linux-kernel - Re: [PATCH -tip ] [BUGFIX] kprobes: Skip kretprobe hit in NMI context to avoid deadlock

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Date:	Fri, 1 Aug 2014 13:00:29 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	linux-kernel@...r.kernel.org,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH -tip ] [BUGFIX] kprobes: Skip kretprobe hit in NMI
 context to avoid deadlock


* Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> wrote:

> Skip kretprobe hit in NMI context, because if an NMI happens
> inside the critical section protected by kretprobe_table.lock
> and another(or same) kretprobe hit, pre_kretprobe_handler
> tries to lock kretprobe_table.lock again.
> Normal interrupts have no problem because they are disabled
> with the lock.
> 
> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> Cc: Ananth N Mavinakayanahalli <ananth@...ibm.com>
> Cc: "David S. Miller" <davem@...emloft.net>
> ---
>  kernel/kprobes.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 734e9a7..a537029 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1778,6 +1778,12 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>  	unsigned long hash, flags = 0;
>  	struct kretprobe_instance *ri;
>  
> +	/* To avoid deadlock, prohibit return probing in NMI context */
> +	if (in_nmi()) {

Should be unlikely()?

> +		rp->nmissed++;
> +		return 0;

In another place in this function we do:

        } else {
                rp->nmissed++;
                raw_spin_unlock_irqrestore(&rp->lock, flags);
        }

Is it safe to modify rp-> without locking?

> +	}
> +
>  	/*TODO: consider to only swap the RA after the last pre_handler fired */

Nit: That comment is oddly formatted.

Thanks,

	Ingo
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