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

[Frederic Weisbecker <fweisbec@gmail.com>]

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?