I spent some more time on this.
I managed to get a trace that exhibited the bug practically right
away, but still unable to generate a reproducible trace :(
So instead I'm adding WARN's and trace_printks to see what I can find out.
Here's a summary of what I think is happneing. Please let me know if I'm
wildly wrong in analyzing this.
Userspace accesses the perf
ring-buffer-user-page of an event for the first time (in this trace
it's a SW_CPU_CLOCK event).
This triggers a page_fault to bring in the page.
This triggers a TRACEPOINT event (task/task_newtask) which
has
PERF_SAMPLE_IP|PERF_SAMPLE_TID|PERF_SAMPLE_TIME|PERF_SAMPLE_CALLCHAIN|PERF_SAMPLE_ID|PERF_SAMPLE_CPU|PERF_SAMPLE_PERIOD|PERF_SAMPLE_STREAM_ID|PERF_SAMPLE_WEIGHT|PERF_SAMPLE_DATA_SRC|PERF_SAMPLE_TRANSACTION;
and exclude_callchain_kernel=1, triggering a perf_callchain().
Here is a dump of the stack right after we enter perf_callchain()
vince@core2:~$ [ 202.320444] ------------[ cut here ]------------
[ 202.324001] WARNING: CPU: 1 PID: 2873 at kernel/events/callchain.c:168
perf_callchain+0x67/0x211()
hermal_sys ehci_pci ehci_hcd sg sd_mod usbcore usb_common
[ 202.324001] CPU: 1 PID: 2873 Comm: perf_fuzzer Not tainted 3.14.0-rc3+ #24
[ 202.324001] Hardware name: AOpen DE7000/nMCP7ALPx-DE R1.06 Oct.19.2012,
BIOS 080015 10/19/2012
[ 202.324001] 00000000000000a8 ffff880119ec1b48 ffffffff81531dc1
00000000000000a8
[ 202.324001] 0000000000000000 ffff880119ec1b88 ffffffff81040ce4
ffff880119ec1b88
[ 202.324001] ffffffff810d416f ffff880119ec1e38 ffff8800cba67800
0000000000000000
[ 202.324001] Call Trace:
[ 202.324001] [<ffffffff81531dc1>] dump_stack+0x49/0x60
[ 202.324001] [<ffffffff81040ce4>] warn_slowpath_common+0x81/0x9b
[ 202.324001] [<ffffffff810d416f>] ? perf_callchain+0x67/0x211
[ 202.324001] [<ffffffff81040d18>] warn_slowpath_null+0x1a/0x1c
[ 202.324001] [<ffffffff810d416f>] perf_callchain+0x67/0x211
[ 202.324001] [<ffffffff8106b0ab>] ? local_clock+0x1b/0x24
[ 202.324001] [<ffffffff810d0872>] perf_prepare_sample+0x7b/0x304
[ 202.324001] [<ffffffff810d1079>] __perf_event_overflow+0x156/0x1c1
[ 202.324001] [<ffffffff810f4613>] ? free_pgtables+0xa7/0xc9
[ 202.324001] [<ffffffff810d125e>] perf_swevent_overflow+0x41/0x5b
[ 202.324001] [<ffffffff810d12ea>] perf_swevent_event+0x72/0x74
[ 202.324001] [<ffffffff810d1478>] perf_tp_event+0xea/0x1ef
[ 202.324001] [<ffffffff8103515a>] ? perf_trace_x86_exceptions+0x4c/0xba
[ 202.324001] [<ffffffff810351b6>] perf_trace_x86_exceptions+0xa8/0xba
[ 202.324001] [<ffffffff8103515a>] ? perf_trace_x86_exceptions+0x4c/0xba
[ 202.324001] [<ffffffff81538439>] trace_do_page_fault+0x48/0x99
[ 202.324001] [<ffffffff815354b2>] trace_page_fault+0x22/0x30
[ 202.324001] ---[ end trace 31ccd31b4e82cb42 ]---
This triggers a kernel pagefault because the BP register is not valid and
copy_from_user_nmi() tries to copy the user stack area from there.
This *should* be OK? And it maybe looks like it works, but then
copy_user_handle_tail()
causes another page fault again at the invaid BP register.
But then the code continues to
perf_output_begin()
237.265689: page_fault_kernel: address=0x17a0 ip=copy_user_handle_tail
error_code=0x0
237.265689: function: __do_page_fault
237.265689: function: bad_area_nosemaphore
237.265690: function: __bad_area_nosemaphore
237.265690: function: no_context
237.265690: function: fixup_exception
237.265690: function: search_exception_tables
237.265690: function: search_extable
237.265691: function: perf_output_begin
237.265692: bprint: perf_output_begin: VMW: event type 2 config
2a st: 2c3e7
how are we back in __do_page_fault again here?
237.265692: function: __do_page_fault
237.265692: function: down_read_trylock
237.265692: function: _cond_resched
237.265693: function: find_vma
237.265693: function: bad_area
237.265693: function: up_read
237.265693: function: __bad_area_nosemaphore
237.265694: function: is_prefetch
237.265694: function: convert_ip_to_linear
237.265695: function: unhandled_signal
237.265695: function: __printk_ratelimit
237.265695: function: _raw_spin_trylock
237.265695: function: _raw_spin_unlock_irqrestore
237.265696: function: printk
237.265696: function: vprintk_emit
[ 202.877004] perf_fuzzer[2873]: segfault at 17a0 ip 00000000004017fd sp
00000000ffd19d10 error 6 in perf_fuzzer[400000+d1000]
Vince
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