Re: [bug, netconsole, SLUB] BUG skbuff_head_cache: Poison overwritten

Vegard Nossum
2008-07-17T23:35:49Z

On Fri, Jul 18, 2008 at 1:15 AM, Vegard Nossum <vegard.nossum@gmail.com> wrote:
> On Thu, Jul 17, 2008 at 11:42 PM, Ingo Molnar <mingo@elte.hu> wrote:
>>
>> A regression to v2.6.26:
>>
>> I started getting this skb-head corruption message today, on a T60
>> laptop with e1000:
>>
>> PM: Removing info for No Bus:vcs11
>> device: 'vcs11': device_create_release
>> =============================================================================
>> BUG skbuff_head_cache: Poison overwritten
>> -----------------------------------------------------------------------------
>>
>> INFO: 0xf658ae9c-0xf658ae9c. First byte 0x6a instead of 0x6b
>
> 1. Notice the range. It's just a single byte.
> 2. Notice the value. It's just a ++.
>
> Probably a stray increment of a uint8_t somewhere on a freed object?
>
> The offset from the beginning of the object is 0xf658ae9c - 0xf658ae00 = 0x9c.
>
> How big is a struct sk_buff? Hm.. it is in fact quite big. Now what
> member has offset 0x9c? Seems to depend on your config. Is there any
> way you can figure it out, Ingo? I'll try it with your config too.

With your config:

(gdb) p ((struct sk_buff *) 0)->truesize
Cannot access memory at address 0x9c

Now just audit users of ->truesize... There are quite a few.

Which one would only += 1?

Vegard

PS: I might be on the completely wrong track. So far I only have bad
experiences with this sk_buff...

--
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
-- E. W. Dijkstra, EWD1036