On Mon, Sep 13, 2021 at 11:00:47AM +0800, 王贇 wrote:

On 2021/9/10 下午11:38, Peter Zijlstra wrote:

quoted

On Thu, Sep 09, 2021 at 11:13:21AM +0800, 王贇 wrote:

quoted

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.