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SubjectRe: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features
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:
>
> > So the syntax will be
> >
> > p[:EVENT] SYM[(CAST)|+OFFS] [FETCHARG]
> >
> > And here is an example;
> >
> > p:myevent vfs_read(void *file, char *buf, size_t count, void *pos) $arg1 $arg2
>
> If we do this, why bother with $arg1 $arg2?

User may want to trace only some of them. :)

>
> We could allow this to be an alternative format?

I think we can skip passing $args, which implies trace all arguments.

p[:EVENT] SYM[(CAST)|+OFFS] [FETCHARG(*)]

*) if SYM(CAST) is given but no FETCHARG, which implies to trace
all arguments in the CAST.

> > In this case inside '()' will be analyzed and packed as something
> > like "reference type" data and it is used when converting "$argN".
> > And maybe we can provide $args special variable to record all
> > arguments (it can be available only when the (CAST) is given).
> >
> > This gives the user a consistent model; if you just give a symbol
> > the arguments may not be correctly translated. but if you give a
> > type-casting information, it will be much better.
> >
> > >
> > > 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.

Thanks!

--
Masami Hiramatsu <mhiramat@kernel.org>

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