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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
