Date: Tue, 29 Jan 2008 18:48:21 +0530
From: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
To: "Abhishek Sagar" <sagar.abhishek@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, jkenisto@us.ibm.com,
       "Masami Hiramatsu" <mhiramat@redhat.com>, "Ingo Molnar" <mingo@elte.hu>
Subject: Re: [PATCH 0/3][RFC] x86: Catch stray non-kprobe breakpoints
Message-ID: <20080129131821.GA27312@in.ibm.com>
Reply-To: ananth@in.ibm.com
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On Tue, Jan 29, 2008 at 04:10:58PM +0530, Abhishek Sagar wrote:
> On 1/29/08, Ananth N Mavinakayanahalli <ananth@in.ibm.com> wrote:
> > > Non kprobe breakpoints in the kernel might lie inside the .kprobes.text section. Such breakpoints can easily be identified by in_kprobes_functions and can be caught early. These are problematic and a warning should be emitted to discourage them (in any rare case, if they actually occur).
> >
> > Why? As Masami indicated in an earlier reply, the annotation is to
> > prevent *only* kprobes.
> 
> May be I'm completely off the mark here, but shouldn't a small subset
> of this section simply be 'breakpoint-free' rather than 'kprobe-free'?
> Placing a breakpoint on kprobe_handler (say) can loop into a recursive
> trap without allowing the debugger's notifier chain to be invoked.

A well heeled debugger will necessarily take care of saving contexts
(using techniques like setjmp/longjmp, etc) to help it recover from such
nested cases (See xmon for example).

> I'm assuming that non-kprobe exception notifiers may (or even should) run
> after kprobe's notifier callback (kprobe_exceptions_notify).

Yes, any such notifier is invoked after kprobe's callback as the kprobe
notifier is always registered with the highest priority.

> > > For this, a check can route the trap handling of such breakpoints away from kprobe_handler (which ends up calling even more functions marked as __kprobes) from inside kprobe_exceptions_notify.
> >
> > Well.. we pass on control of a !kprobe breakpoint to the kernel. This is
> > exactly what permits debuggers like xmon to work fine now.
> 
> This will still happen. It doesn't stop non-kprobe breakpoints from
> being handled, wherever they may be.
> 
> > I don't see any harm in such breakpoints being handled autonomously
> > without any sort of kprobe influence.
> 
> Here's what seems to be happening currently:
> 
> int3 (non-kprobe) -> do_int3 ->kprobe_exceptions_notify ->
> kprobe_handler (passes the buck to the kernel) -> non-krpobe/debugger
> exception handler.
> 
> Here's what the patch will do:
> 
> int3 (non-kprobe) -> do_int3 ->kprobe_exceptions_notify ->
> WARN_ON/kprobe_handler -> non-kprobe/debugger exception handler.
> 
> The WARN_ON (and not a BUG_ON) will be hit iff:
> (in_kprobes_functions(addr) && !is_jprobe_bkpt(addr))

But that still is unneeded dmesg clutter, IMHO.

Ananth
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