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

[Masami Hiramatsu]

On Sat, 05 May 2018 13:16:04 +0530
"Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:

> 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.

Yes, kprobe_ftrace_handler itself must be run under preempt disabled
because it depends on a per-cpu variable. What I will remove is the
redundant preempt disable/enable_noresched (unbalanced) pair in the
kprobe_ftrace_handler, and jprobe x86 ports which is no more used.

Thank you,


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>