Messages in this thread | | | Date | Thu, 22 Aug 2019 15:52:05 +0530 | From | "Naveen N. Rao" <> | Subject | Re: [PATCH v4] arm64: implement KPROBES_ON_FTRACE |
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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
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