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

From: Masami Hiramatsu
Date: Tue Mar 03 2015 - 03:21:33 EST


(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@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxxx


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