linux-kernel - Re: kernel BUG at mm/slab.c:601

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Date:	Sat, 22 Nov 2008 02:50:54 +0100
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Brian Phelps" <lm317t@...il.com>,
	"Mauro Carvalho Chehab" <mchehab@...radead.org>,
	"Julia Lawall" <julia@...u.dk>
Cc:	linux-kernel@...r.kernel.org, "Al Viro" <viro@...iv.linux.org.uk>,
	"Mikael Pettersson" <mikpe@...uu.se>,
	"Alexander Shaduri" <ashaduri@...il.com>,
	"Alexey Dobriyan" <adobriyan@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: kernel BUG at mm/slab.c:601

On Sat, Nov 22, 2008 at 2:26 AM, Vegard Nossum <vegard.nossum@...il.com> wrote:
> [Resend with (I hope) working e-mail address for Mauro]
>
> On Sat, Nov 22, 2008 at 2:25 AM, Vegard Nossum <vegard.nossum@...il.com> wrote:
>> On Sat, Nov 22, 2008 at 2:10 AM, Vegard Nossum <vegard.nossum@...il.com> wrote:
>>>> [  527.562373]  ffffffff8043b157 0000000000200200 ffffffffa02810d4
>>>> ffff88001e13c600
>>>
>>> LIST_POISON2 on the stack:
>>>
>>> include/linux/poison.h:#define LIST_POISON2  ((void *) 0x00200200)
>>
>> So looking at bttv source code, I wonder what the codes like these are
>> trying to do:
>>
>>                        if (set->top->vb.queue.next)
>>                                list_del(&set->top->vb.queue);
>>
>> Code is ancient, I'll ask Mauro.

A semantic patch that finds such invalid list constructs (it is
invalid, isn't it?) would look like this:

@@
expression E;
statement S;
@@

(
* if (E.next) S
|
* if (!E.next) S
|
* if (E.prev) S
|
* if (!E.prev) S
)

I am guessing that the original code wanted to check whether the
object was the last in a list? (The invalid assumption is that NULL
ends the list. Or NULL means that the node is not on a list? But if
so, why take it off the list? In either case, the NULL test looks
wrong because of the poison pointers which prevent intent of the code
-- to check if there is a valid next entry.)

But for this test we actually need to know the list head too. And I
don't know where to find it.


Vegard

-- 
"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
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