netdev - RE: [PANIC] lro + iscsi or lro + skb text search causes panic

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Date:	Wed, 28 Jan 2009 18:06:57 +0530
From:	<Shyam_Iyer@...l.com>
To:	<michaelc@...wisc.edu>, <herbert@...dor.apana.org.au>
Cc:	<davem@...emloft.net>, <jesse.brandeburg@...el.com>,
	<netdev@...r.kernel.org>, <olaf.kirch@...cle.com>, <tgraf@...g.ch>,
	<kkeil@...e.de>
Subject: RE: [PANIC] lro + iscsi or lro + skb text search causes panic



Mike Christie wrote:
> Herbert Xu wrote:
>> On Sun, Jan 25, 2009 at 09:32:22PM -0800, David Miller wrote:
>>> From: Mike Christie <michaelc@...wisc.edu>
>>> Date: Thu, 22 Jan 2009 18:04:11 -0600
>>>
>>>> With the patch running against linus's git tree, my box locks up. 
>>>> You cannot ping it. I do not get a oops or anything in the logs,
and 
>>>> the keyboard does not respond. I will try to get some oops output 
>>>> and more info.
>>> Herbert, any idea offhand?
>> 
>> Yeah, I missed an offset update in there :) Here's a better version.
>> 
>> net: Fix frag_list handling in skb_seq_read
>> 
>> The frag_list handling was broken in skb_seq_read:
>> 
>> 1) We didn't add the stepped offset when looking at the head are of 
>> fragments other than the first.
>> 
>> 2) We didn't take the stepped offset away when setting the data 
>> pointer in the head area.
>> 
>> 3) The frag index wasn't reset.
>> 
>> This patch fixes both issues.
>> 

>It oopsd for me in skb_seq_read. addr2line said it was
linux-2.6/net/core/skbuff.c:2228, which is this line:


>while (st->frag_idx < skb_shinfo(st->cur_skb)->nr_frags) {


>I added some printks in there and it looks like we hit this:

>         } else if (st->root_skb == st->cur_skb &&
>                   skb_shinfo(st->root_skb)->frag_list) {
>                 st->cur_skb = skb_shinfo(st->root_skb)->frag_list;
>                 st->frag_idx = 0;
>                 goto next_skb;
>        }



Actually I did some testing and added a few printks and found that the
st->cur_skb->data was 0 and hence the ptr used by iscsi_tcp was null.
This caused the kernel panic.

 	if (abs_offset < block_limit) {
-		*data = st->cur_skb->data + abs_offset;
+		*data = st->cur_skb->data + (abs_offset -
st->stepped_offset);

 I enabled the debug_tcp and with a few printks found that the code in
my scenario did not go to the next_skb label as described by Mike and
could find that the sequence being followed was this -

It hit this if condition -
 if (st->cur_skb->next) {
                st->cur_skb = st->cur_skb->next;
                st->frag_idx = 0;
                goto next_skb;

And so, now the st pointer is shifted to the next skb whereas actually
it should have hit the second else if first since the data is in the
frag_list.

else if (st->root_skb == st->cur_skb &&
                   skb_shinfo(st->root_skb)->frag_list) {
                st->cur_skb = skb_shinfo(st->root_skb)->frag_list;
                goto next_skb;
        }


Reversing the two conditions the attached patch fixes the issue for me
on top of Herbert's patches.  I have done the testing on the ixgbe
adapter itself and verified the fix using some amount of data transfer
as well.

Signed-off-by: Shyam Iyer <shyam_iyer@...l.com>

--- skbuff.c.orig	2009-01-29 01:12:03.000000000 +0530
+++ skbuff.c	2009-01-29 01:34:57.000000000 +0530
@@ -2039,15 +2039,15 @@
 		st->frag_data = NULL;
 	}
 
-	if (st->cur_skb->next) {
-		st->cur_skb = st->cur_skb->next;
-		st->frag_idx = 0;
-		goto next_skb;
-	} else if (st->root_skb == st->cur_skb &&
+	if (st->root_skb == st->cur_skb &&
 		   skb_shinfo(st->root_skb)->frag_list) {
 		st->cur_skb = skb_shinfo(st->root_skb)->frag_list;
 		st->frag_idx=0;
 		goto next_skb;
+	} else if (st->cur_skb->next) {
+		st->cur_skb = st->cur_skb->next;
+		st->frag_idx = 0;
+		goto next_skb;
 	}
 
 	return 0;


 
P.S: Currently sending through an email client that can't do proper
patch formatting so please excuse the patch in an attachement.


Download attachment "skbuff_patch" of type "application/octet-stream" (604 bytes)