linux-kernel - Re: [PATCH 3/3] x86: WARN_ON breakpoints from .kprobes.text section

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Date:	Mon, 28 Jan 2008 12:22:36 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Abhishek Sagar <sagar.abhishek@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>, jkenisto@...ibm.com,
	ananth@...ibm.com, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 3/3] x86: WARN_ON breakpoints from .kprobes.text section

Hi,

Abhishek Sagar wrote:
> On 1/28/08, Masami Hiramatsu <mhiramat@...hat.com> wrote:
>> Thank you for explanation, I hope I can understand it.
>> Even if it causes a trap recursively, it could be checked (and ignored) by
>> longjump_break_handler(), and passed to the debugger correctly.
> 
> Yes, all non-kprobe breakpoints are left to the kernel to be handled.
> The objective here is to intercept the trap handling of a certain
> category of such breakpoints and emit a warning. The premise being
> that .kprobes.text section is a logical breakpoint-free zone.

Oh, I think I've gotten what misleads you.
The .kprobes.text section is a KPROBES-FREE zone. There may be
breakpoints owned by other debuggers and hand-coded breakpoints
(like as jprobe_return).

>> Please consider that someone expands jprobe(jprobe2) which uses
>> jprobe_return2() instead of jprobe_return(), how would you handle it?
> 
> By a simple modification of is_jprobe_bkpt() (defined in patch #2 of
> this series).

IMO, one of advantages of current logic is that you can add another break_handler-based
probe as an module without any patches. Even if someone makes fooprobe which is
not a jprobe variant, current logic can treat it correctly.
(Another advantage is the performance. Current logic checks only if there is a
 running kprobe and there is no kprobes related to the trapped address, instead of
 checking address section every time when each breakpoint is hit.)

>> Current kprobes provides an opportunity to those external probe frameworks
>> for checking it by themselves.
> 
> Could you clarirfy this with some example. For now I'm assuming that
> by external probe frameworks you mean kernel modules using kprobes.

Yes, I mentioned it above.

> If
> they embed breakpoints in their handlers, then they will simply not be
> caught by this check because thay cannot lie in the .kprobes.text
> section.

They cannot lie kprobes in the .kprobes.text section, but can put
breakpoints by hand. this is the reason why kprobes provides break_handler.

Thanks,
Best Regards,

-- 
Masami Hiramatsu

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

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

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