kprobe pre_handler change return IP

kprobe pre_handler change return IP

[Liuyongan]

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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 ?
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Re: kprobe pre_handler change return IP

[Masami Hiramatsu]

(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

RE: kprobe pre_handler change return IP

[Liuyongan]

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> -----Original Message-----
> From: Masami Hiramatsu [mailto:masami.hiramatsu.pt@hitachi.com]
> Sent: Sunday, October 20, 2013 12:00 AM
> To: Liuyongan
> Cc: linux-kernel@vger.kernel.org; Qianhuibin
> Subject: Re: kprobe pre_handler change return IP
> 
> (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

Great! It works. 
As my_pre_handler() in another modules,  I should export current_kprobe using

EXPORT_PER_CPU_SYMBOL(current_kprobe);

in arch/x86/kernel/kprobes.c right after current_kprobe's definition.

> 
> >    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
> 

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