Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 10 Jan 2014 11:56:07 -0800 | Subject | Re: SIGSEGV when using "perf record -g" with 3.13-rc* kernel |
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On Fri, Jan 10, 2014 at 11:43 AM, Waiman Long <waiman.long@hp.com> wrote: > On 01/10/2014 01:54 PM, Andy Lutomirski wrote: >> >> On Fri, Jan 10, 2014 at 9:41 AM, Peter Zijlstra<peterz@infradead.org> >> wrote: >>> >>> On Fri, Jan 10, 2014 at 06:02:23PM +0100, Peter Zijlstra wrote: >>>> >>>> On Fri, Jan 10, 2014 at 05:58:22PM +0100, Peter Zijlstra wrote: >>>>> >>>>> On Fri, Jan 10, 2014 at 10:29:13AM -0500, Waiman Long wrote: >>>>>> >>>>>> Peter, >>>>>> >>>>>> Call Trace: >>>>>> <NMI> [<ffffffff815710af>] dump_stack+0x49/0x62 >>>>>> [<ffffffff8104e3bc>] warn_slowpath_common+0x8c/0xc0 >>>>>> [<ffffffff8104e40a>] warn_slowpath_null+0x1a/0x20 >>>>>> [<ffffffff8105f1f1>] force_sig_info+0x131/0x140 >>>>>> [<ffffffff81042a4f>] force_sig_info_fault+0x5f/0x70 >>>>>> [<ffffffff8106d8da>] ? search_exception_tables+0x2a/0x50 >>>>>> [<ffffffff81043b3d>] ? fixup_exception+0x1d/0x70 >>>>>> [<ffffffff81042cc9>] no_context+0x159/0x1f0 >>>>>> [<ffffffff81042e8d>] __bad_area_nosemaphore+0x12d/0x230 >>>>>> [<ffffffff81042e8d>] ? __bad_area_nosemaphore+0x12d/0x230 >>>>>> [<ffffffff81042fa3>] bad_area_nosemaphore+0x13/0x20 >>>>>> [<ffffffff81578fc2>] __do_page_fault+0x362/0x480 >>>>>> [<ffffffff81578fc2>] ? __do_page_fault+0x362/0x480 >>>>>> [<ffffffff815791be>] do_page_fault+0xe/0x10 >>>>>> [<ffffffff81575962>] page_fault+0x22/0x30 >>>>>> [<ffffffff815817e4>] ? bad_to_user+0x5e/0x66b >>>>>> [<ffffffff81285316>] copy_from_user_nmi+0x76/0x90 >>>>>> [<ffffffff81017a20>] perf_callchain_user+0xd0/0x360 >>>>>> [<ffffffff8111f64f>] perf_callchain+0x1af/0x1f0 >>>>>> [<ffffffff81117693>] perf_prepare_sample+0x2f3/0x3a0 >>>>>> [<ffffffff8111a2af>] __perf_event_overflow+0x10f/0x220 >>>>>> [<ffffffff8111ab14>] perf_event_overflow+0x14/0x20 >>>>>> [<ffffffff8101f69e>] intel_pmu_handle_irq+0x1de/0x3c0 >>>>>> [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390 >>>>>> [<ffffffff81576e64>] perf_event_nmi_handler+0x34/0x60 >>>>>> [<ffffffff8157664a>] nmi_handle+0x8a/0x170 >>>>>> [<ffffffff81576848>] default_do_nmi+0x68/0x210 >>>>>> [<ffffffff81576a80>] do_nmi+0x90/0xe0 >>>>>> [<ffffffff81575c67>] end_repeat_nmi+0x1e/0x2e >>>>>> [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390 >>>>>> [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390 >>>>>> [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390 >>>>>> <<EOE>> [<ffffffff81042f7d>] __bad_area_nosemaphore+0x21d/0x230 >>>>>> [<ffffffff81042fa3>] bad_area_nosemaphore+0x13/0x20 >>>>>> [<ffffffff81578fc2>] __do_page_fault+0x362/0x480 >>>>>> [<ffffffff8113cfbc>] ? vm_mmap_pgoff+0xbc/0xe0 >>>>>> [<ffffffff815791be>] do_page_fault+0xe/0x10 >>>>>> [<ffffffff81575962>] page_fault+0x22/0x30 >>>>>> ---[ end trace 037bf09d279751ec ]--- >>>>>> >>>>>> So this is a double page faults. Looking at relevant changes in >>>>>> 3.13 kernel, I spotted the following one patch that modified the >>>>>> perf_callchain_user() function shown up in the stack trace above: >>>>>> >>>>> Hurm, that's an expected double fault, not something we should take the >>>>> process down for. >>>>> >>>>> I'll have to look at how all that works for a bit. >>> >>> Andy, introduced all this in 4fc3490114bb ("x86-64: Set siginfo and >>> context on vsyscall emulation faults"). >>> >>> It looks like your initial userspace fault hit the magic button and ends >>> up in emulate_vsyscall. Right at that point we trigger a PMI, which >>> tries to do a stack-trace. That stack-trace also stumbles into unmapped >>> memory (might be the same) and faults again. >>> >>> Now at that point, we usually just give up on the callchain and proceed >>> like normal, however because of this double fault emulate-vsyscall >>> SIGSEGV magic you loose. >>> >>> So the below might well be a valid fix.. Anybody? Andy? >> >> Yuck -- when I wrote that thing, I hadn't imagined that an interrupt >> (there's nothing particularly special about NMIs here, I think) would >> try to access user memory. The fix below looks okay, but IMO it needs >> a big fat comment explaining what's going on. >> >> Is there a way to ask whether the previous entry into the kernel came >> from user space? The valid "sig_on_uaccess_error" case happens when >> the current fault was triggered by a fault from userspace. The >> invalid case (and any invalid case from, say, an int3 that a >> tracepoint stuck in there) would be a page fault triggered by a fault >> handler that in turn started in kernel space (in particular, in >> emulate_vsyscall). > > > The processes that got the SIGSEGV were all running shell scripts. I am not > totally sure that they were running in user space when getting the PMIs, but > are likely the case.
The error you're seeing is:
- Userspace code calls into the vsyscall page (e.g. old code calling gettimeofday). - Calls to the vsyscall page trap, so you end up executing in kernel space in emulate_vsyscall. - perf is running, so you end up in an NMI handler that triggers while sig_on_uaccess_fault is true. - The perf callchain code tries to read from a bad userspace pointer (not sure why -- the ip value in the vsyscall page *is* readable). That traps (as expected), but the trap handler injects SIGSEGV due to sig_on_uaccess_fault==1.
The cleanest fix I can think of is to change the condition to sig_on_uaccess_fault==1 && (the get_user caller was a single kernel entry away from userspace). In the failure you're seeing, there was an NMI in there.
--Andy
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