tipc: fix error handling of expanding buffer headroom

Coverity says:

*** CID 1338065:  Error handling issues  (CHECKED_RETURN)
/net/tipc/udp_media.c: 162 in tipc_udp_send_msg()
156     	struct udp_media_addr *dst = (struct udp_media_addr *)&dest->value;
157     	struct udp_media_addr *src = (struct udp_media_addr *)&b->addr.value;
158     	struct sk_buff *clone;
159     	struct rtable *rt;
160
161     	if (skb_headroom(skb) < UDP_MIN_HEADROOM)
>>>     CID 1338065:  Error handling issues  (CHECKED_RETURN)
>>>     Calling "pskb_expand_head" without checking return value (as is done elsewhere 51 out of 56 times).
162     		pskb_expand_head(skb, UDP_MIN_HEADROOM, 0, GFP_ATOMIC);
163
164     	clone = skb_clone(skb, GFP_ATOMIC);
165     	skb_set_inner_protocol(clone, htons(ETH_P_TIPC));
166     	ub = rcu_dereference_rtnl(b->media_ptr);
167     	if (!ub) {

When expanding buffer headroom over udp tunnel with pskb_expand_head(),
it's unfortunate that we don't check its return value. As a result, if
the function returns an error code due to the lack of memory, it may
cause unpredictable consequence as we unconditionally consider that
it's always successful.

Fixes: e53567948f82 ("tipc: conditionally expand buffer headroom over udp tunnel")
Reported-by: <scan-admin@coverity.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Ying Xue <ying.xue@windriver.com>
---
 net/tipc/udp_media.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

From: Ying Xue <ying.xue@windriver.com>
Date: Sun, 22 Nov 2015 15:38:39 +0800

> @@ -158,8 +158,11 @@ static int tipc_udp_send_msg(struct net *net, struct sk_buff *skb,
>  	struct udp_media_addr *src = (struct udp_media_addr *)&b->addr.value;
>  	struct rtable *rt;
>  
> -	if (skb_headroom(skb) < UDP_MIN_HEADROOM)
> -		pskb_expand_head(skb, UDP_MIN_HEADROOM, 0, GFP_ATOMIC);
> +	if (skb_headroom(skb) < UDP_MIN_HEADROOM) {
> +		err = pskb_expand_head(skb, UDP_MIN_HEADROOM, 0, GFP_ATOMIC);
> +		if (!err)
> +			goto tx_error;
> +	}

pskb_expand_head() returns negative error codes, therefore zero means
success and therefore this "!err" check is reversed.
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On 11/24/2015 12:35 PM, David Miller wrote:
> pskb_expand_head() returns negative error codes, therefore zero means
> success and therefore this "!err" check is reversed.

Thank you for pointing out such a stupid mistake I made.

I will send v2 to correct it.

Regards,
Ying

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