Hi,
On Thu, 22 Aug 2019 12:23:58 +0530
"Naveen N. Rao" <naveen.n....@linux.vnet.ibm.com> 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 <jisheng.zh...@synaptics.com>
>
> This looks good to me. Except for a small confirmation below:
> Reviewed-by: Naveen N. Rao <naveen.n....@linux.vnet.ibm.com>
>
>
<...>
> > +/* 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.
>
>
