Re: [PATCH] kprobes: x86: cleanup __recover_probed_insn().

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

(2015/03/03 15:39), Wang Nan wrote:
> Since kernel kconfig forbids turning off KPROBES_ON_FTRACE for x86, we
> don't need to consider the situation that a kprobe probing on a ftrace
> location. The only exception should be early kprobe with
> KPROBES_ON_FTRACE enabled. However, it is still impossible for it to get
> a tainted by ftrace if it is registered before ftrace is ready.
> 
> Thus this patch removes unneed logic to make code simpler.

Nak.
Please make sure why this is introduced (and try to check by reproducing it).
https://lkml.org/lkml/2015/2/20/208

Thank you,

> 
> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> ---
>  arch/x86/kernel/kprobes/core.c | 62 ++++++++----------------------------------
>  1 file changed, 12 insertions(+), 50 deletions(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 4e3d5a9..88a99c0 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -219,55 +219,6 @@ retry:
>  	}
>  }
>  
> -static unsigned long
> -__recover_probed_insn(kprobe_opcode_t *buf, unsigned long addr)
> -{
> -	struct kprobe *kp;
> -	unsigned long faddr;
> -
> -	kp = get_kprobe((void *)addr);
> -	faddr = ftrace_location(addr);
> -	/*
> -	 * Addresses inside the ftrace location are refused by
> -	 * arch_check_ftrace_location(). Something went terribly wrong
> -	 * if such an address is checked here.
> -	 */
> -	if (WARN_ON(faddr && faddr != addr))
> -		return 0UL;
> -	/*
> -	 * Use the current code if it is not modified by Kprobe
> -	 * and it cannot be modified by ftrace.
> -	 */
> -	if (!kp && !faddr)
> -		return addr;
> -
> -	/*
> -	 * Basically, kp->ainsn.insn has an original instruction.
> -	 * However, RIP-relative instruction can not do single-stepping
> -	 * at different place, __copy_instruction() tweaks the displacement of
> -	 * that instruction. In that case, we can't recover the instruction
> -	 * from the kp->ainsn.insn.
> -	 *
> -	 * On the other hand, in case on normal Kprobe, kp->opcode has a copy
> -	 * of the first byte of the probed instruction, which is overwritten
> -	 * by int3. And the instruction at kp->addr is not modified by kprobes
> -	 * except for the first byte, we can recover the original instruction
> -	 * from it and kp->opcode.
> -	 *
> -	 * In case of Kprobes using ftrace, we do not have a copy of
> -	 * the original instruction. In fact, the ftrace location might
> -	 * be modified at anytime and even could be in an inconsistent state.
> -	 * Fortunately, we know that the original code is the ideal 5-byte
> -	 * long NOP.
> -	 */
> -	memcpy(buf, (void *)addr, MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
> -	if (faddr)
> -		memcpy(buf, ideal_nops[NOP_ATOMIC5], 5);
> -	else
> -		buf[0] = kp->opcode;
> -	return (unsigned long)buf;
> -}
> -
>  /*
>   * Recover the probed instruction at addr for further analysis.
>   * Caller must lock kprobes by kprobe_mutex, or disable preemption
> @@ -282,7 +233,18 @@ unsigned long recover_probed_instruction(kprobe_opcode_t *buf, unsigned long add
>  	if (__addr != addr)
>  		return __addr;
>  
> -	return __recover_probed_insn(buf, addr);
> +	/*
> +	 * If KPROBES_ON_FTRACE is off, we are not allowed probing at
> +	 * ftrace location. If it is on, we should use
> +	 * arm_kprobe_ftrace() and never get here.  As a result, there
> +	 * is no need to care about confliction between kprobe and
> +	 * ftrace. The only exception should be early kprobes. However,
> +	 * for such kprobes registered before ftrace is ready, it is
> +	 * impossible to get a tainted instruction; for such kprobes
> +	 * registered after ftrace ready, it will use
> +	 * arm_kprobe_ftrace() and won't get here.
> +	 */
> +	return addr;
>  }
>  
>  /* Check if paddr is at an instruction boundary */
> 


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