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SubjectRe: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features
On Mon, 07 May 2018 13:41:53 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

> Masami Hiramatsu wrote:
> > On Sat, 05 May 2018 13:16:04 +0530
> > "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:
> >
> >> Masami Hiramatsu wrote:
> >> > On Fri, 4 May 2018 12:06:42 -0400
> >> > Steven Rostedt <rostedt@goodmis.org> wrote:
> >> >
> >> >> On Sat, 5 May 2018 00:48:28 +0900
> >> >> Masami Hiramatsu <mhiramat@kernel.org> 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.
>
> Won't that break out-of-tree users depending on returning a non-zero
> value to handle preemption differently? You seem to have alluded to it
> earlier in the mail chain above where you said that this is not just for
> jprobes (though it was added for jprobes as the main use case).

No, all users are in tree already (function override for bpf and error-injection).
And also, for changing execution path by using kprobes, user handler must call
not only preempt_enable(), but also clear current_kprobe per-cpu variable which
is not exported to kmodules.

This means if there is such out-of-tree module, that must change the kernel or
hack the kernel to identify the address of curent_kprobe. If it requires such
a change or hack for the kernel, it is very easy to update the module too.

Thank you,

>
> - Naveen
>
>


--
Masami Hiramatsu <mhiramat@kernel.org>

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