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Re: [PATCH -tip v4 0/6] kprobes: introduce NOKPROBE_SYMBOL() and fixes crash bugs
- From: Ingo Molnar <mingo at kernel dot org>
- To: Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>
- Cc: Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, Sandeepa Prabhu <sandeepa dot prabhu at linaro dot org>, x86 at kernel dot org, lkml <linux-kernel at vger dot kernel dot org>, "Steven Rostedt (Red Hat)" <rostedt at goodmis dot org>, systemtap at sourceware dot org, "David S. Miller" <davem at davemloft dot net>
- Date: Wed, 11 Dec 2013 14:34:23 +0100
- Subject: Re: [PATCH -tip v4 0/6] kprobes: introduce NOKPROBE_SYMBOL() and fixes crash bugs
- Authentication-results: sourceware.org; auth=none
- References: <20131204012841 dot 22118 dot 82992 dot stgit at kbuild-fedora dot novalocal> <20131204084551 dot GA31772 at gmail dot com> <529FBA71 dot 6070107 at hitachi dot com> <20131205102127 dot GA19923 at gmail dot com> <52A137B6 dot 6030307 at hitachi dot com> <20131210152811 dot GA1195 at gmail dot com> <52A7CA0A dot 9060009 at hitachi dot com>
* Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
> > So why are annotations needed at all? What can happen if an
> > annotation is missing and a piece of code is probed which is also
> > used by the kprobes code internally - do we crash, lock up,
> > misbehave or handle it safely?
>
> The kprobe has recursion detector, [...]
It's the 'current_kprobe' percpu variable, checked via
kprobe_running(), right?
> [...] but it is detected in the kprobe exception(int3) handler, this
> means that if we put a probe before detecting the recursion, we'll
> do an infinite recursion.
So only the (presumably rather narrow) code path leading to the
recursion detection code has to be annotated, correct?
> And also, even if we can detect the recursion, we can't stop the
> kernel, we need to skip the probe. This means that we need to
> recover to the main execution path by doing single step. As you may
> know, since the single stepping involves the debug exception, we
> have to avoid proving on that path too. Or we'll have an infinite
> recursion again.
I don't see why this is needed: if a "probing is disabled" recursion
flag is set the moment the first probe fires, and if it's only cleared
once all processing is finished, then any intermediate probes should
simply return early from int3 and not fire.
What am I missing?
Thanks,
Ingo