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Date:	Sun, 10 Aug 2014 11:35:07 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Kees Cook <keescook@...omium.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: Runtime trouble with commit dbd952127d (seccomp: introduce writer
 locking)

On 08/09/2014 08:18 PM, Kees Cook wrote:
> On Sat, Aug 9, 2014 at 6:47 PM, Guenter Roeck <private@...ck-us.net> wrote:
>> Hi folks,
>>
>> I am having some trouble with commit dbd952127d (seccomp: introduce
>> writer locking) when running my qemu tests on the upstream kernel.
> Eek, sorry this is causing you trouble!
>
>> With powerpc, I get the following crash.
>>
>> ftrace: allocating 20093 entries in 59 pages
>> ------------[ cut here ]------------
>> kernel BUG at kernel/fork.c:1108!
> For your tree, does this resolve to copy_seccomp()'s
>
>          BUG_ON(!spin_is_locked(&current->sighand->siglock));
>
> line?
>
>> Oops: Exception in kernel mode, sig: 5 [#1]
>> PREEMPT PowerMac
>> Modules linked in:
>> CPU: 0 PID: 0 Comm: swapper Not tainted 3.16.0-yocto-standard+ #1
>> task: c08a03d0 ti: c08e4000 task.ti: c08e4000
>> NIP: c002b98c LR: c002b988 CTR: c03837ec
>> REGS: c08e5e30 TRAP: 0700   Not tainted  (3.16.0-yocto-standard+)
>> MSR: 00021032 <ME,IR,DR,RI>  CR: 22282048  XER: 20000000
>>
>> GPR00: c002b988 c08e5ee0 c08a03d0 00000001 c0a9b878 00000800 c783603c
>> 00000020
>> GPR08: c783603c 00000003 00000001 c08f61fc 22222044 00000000 07c5ac10
>> 00800300
>> GPR16: c7870000 00000010 00000000 07de7fa4 00000000 c08f594c c08f0000
>> c78702d0
>> GPR24: 00000000 c08f0000 c08f5940 c000422c c08a8ea0 c7870310 00000000
>> c7808400
>> NIP [c002b98c] copy_process.part.52+0x794/0x13e4
>> LR [c002b988] copy_process.part.52+0x790/0x13e4
>> Call Trace:
>> [c08e5ee0] [c002b988] copy_process.part.52+0x790/0x13e4 (unreliable)
>> [c08e5f60] [c002c768] do_fork+0xb8/0x3bc
>> [c08e5fa0] [c00041c8] rest_init+0x30/0x94
>> [c08e5fb0] [c083cf98] start_kernel+0x368/0x37c
>> [c08e5ff0] [00003438] 0x3438
>> Instruction dump:
>> 7d400124 38600001 48027e95 55e903e1 41820b84 80e202c0 90f002c0 81420570
>> 91500570 38600001 48027e75 39400001 <0f0a0000> 81420568 8162056c 91500568
>> ---[ end trace dc8fa200cb88537f ]---
>>
>> With  mips (both 32 bit and 64 bit), I get
>>
>> NR_IRQS:256
>> CPU frequency 200.00 MHz
>> Console: colour dummy device 80x25
>> console [tty0] enabled
>> bootconsole [uart0] disabled
>>
>> [ and then there is silence ]
>>
>> In all cases I bisected the problem to the above mentioned commit.
>>
>> Failures are seen in both linux-next and the tip of Linus' tree.
>> Test results are on http://server.roeck-us.net:8010/builders
>> (look for qwmu results). Build and test scripts are in the mips,
>> mips64, and powerpc subdirectories of
>> https://github.com/groeck/linux-build-test/tree/master/rootfs.
>>
>> Any idea, anyone, what might be going on, or how I could do to
>> help tracking this down ?
> Can you send me your .config for the powerpc case? There must be
> something going on here with the spinlock, but I don't see how, since
> the lines immediately before the copy_seccomp call take the lock...
>
> Is there something special going on here for the "pid 0" case?
>
Here is another data point: CONFIG_DEBUG_SPINLOCK is not enabled
in my ppc configuration. The problem disappears if I enable it.

Guenter

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