Re: [PATCH v4] arm64: implement KPROBES_ON_FTRACE

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

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

> 
> 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.

> 
> 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.


- Naveen