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

[Waiman Long]

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.

-Longman

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