RE: kprobe pre_handler change return IP

[Liuyongan]

> -----Original Message-----
> From: Masami Hiramatsu [mailto:masami.hiramatsu.pt@xxxxxxxxxxx]
> Sent: Sunday, October 20, 2013 12:00 AM
> To: Liuyongan
> Cc: linux-kernel@xxxxxxxxxxxxxxx; 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@xxxxxxxxxxx
> 

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