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SubjectRe: 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

> 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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