kprobe(addr);
> >
> > 	if (p) {
> > 		if (kprobe_running()) {
> > 			if (reenter_kprobe(p, regs, kcb))
> > 				return 1;
> > 		} else {
> > 			set_current_kprobe(p, regs, kcb);
> > 			kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> >
> > 			/*
> > 			 * If we have no pre-handler or it returned 0, we
> > 			 * continue with normal processing.  If we have a
> > 			 * pre-handler and it returned non-zero, it prepped
> > 			 * for calling the break_handler below on re-entry
> > 			 * for jprobe processing, so get out doing nothing
> > 			 * more here.
> > 			 */
> > 			if (!p->pre_handler || !p->pre_handler(p, regs))
> > 				setup_singlestep(p, regs, kcb, 0);
> > 			return 1;
> >
> >
> > Which is why I thought it was for jprobes. I'm a bit confused about
> > where preemption is enabled again.
>
> You're right. So I would like to remove it with x86 jprobe support
> code to avoid inconsistency.

I didn't understand that. Which code are you planning to remove? Can you 
please elaborate? I thought we still need to disable preemption in the 
ftrace handler.

Thanks,
Naveen