linux-kernel - Re: tracing ring_buffer

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Message-ID: <4FBEC9E6.8040301@linux.intel.com>
Date:	Thu, 24 May 2012 16:53:10 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: tracing ring_buffer_resize oops.

On 05/24/2012 04:40 PM, Steven Rostedt wrote:
> On Thu, 2012-05-24 at 13:22 -0400, Dave Jones wrote:
> 
> I found a clue!
> 
> 
>> [ 1013.243754] BUG: unable to handle kernel NULL pointer dereference at 0000000000000002
>> [ 1013.272665] IP: [<ffff880145cc0000>] 0xffff880145cbffff
>> [ 1013.285186] PGD 1401b2067 PUD 14324c067 PMD 0 
>> [ 1013.298832] Oops: 0010 [#1] PREEMPT SMP 
>> [ 1013.310600] CPU 2 
>> [ 1013.317904] Modules linked in: ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 xt_state nf_conntrack ip6table_filter ip6_tables crc32c_intel ghash_clmulni_intel microcode usb_debug serio_raw pcspkr iTCO_wdt i2c_i801 iTCO_vendor_support e1000e nfsd nfs_acl auth_rpcgss lockd sunrpc i915 video i2c_algo_bit drm_kms_helper drm i2c_core [last unloaded: scsi_wait_scan]
>> [ 1013.401848] 
>> [ 1013.407399] Pid: 112, comm: kworker/2:1 Not tainted 3.4.0+ #30 
>> [ 1013.437943] RIP: 8eb8:[<ffff88014630a000>]  [<ffff88014630a000>] 0xffff880146309fff
> 
> RIP is always near the GS segment. As GS points to the per_cpu area, we
> may somehow be getting our GS screwed up. I'm not sure why that would
> affect the RIP. Maybe stacks are not being processed properly somewhere?
> 
> It's strange because I can either trigger it on the first try, or it
> never triggers at all??
> 

Much more fundamentally, RIP should never leave the range [-2G, 0).
What is happening here is almost certainly that we jump through
something which isn't a function pointer.

The other thing worth noting is that the code segment is not the
standard Linux code segment, not even close; it *also* doesn't look like
the typical Xen code segment.  This makes be believe that we did an IRET
with the stack pointer set to something other than a valid interrupt
stack frame.  Specifically, note that the value of R12 is the same
value; R12 is a preserved register and may have been pushed onto the
stack by something that wants to save it.

Waitaminute... this isn't related to your using R12 to save a pointer to
something, is it?

> [ 1013.459871] RSP: ffffffff8165e919:ffff88014780f408  EFLAGS: 00010046

	-hpa

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