linux-kernel - Re: [PATCH 18/18] tracing/kprobes: Dump the culprit kprobe in case of kprobe recursion

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Date:	Thu, 27 Aug 2009 11:52:09 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Subject: Re: [PATCH 18/18] tracing/kprobes: Dump the culprit kprobe in case
 of kprobe recursion

Frederic Weisbecker wrote:
> On Thu, Aug 27, 2009 at 11:30:24AM -0400, Masami Hiramatsu wrote:
>> Hi Frederic,
>>
>> Frederic Weisbecker wrote:
>>> Kprobes can enter into a probing recursion, ie: a kprobe that does an
>>> endless loop because one of its core mechanism function used during
>>> probing is also probed itself.
>>>
>>> This patch helps pinpointing the kprobe that raised such recursion
>>> by dumping it and raising a BUG instead of a warning (we also disarm
>>> the kprobe to try avoiding recursion in BUG itself). Having a BUG
>>> instead of a warning stops the stacktrace in the right place and
>>> doesn't pollute the logs with hundreds of traces that eventually end
>>> up in a stack overflow.
>>
>> Thanks, but I also found similar bug cases.
>>
>>>
>>> Signed-off-by: Frederic Weisbecker<fweisbec@...il.com>
>>> Cc: Masami Hiramatsu<mhiramat@...hat.com>
>>> Cc: Ananth N Mavinakayanahalli<ananth@...ibm.com>
>>> ---
>>>    arch/x86/kernel/kprobes.c |    8 ++++++--
>>>    include/linux/kprobes.h   |    2 ++
>>>    kernel/kprobes.c          |    7 +++++++
>>>    3 files changed, 15 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
>>> index 16ae961..ecee3d2 100644
>>> --- a/arch/x86/kernel/kprobes.c
>>> +++ b/arch/x86/kernel/kprobes.c
>>> @@ -490,9 +490,13 @@ static int __kprobes reenter_kprobe(struct kprobe *p, struct pt_regs *regs,
>>
>> Before this, kprobes checks p != kprobe_running(), but it's a
>> meaningless branch. Hitting a kprobe while KPROBES_HIT_SS always
>> treated as unrecoverable.
>
>
>
> Yeah, but that's the place where a probe ends up when bad reentrancy happens
> right?

No, a place which is shared by kprobes and other subsystems, will cause a
problem.

for example, I found an irq_return case which will be p == kprobe_running()
on x86-64.

-> <some irq occurs>
  -> irq_return
    -> <hit int3>
      -> do_int3
      -> <handling kprobe (set kprobe_running)>
    -> irq_return (from do_int3)
      -> <hit int3>
        -> do_int3
        <handling kprobe (kprobe_running == p)> <- here!


Perhaps, the original code assumes that it will be caused by an int3
which another subsystem inserted on out-of-line singlestep buffer
if the hitting probe is same as current probe.

However, in that case, int3 hitting address is on the out-of-line
buffer and should be different from first (current) int3 address.

So, I think this part should also be removed.

               if (p == kprobe_running()) {
                       regs->flags &= ~X86_EFLAGS_TF;
                       regs->flags |= kcb->kprobe_saved_flags;
                       return 0;
               } else {

Thank you,

>
>
>
>>>    			/* A probe has been hit in the codepath leading up
>>>    			 * to, or just after, single-stepping of a probed
>>>    			 * instruction. This entire codepath should strictly
>>> -			 * reside in .kprobes.text section. Raise a warning
>>> -			 * to highlight this peculiar case.
>>> +			 * reside in .kprobes.text section.
>>> +			 * Raise a BUG or we'll continue in an endless
>>> +			 * reentering loop and eventually a stack overflow.
>>>    			 */
>>> +			arch_disarm_kprobe(p);
>>> +			dump_kprobe(p);
>>> +			BUG();
>>>    		}
>>>    	default:
>>>    		/* impossible cases */
>>> diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
>>> index bcd9c07..87eb79c 100644
>>> --- a/include/linux/kprobes.h
>>> +++ b/include/linux/kprobes.h
>>> @@ -296,6 +296,8 @@ void recycle_rp_inst(struct kretprobe_instance *ri, struct hlist_head *head);
>>>    int disable_kprobe(struct kprobe *kp);
>>>    int enable_kprobe(struct kprobe *kp);
>>>
>>> +void dump_kprobe(struct kprobe *kp);
>>> +
>>>    #else /* !CONFIG_KPROBES: */
>>>
>>>    static inline int kprobes_built_in(void)
>>> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
>>> index ef177d6..f72e96c 100644
>>> --- a/kernel/kprobes.c
>>> +++ b/kernel/kprobes.c
>>> @@ -1141,6 +1141,13 @@ static void __kprobes kill_kprobe(struct kprobe *p)
>>>    	arch_remove_kprobe(p);
>>>    }
>>>
>>> +void __kprobes dump_kprobe(struct kprobe *kp)
>>> +{
>>> +	printk(KERN_WARNING "Dumping kprobe:\n");
>>> +	printk(KERN_WARNING "Name: %s\nAddress: %p\nOffset: %x\n",
>>> +	       kp->symbol_name, kp->addr, kp->offset);
>>> +}
>>
>> Since kp->symbol_name + kp->offset = kp->addr, I recommend to show it
>> as "Kprobe at %s+%x:<%p>\n", kp->symbol_name, kp->offset, kp->addr.
>
>
> Ok I'll fix this, thanks.
>
>
>>> +
>>>    /* Module notifier call back, checking kprobes on the module */
>>>    static int __kprobes kprobes_module_callback(struct notifier_block *nb,
>>>    					     unsigned long val, void *data)
>>
>> Thank you,
>>
-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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