Re: [vserver] Again: [vserver] Linux vServer: general protection fault with apache2 and kernel 2.6.38.6

From: Herbert Poetzl <herbert_at_13thfloor.at>
Date: Sat 06 Aug 2011 - 13:38:16 BST
Message-ID: <20110806123816.GD12671@MAIL.13thfloor.at>

On Sat, Aug 06, 2011 at 02:13:37PM +0200, Urban Loesch wrote:

> Hi Herbert,
>> I just tried to pinpoint the location based on my
>> 2.6.38.8-vs2.3.0.37-rc17 kernel and I suspect that
>> task_rq(p) is causing this (for certain p), but
>> I was wondering why your task_rq_lock() is 0xa0
>> bytes in size, where mine is just 0x65 bytes ...

>> especially as the task_rq_lock function is quite
>> compact ...

>> could you upload the output of the folling commands
>> for me (executed in the build directory of your
>> kernel or with the vmlinux object file)

>> # objdump -t vmlinux | grep task_rq_lock

> gives me only on line:

that's fine, nothing more was expected :)

> /usr/src/linux-2.6.38.8 # objdump -t vmlinux | grep task_rq_lock
> ffffffff8104ec60 l F .text 000000000000009c task_rq_lock

>> # objdump -d vmlinux --start-address=0x`objdump -t vmlinux | sed -n
>> '/task_rq_lock/ {s/ .*//; p}'` | sed '/task>:/ Q'

> more lines :-)
> You can download it at http://www.enas.net/objdump.txt

I copied the interesting part here:

ffffffff8104ec60 <task_rq_lock>:
ffffffff8104ec60: 55 push %rbp
ffffffff8104ec61: 48 89 e5 mov %rsp,%rbp
ffffffff8104ec64: 48 83 ec 20 sub $0x20,%rsp
ffffffff8104ec68: 48 89 1c 24 mov %rbx,(%rsp)
ffffffff8104ec6c: 4c 89 64 24 08 mov %r12,0x8(%rsp)
ffffffff8104ec71: 4c 89 6c 24 10 mov %r13,0x10(%rsp)
ffffffff8104ec76: 4c 89 74 24 18 mov %r14,0x18(%rsp)
ffffffff8104ec7b: e8 40 d0 fb ff callq ffffffff8100bcc0 <mcount>

this suggests that you have FTRACE enabled in your
kernel and probably a bunch of related TRACERS

ffffffff8104ec80: 48 c7 c3 40 3c 01 00 mov $0x13c40,%rbx
ffffffff8104ec87: 49 89 fc mov %rdi,%r12
ffffffff8104ec8a: 49 89 f5 mov %rsi,%r13
ffffffff8104ec8d: ff 14 25 00 34 a1 81 callq *0xffffffff81a13400
ffffffff8104ec94: 48 89 c2 mov %rax,%rdx
ffffffff8104ec97: ff 14 25 10 34 a1 81 callq *0xffffffff81a13410
ffffffff8104ec9e: 49 89 55 00 mov %rdx,0x0(%r13)
ffffffff8104eca2: 49 8b 44 24 08 mov 0x8(%r12),%rax
ffffffff8104eca7: 49 89 de mov %rbx,%r14
ffffffff8104ecaa: 8b 40 18 mov 0x18(%rax),%eax
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

this is where your problem happens, and together with
the information from the original post (which you
removed) this looks like task_rq looking for a task at
0x9066669066666605 which is rather unlikely to be a
valid address, so to me this looks like some kind of
memory corruption, maybe caused by a completely
different subsystem ...

ffffffff8104ecad: 4c 03 34 c5 00 d4 ab add -0x7e542c00(,%rax,8),%r14
ffffffff8104ecb4: 81
ffffffff8104ecb5: 4c 89 f7 mov %r14,%rdi
ffffffff8104ecb8: e8 43 14 54 00 callq ffffffff81590100 <_raw_spin_lock>
ffffffff8104ecbd: 49 8b 44 24 08 mov 0x8(%r12),%rax
ffffffff8104ecc2: 8b 40 18 mov 0x18(%rax),%eax
ffffffff8104ecc5: 48 8b 14 c5 00 d4 ab mov -0x7e542c00(,%rax,8),%rdx
ffffffff8104eccc: 81
ffffffff8104eccd: 48 8d 04 13 lea (%rbx,%rdx,1),%rax
ffffffff8104ecd1: 49 39 c6 cmp %rax,%r14
ffffffff8104ecd4: 75 18 jne ffffffff8104ecee <task_rq_lock+0x8e>
ffffffff8104ecd6: 4c 89 f0 mov %r14,%rax
ffffffff8104ecd9: 48 8b 1c 24 mov (%rsp),%rbx
ffffffff8104ecdd: 4c 8b 64 24 08 mov 0x8(%rsp),%r12
ffffffff8104ece2: 4c 8b 6c 24 10 mov 0x10(%rsp),%r13
ffffffff8104ece7: 4c 8b 74 24 18 mov 0x18(%rsp),%r14
ffffffff8104ecec: c9 leaveq
ffffffff8104eced: c3 retq
ffffffff8104ecee: 49 8b 75 00 mov 0x0(%r13),%rsi
ffffffff8104ecf2: 4c 89 f7 mov %r14,%rdi
ffffffff8104ecf5: e8 86 14 54 00 callq ffffffff81590180 <_raw_spin_unlock_irqrestore>
ffffffff8104ecfa: eb 91 jmp ffffffff8104ec8d <task_rq_lock+0x2d>
ffffffff8104ecfc: 0f 1f 40 00 nopl 0x0(%rax)

but that's just a wild guess and some hand waving, to
get better information and more debug info we would
first need to find a way to trigger this issue within
a short amount of time, let's say a few minutes up to
an hour or so ...

btw, does it happen on more than one system or is it
always the same system?

best,
Herbert

> thanks,
> Urban
Received on Sat Aug 6 13:38:29 2011

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