Subject: Re: [PATCH v4] arm64: implement KPROBES_ON_FTRACE

From: Jisheng Zhang <[email-masked]>
Date: Thu, 22 Aug 2019 09:47:13 +0000

Hi,

On Thu, 22 Aug 2019 12:23:58 +0530
"Naveen N. Rao" <[email-masked]> wrote:

> 
> 
> Jisheng Zhang wrote:
> > KPROBES_ON_FTRACE avoids much of the overhead with regular kprobes as it
> > eliminates the need for a trap, as well as the need to emulate or
> > single-step instructions.
> >
> > Tested on berlin arm64 platform.
> >
> > ~ # mount -t debugfs debugfs /sys/kernel/debug/
> > ~ # cd /sys/kernel/debug/
> > /sys/kernel/debug # echo 'p _do_fork' > tracing/kprobe_events
> >
> > before the patch:
> >
> > /sys/kernel/debug # cat kprobes/list
> > ffffff801009fe28  k  _do_fork+0x0    [DISABLED]
> >
> > after the patch:
> >
> > /sys/kernel/debug # cat kprobes/list
> > ffffff801009ff54  k  _do_fork+0x4    [DISABLED][FTRACE]
> >
> > Signed-off-by: Jisheng Zhang <[email-masked]>  
> 
> This looks good to me. Except for a small confirmation below:
> Reviewed-by: Naveen N. Rao <[email-masked]>
> 
> 

<...>

> > +/* 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 = instruction_pointer(regs);
> > +             /* Kprobe handler expects regs->pc = pc + 4 as breakpoint hit */
> > +             instruction_pointer_set(regs, ip + sizeof(kprobe_opcode_t));  
> 
> Just want to make sure that you've confirmed that this is what happens
> with a regular trap/brk based kprobe on ARM64. The reason for setting
> the instruction pointer here is to ensure that it is set to the same
> value as would be set if there was a trap/brk instruction at the ftrace
> location. This ensures that the kprobe pre handler sees the same value
> regardless.

Due to the arm64's DYNAMIC_FTRACE_WITH_REGS implementation, the code itself
is correct. But this doesn't look like "there was a trap instruction at
the ftrace location".

W/O KPROBE_ON_FTRACE:

foo:
00	insA
04	insB
08	insC

kprobe's pre_handler() will see pc points to 00.

W/ KPROBE_ON_FTRACE:

foo:
00	lr saver
04	nop     // will be modified to ftrace call ins when KPROBE is armed
08	insA
0c	insB

later, kprobe_ftrace_handler() will see pc points to 04, so pc + 4 will
point to 08 the same as the one w/o KPROBE_ON_FTRACE.

It seems I need to fix the comment.

> 
> Further changes to the instruction pointer are to achieve the same
> effect for kprobe post handlers.
> 
> 

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