Re: [RFC PATCH] perf: fix panic by mark recursion inside perf_log_throttle

[王贇 <yun.wang@linux.alibaba.com>]

On 2021/9/13 下午6:24, Peter Zijlstra wrote:
> On Mon, Sep 13, 2021 at 11:00:47AM +0800, 王贇 wrote:
>>
>>
>> On 2021/9/10 下午11:38, Peter Zijlstra wrote:
>>> On Thu, Sep 09, 2021 at 11:13:21AM +0800, 王贇 wrote:
>>>> When running with ftrace function enabled, we observed panic
>>>> as below:
>>>>
>>>>   traps: PANIC: double fault, error_code: 0x0
>>>>   [snip]
>>>>   RIP: 0010:perf_swevent_get_recursion_context+0x0/0x70
>>>>   [snip]
>>>>   Call Trace:
>>>>    <NMI>
>>>>    perf_trace_buf_alloc+0x26/0xd0
>>>>    perf_ftrace_function_call+0x18f/0x2e0
>>>>    kernelmode_fixup_or_oops+0x5/0x120
>>>>    __bad_area_nosemaphore+0x1b8/0x280
>>>>    do_user_addr_fault+0x410/0x920
>>>>    exc_page_fault+0x92/0x300
>>>>    asm_exc_page_fault+0x1e/0x30
>>>>   RIP: 0010:__get_user_nocheck_8+0x6/0x13
>>>>    perf_callchain_user+0x266/0x2f0
>>>>    get_perf_callchain+0x194/0x210
>>>>    perf_callchain+0xa3/0xc0
>>>>    perf_prepare_sample+0xa5/0xa60
>>>>    perf_event_output_forward+0x7b/0x1b0
>>>>    __perf_event_overflow+0x67/0x120
>>>>    perf_swevent_overflow+0xcb/0x110
>>>>    perf_swevent_event+0xb0/0xf0
>>>>    perf_tp_event+0x292/0x410
>>>>    perf_trace_run_bpf_submit+0x87/0xc0
>>>>    perf_trace_lock_acquire+0x12b/0x170
>>>>    lock_acquire+0x1bf/0x2e0
>>>>    perf_output_begin+0x70/0x4b0
>>>>    perf_log_throttle+0xe2/0x1a0
>>>>    perf_event_nmi_handler+0x30/0x50
>>>>    nmi_handle+0xba/0x2a0
>>>>    default_do_nmi+0x45/0xf0
>>>>    exc_nmi+0x155/0x170
>>>>    end_repeat_nmi+0x16/0x55
>>>
>>> kernel/events/Makefile has:
>>>
>>> ifdef CONFIG_FUNCTION_TRACER
>>> CFLAGS_REMOVE_core.o = $(CC_FLAGS_FTRACE)
>>> endif
>>>
>>> Which, afaict, should avoid the above, no?
>>
>> I'm afraid it's not working for this case, the
>> start point of tracing is at lock_acquire() which
>> is not from 'kernel/events/core', the following PF
>> related function are also not from 'core', prevent
>> ftrace on 'core' can't prevent this from happen...
> 
> I'm confused tho; where does the #DF come from? Because taking a #PF
> from NMI should be perfectly fine.
> 
> AFAICT that callchain is something like:
> 
> 	NMI
> 	  perf_event_nmi_handler()
> 	    (part of the chain is missing here)
> 	      perf_log_throttle()
> 	        perf_output_begin() /* events/ring_buffer.c */
> 		  rcu_read_lock()
> 		    rcu_lock_acquire()
> 		      lock_acquire()
> 		        trace_lock_acquire() --> perf_trace_foo
> 
> 			  ...
> 			    perf_callchain()
> 			      perf_callchain_user()
> 			        #PF (fully expected during a userspace callchain)
> 				  (some stuff, until the first __fentry)
> 				    perf_trace_function_call
> 				      perf_trace_buf_alloc()
> 				        perf_swevent_get_recursion_context()
> 					  *BOOM*
> 
> Now, supposedly we then take another #PF from get_recursion_context() or
> something, but that doesn't make sense. That should just work...
> 
> Can you figure out what's going wrong there? going with the RIP, this
> almost looks like 'swhash->recursion' goes splat, but again that makes
> no sense, that's a per-cpu variable.

That's true, I actually have tried several approach to avoid the issue, but
it trigger panic as long as we access 'swhash->recursion', the array should
be accessible but somehow broken, that's why I consider this a suspected
stack overflow, since nmi repeated and trace seems very long, but just a
suspect...

Regards,
Michael Wang

>