linux-kernel - Re: PROBLEM: null pointer dereference in cfq_dispatch

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Message-Id: <200703011308.28266.linux@f-seidel.de>
Date:	Thu, 1 Mar 2007 13:08:27 +0100
From:	Frank Seidel <linux@...eidel.de>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: null pointer dereference in cfq_dispatch_requests (2.6.21-rc2 and 2.6.20)

Am Mittwoch, 28. Februar 2007 19:02 schrieb Dan Williams:
> I can reliably reproduce a null pointer dereference on 2.6.20 and
> 2.6.21-rc2.  I will keep digging to find the kernel version where
> this last worked, but wanted to see if there were any immediate
> experiments I should try.
> ...
> Kernel 2.6.21-rc2 on an i686
> ...
> [  431.709022] BUG: unable to handle kernel NULL pointer dereference
> at virtual address 0000005c [  431.717993]  printing eip:
> ...
> [  431.825386] EIP is at cfq_dispatch_insert+0xb/0x53
> ...
> [  431.887396]  [<c01e1fc9>] cfq_dispatch_requests+0x138/0x3f0
Hi,
unfortunately i yet don't really have much/enough knowledge of cfq and 
the kernels inwards at the moment...
but looking at cfq_dispatch_insert+0xb it seems the struct request 
pointer given (as second parameter by cfq_dispatch_request) was NULL 
and dereferencing it in the RQ_CFQQ macro leads to this oops.

The "break"-out patch below for __cfq_dispatch_request might be at least 
a possible workaround for this, but it could also be total bullsh.. 
Perhaps someone smarter might pick this up.. and give a real fix.

Have fun,
Frank
---

 block/cfq-iosched.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletion(-)

Index: linux-2.6/block/cfq-iosched.c
===================================================================
--- linux-2.6.orig/block/cfq-iosched.c
+++ linux-2.6/block/cfq-iosched.c
@@ -962,7 +962,8 @@ __cfq_dispatch_requests(struct cfq_data
                 * follow expired path, else get first next available
                 */
                if ((rq = cfq_check_fifo(cfqq)) == NULL)
-                       rq = cfqq->next_rq;
+                       if ((rq = cfqq->next_rq) == NULL)
+                               break;

                /*
                 * finally, insert request into driver dispatch list
-
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