netdev - RE: [EXT] [PATCH net v2] ipv4: ip_gre: Avoid skb_pull() failure in ipgre

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Date: Sun, 3 Dec 2023 15:17:09 +0000
From: Suman Ghosh <sumang@...vell.com>
To: Shigeru Yoshida <syoshida@...hat.com>
CC: "davem@...emloft.net" <davem@...emloft.net>,
        "dsahern@...nel.org"
	<dsahern@...nel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [EXT] [PATCH net v2] ipv4: ip_gre: Avoid skb_pull() failure in
 ipgre_xmit()

>>> 	}
>>>
>>> 	if (dev->header_ops) {
>>>+		int pull_len = tunnel->hlen + sizeof(struct iphdr);
>>>+
>>> 		if (skb_cow_head(skb, 0))
>>> 			goto free_skb;
>>>
>>> 		tnl_params = (const struct iphdr *)skb->data;
>>>
>>>-		/* Pull skb since ip_tunnel_xmit() needs skb->data pointing
>>>-		 * to gre header.
>>>-		 */
>>>-		skb_pull(skb, tunnel->hlen + sizeof(struct iphdr));
>>>+		if (!pskb_network_may_pull(skb, pull_len))
>> [Suman] Since this is transmit path, should we add unlikely() here?
>
>Thanks for your comment.
>
>I traced this function and found that pskb_may_pull_reason() seems to
>have appropriate likely() and unlikely() as Eric says.
>
>I'm new to Linux networking. Could you kindly explain the background of
>your suggestion?
>
>I understand that a transmit path must be as fast as possible, so we
>should use unlikely() for rare cases such like this error path. Am I
>correct?
>
>Thanks,
>Shigeru
[Suman] Yes. Likely()/unlikely() helps the compiler for branch prediction and we use it mostly on the data path.
But I cross checked that this is static inline and the function pskb_may_pull() already have likely()/unlikely() in place.
So, you can ignore my comment here.
>
>>>+			goto free_skb;
>>>+
>>>+		/* ip_tunnel_xmit() needs skb->data pointing to gre header. */
>>>+		skb_pull(skb, pull_len);
>>> 		skb_reset_mac_header(skb);
>>>
>>> 		if (skb->ip_summed == CHECKSUM_PARTIAL &&
>>>--
>>>2.41.0
>>>
>>