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    SubjectRe: [PATCH -tip v5 2/7] kprobes: checks probe address is instruction boudary on x86

    On Mon, 11 May 2009, Masami Hiramatsu wrote:
    >
    > >
    > >> + * by fix_riprel().
    > >> + */
    > >> + memcpy(buf, kp->addr, MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
    > >> + buf[0] = kp->opcode;
    > >
    > > Why is it OK to copy addr to "buf" and then rewrite the first character of
    > > buf? Does it have something to do with the above "p"?
    >
    > Yes, each kprobe copied probed instruction to kp->ainsn.insn,
    > which is an executable buffer for single stepping.
    > So, basically, kp->ainsn.insn has an original instruction.
    > However, RIP-relative instruction can not do single-stepping
    > at different place, fix_riprel() tweaks the displacement of
    > that instruction. In that case, we can't recover the instruction
    > from the kp->ainsn.insn.
    >
    > On the other hand, 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.

    For code that is awkward, complex or non-trivial, don't be afraid to put
    in a paragraph explaining the code. The above explanation should be a
    comment in the code. Otherwise people like me would just look at it and
    say "huh?".

    Note, I'm a bit cranky this morning, so I hope I don't offend anyone.

    -- Steve


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