linux-kernel - Re: [PATCH 0/3][RFC] x86: Catch stray non-kprobe breakpoints

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Message-ID: <20080130040712.GA6762@in.ibm.com>
Date:	Wed, 30 Jan 2008 09:37:13 +0530
From:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Abhishek Sagar <sagar.abhishek@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, jkenisto@...ibm.com,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 0/3][RFC] x86: Catch stray non-kprobe breakpoints

On Tue, Jan 29, 2008 at 02:29:41PM -0500, Masami Hiramatsu wrote:
> Abhishek Sagar wrote:
> > On 1/29/08, Masami Hiramatsu <mhiramat@...hat.com> wrote:
> >> In that case, why don't you just reduce the priority of kprobe_exceptions_nb?
> >> Then, the execution path becomes very simple.
> > 
> > Ananth mentioned that the kprobe notifier has to be the first to run.
> 
> (Hmm.. I think he has just explained current implementation:))
> IMHO, since kprobes itself can not know what the external debugger
> wants to do, the highest priority should be reserved for those external tools.

The reason why kprobes needs to be the first to run is simple: it
doesn't need user intervention and if it isn't the intended recepient of
the breakpoint, it just lets the kernel take over (unlike a debugger,
which would potentially need user attention). Also, if the underlying
instruction itself is a breakpoint, we have the facility in kprobes to
single-step inline so the kernel can take control and notify any other
intended recepient of the underlying breakpoint.

As such, I believe the current situation is fine, has worked fine for
close to 4 years now and doesn't warrant any change.

Ananth
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