Re: SIGSEGV when using "perf record -g" with 3.13-rc* kernel

[Andy Lutomirski]

On Fri, Jan 10, 2014 at 11:43 AM, Waiman Long <waiman.long@xxxxxx> wrote:
> On 01/10/2014 01:54 PM, Andy Lutomirski wrote:
>>
>> On Fri, Jan 10, 2014 at 9:41 AM, Peter Zijlstra<peterz@xxxxxxxxxxxxx>
>> 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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