On Thu, 27 Feb 2014 17:06:36 -0500 (EST) Vince Weaver <vincent.wea...@maine.edu> wrote:
> > 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 You can also allow perf to be traced. Remove the CFLAGS_REMOVE_core.o line from kernel/events/Makefile. > 237.265692: bprint: perf_output_begin: VMW: event type 2 config > 2a st: 2c3e7 > > how are we back in __do_page_fault again here? Well, the perf ring buffer is vmalloced, right? That can cause a page fault too. > > 237.265692: function: __do_page_fault Try adding a trace_printk() to see all the page faults? > 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] > Yeah, something is getting mesed up. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
