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

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Date:	Mon, 11 May 2009 11:14:36 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Ingo Molnar <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	kvm <kvm@...r.kernel.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>
Subject: Re: [PATCH -tip v5 2/7] kprobes: checks probe address is instruction
 boudary on x86

Masami Hiramatsu wrote:
>>> +	if (!kallsyms_lookup(paddr, NULL, &offset, NULL, __dummy_buf))
>>> +		return 0;
>>> +
>>> +	/* Decode instructions */
>>> +	addr = paddr - offset;
>>> +	while (addr < paddr) {
>>> +		insn_init_kernel(&insn, (void *)addr);
>>> +		insn_get_opcode(&insn);
>>> +		if (OPCODE1(&insn) == BREAKPOINT_INSTRUCTION) {
>>> +			ret = recover_probed_instruction(buf, addr);
>> Oh, the above puts back the original op code. That is why it is OK?
> 
> Oops, no. I have to use get_kprobe() instead. Thanks!

Ah, I forgot another possibility. There might be another subsystem,
like kgdb, will put their break point on the kernel.
In that case, decoder will decode the instruction is a break point
instruction and the first opcode is int3. So, this part is correct.
In the future, we need to add a generic recover_instruction() code
for those text modification subsystems.

Thank you,
-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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