Steven Rostedt wrote: > 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. No, that's very helpful review for me. Thanks :-) -- Masami Hiramatsu Software Engineer Hitachi Computer Products (America) Inc. Software Solutions Division e-mail: mhiramat@xxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
