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

["Abhishek Sagar" <sagar.abhishek@gmail.com>]

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

> > 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))

> Ananth

I hope I've understood the point you were making, or at least came close :-).

--
Thanks,
Abhishek