Subject: Re: [PATCH net-next] net: Check frag_lists first to prevent data out of order

From: Weilong Chen <[email-masked]>
Date: Fri, 28 Aug 2015 11:35:40 +0800

Thanks for reply.

> On Wed, 2015-08-26 at 19:12 -0700, Eric Dumazet wrote:
>> On Thu, 2015-08-27 at 08:56 +0800, [email-masked] wrote:
>>> From: Weilong Chen <[email-masked]>
>>>
>>> When try to merge several skbs to prior one, if the frag_list is
>>> used and the the last one is a small packet, once the condition
>>> "len <= skb_tailroom(to)" is satisfied, we will get a wrong
>>> packet!
>>> This patch just check frag_lists before the condtion to prevent
>>> this from happening.
>>>
>>> Signed-off-by: Weilong Chen <[email-masked]>
>>> ---
>>>   net/core/skbuff.c | 6 +++---
>>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>> index 8a725cc..d08edcb 100644
>>> --- a/net/core/skbuff.c
>>> +++ b/net/core/skbuff.c
>>> @@ -4133,6 +4133,9 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
>>>   	if (skb_cloned(to))
>>>   		return false;
>>>
>>> +	if (skb_has_frag_list(to) || skb_has_frag_list(from))
>>> +		return false;
>>> +
>>>   	if (len <= skb_tailroom(to)) {
>>>   		if (len)
>>>   			BUG_ON(skb_copy_bits(from, 0, skb_put(to, len), len));
>>> @@ -4140,9 +4143,6 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
>>>   		return true;
>>>   	}
>>>
>>> -	if (skb_has_frag_list(to) || skb_has_frag_list(from))
>>> -		return false;
>>> -
>>>   	if (skb_headlen(from) != 0) {
>>>   		struct page *page;
>>>   		unsigned int offset;
>>
>> Sigh.
>>
>> No idea what problem you tried to solve.
>>
>> This patch is not needed.
>>
>> If (len <= skb_tailroom()), then it is obviously correct to copy_bits()
>> the bytes.
>>
>> Hints :
>>
>> - If @to has a fraglist, then skb_tailroom(to) is 0 so the copy can not
>> be done.

How to make sure it?
In my test, @to has a fraglist, but skb_tailroom(to) is not 0!
The test is about tipc, the function tipc_buf_append will merge 3 skbs 
to one:
packet 1: len = 1420 skb_tailroom = 190
packet 2: len = 1420
	packet 2 will be add to 1' fraglist, but not update skb_tailroom
packet 3: len = 60
	Here's the error!

>>
>> - If @from has a fraglist, it is not relevant as we copy it into @to and
>> will free @from.
>
> This is going to be a FAQ
>
> http://www.spinics.net/lists/netdev/msg315090.html
>
>
>
>
>
> .
>