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>
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