Re: [PATCH v4] arm64: implement KPROBES_ON_FTRACE

[Jisheng Zhang <Jisheng.Zhang@synaptics.com>]

On Thu, 22 Aug 2019 15:52:05 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

> 
> 
> Jisheng Zhang wrote:
> > Hi,
> >
> > On Thu, 22 Aug 2019 12:23:58 +0530
> > "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:  
> >> Jisheng Zhang wrote:  
> ...
> >> > +/* 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.  
> 
> In this case, the probe will be placed at foo+0x00, so pre_handler()
> seeing that address in pt_regs is correct behavior - as long as arm64
> 'brk' instruction causes an exception with the instruction pointer set

Yep, confirmed with regular trap/brk based kprobes, I do see PC set to
the "brk" instruction.

> *to* the 'brk' instruction. This is similar to how powerpc 'trap' works.
> However, x86 'int3' causes an exception *after* execution of the
> instruction.

Got it. I understand where's the comment "expects regs->pc = pc + 1" from.

> 
> >
> > 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  
> 
> In this case, if user asks for a probe to be placed at 'foo', we will
> choose foo+0x04 and from that point on, the behavior should reflect that
> a kprobe was placed at foo+0x04. In particular, the pre_handler() should
> see foo+0x04 in pt_regs. The post_handler() would then see foo+0x08.
> 
> >
> > 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.  
> 
> I didn't mean to compare regular trap/brk based kprobes with
> KPROBES_ON_FTRACE. The only important aspect is that the handlers see
> consistent pt_regs in both cases, depending on where the kprobe was
> placed. Choosing a different address/offset to place a kprobe during its
> registration is an orthogonal aspect.

Indeed, previously, I want to let the PC point to the same instruction, it
seems I misunderstood the "consistent" meaning.

> 
> >
> > It seems I need to fix the comment.  
> 
> Given your explanation above, I think you can simply drop the first
> adjustment to the instruction pointer before the pre handler invocation.
> The rest of the code looks fine.
> 
> 

Yep, thanks a lot. Will send out a new version soon.