linux-kernel - Re: [PATCH 2/4] wan/hdlc

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Date:   Mon, 08 Apr 2019 08:07:02 +0200
From:   Martin Schiller <ms@....tdt.de>
To:     David Miller <davem@...emloft.net>
Cc:     andrew.hendry@...il.com, khc@...waw.pl, isdn@...ux-pingi.de,
        edumazet@...gle.com, linux-x25@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] wan/hdlc_x25: fix skb handling

On 2019-04-05 21:15, David Miller wrote:
> From: Martin Schiller <ms@....tdt.de>
> Date: Fri, 05 Apr 2019 08:56:44 +0200
> 
>> On 2019-04-05 02:32, David Miller wrote:
>>> From: Martin Schiller <ms@....tdt.de>
>>> Date: Wed,  3 Apr 2019 07:01:16 +0200
>>> 
>>>>  	/* X.25 to LAPB */
>>>>  	switch (skb->data[0]) {
>>>>  	case X25_IFACE_DATA:	/* Data to be transmitted */
>>>> -		skb_pull(skb, 1);
>>>> -		if ((result = lapb_data_request(dev, skb)) != LAPB_OK)
>>>> -			dev_kfree_skb(skb);
>>>> -		return NETDEV_TX_OK;
>>>> +		skbn = skb_copy(skb, GFP_ATOMIC);
>>>> +		skb_pull(skbn, 1);
>>>> +		skb_reset_network_header(skbn);
>>>> +		if ((result = lapb_data_request(dev, skbn)) != LAPB_OK)
>>>> +			dev_kfree_skb(skbn);
>>> This leaks 'skb'.
>> 
>> What exactly do you mean?
>> 'skb' will get freed at the end of x25_xmit() function:
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/wan/hdlc_x25.c#n129
> 
> Then why was it freed here in the original code?

In the original code, 'skb' is only freed here if lapb_data_request()
return a value != LAPB_OK, which is the case when the skb can't be
queued for transmission. Otherwise 'skb' won't be freed here in the
"X25_IFACE_DATA" case.

What my change do is, that 'skb' is copied to 'skbn' before the skb_pull
of the first byte, to fix the problem that tracing layer3 (ETH_P_X25)
packets results in a malformed first byte of the packets, because the
original "skb" will get modified before the frame reaches the tcpdump
output.

Everything else works like before.