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

We could allow this to be an alternative format?

>
> 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 */


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.

-- Steve

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