Re: [PATCH v2 6/6] riscv: Add KPROBES_ON_FTRACE supported

Guo Ren
2020-07-11T01:32:17Z

Thx Masami,

On Fri, Jul 10, 2020 at 9:50 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> Hi Guo,
>
> On Thu, 9 Jul 2020 02:19:14 +0000
> guoren@kernel.org 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;
> > +
> > + 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 {
> > + /*
> > + * The regs->epc hasn't been saved by SAVE_ALL in mcount-dyn.S
> > + * So no need to resume it, just for kprobe handler.
> > + */
> > + instruction_pointer_set(regs, ip);
> > + __this_cpu_write(current_kprobe, p);
> > + kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> > + if (!p->pre_handler || !p->pre_handler(p, regs)) {
> > + /*
> > + * Emulate singlestep (and also recover regs->pc)
> > + * as if there is a nop
> > + */
> > + instruction_pointer_set(regs,
> > + (unsigned long)p->addr + MCOUNT_INSN_SIZE);
> > + if (unlikely(p->post_handler)) {
> > + kcb->kprobe_status = KPROBE_HIT_SSDONE;
> > + p->post_handler(p, regs, 0);
> > + }
>
> Hmm, don't you need restoring the previous instruction pointer here?
look at riscv mcount-dyn.S SAVE_ALL function, sp frame lay out like this:
-----------------------
| return address |
-----------------------
| frame pointer |
-----------------------
| pt_regs x1-x31|
-----------------------
It's not a complete pt_regs for the handler, so modifing regs->ip is no use.

> If you don't support modifying the instruction pointer in the handler,
We can modify ip like this if necessary:
*(unsigned long *)((unsigned long)regs + sizeof(struct pt_regs) + 8) = xxx;

> it must not be compatible with kprobes.
Why, can you show related codes? thank you very much.

>
> Now BPF function override and function error injection depends on
> this behevior, so could you consider to support it in the "ftrace"
> implementation at first? (And if it is enabled, you can enable the
> livepatch on RISCV too)
Great message!

But can you show me codes that bpf and err-jnject using the behavior? Thx

I'll try to fix up it :)

--
Best Regards
Guo Ren

ML: https://lore.kernel.org/linux-csky/