Re: [PATCH -tip v14 03/12] kprobes: checks probe address is instruction boudary on x86

Frederic Weisbecker
2009-08-18T23:43:47Z

On Tue, Aug 18, 2009 at 07:17:39PM -0400, Masami Hiramatsu wrote:
> Frederic Weisbecker wrote:
> >> + while (addr < paddr) {
> >> + kernel_insn_init(&insn, (void *)addr);
> >> + insn_get_opcode(&insn);
> >> +
> >> + /* Check if the instruction has been modified. */
> >> + if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION) {
> >> + ret = recover_probed_instruction(buf, addr);
> >
> >
> >
> > I'm confused about the reason of this recovering. Is it to remove
> > kprobes behind the current setting one in the current function?
>
> No, it recovers just an instruction which is probed by a kprobe,
> because we need to know the first byte of this instruction for
> decoding it.
>
> Perhaps we'd better to have more generic interface (text_peek?)
> for it because another subsystem (e.g. kgdb) may want to insert int3...
>
> Thank you,

Aah, I see now, it's to keep a sane check of the instructions
boundaries without int 3 artifacts in the middle.

But in that case, you should re-arm the breakpoint after your
check, right?

Or may be you could do the check without repatching?
May be by doing a copy of insn.opcode.bytes and replacing bytes[0]
with what a random kprobe has stolen?