Re: kprobe pre_handler change return IP

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2013/10/17 21:57), Liuyongan wrote:
> I use kprobe to probe a function suppose: 
>           int is_winter(int num) { ... }
>           int replace_is_winter(int num) { ...} 
> I want to replace is_winter() with replace_is_winter(), so when we call is_winter, replace_is_winter will be called.
>                                                                                                                   
> so:  
>    int my_pre_handler(struct kprobe *p, struct pt_regs *regs)
>    { 
>         regs->ip = (unsigned long)&replace_is_winter;
>         return 1;
>    } 
> 
> and echo 0 > /proc/sys/debug/kprobes-optimization so that jump instruction will not be used.
> 
> I got a exception in fault_handler, and trap number is 14.
> 
>    fault_handler: p->addr = 0xffffffffa08e201a, ip = ffffffff8021c59d, trap #14n
> 
> Anyone here can help me ?

If you want to replace something with kprobes, the pre_handler must clean current_kprobe up.
Actually the same thing has been done in setup_detour_execution(). So, what you need to do is

>    int my_pre_handler(struct kprobe *p, struct pt_regs *regs)
>    {
>         regs->ip = (unsigned long)&replace_is_winter;

          reset_current_kprobe();
          preempt_enable_no_resched();

>         return 1;
>    }

Happy hacking! ;)


Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com