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

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Date: Mon, 04 Dec 2023 00:38:19 +0900 (JST)
From: Shigeru Yoshida <syoshida@...hat.com>
To: sumang@...vell.com
Cc: davem@...emloft.net, dsahern@...nel.org, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [EXT] [PATCH net v2] ipv4: ip_gre: Avoid skb_pull() failure in
 ipgre_xmit()

On Sun, 3 Dec 2023 15:17:09 +0000, Suman Ghosh wrote:
>>>> 	}
>>>>
>>>> 	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.

Thank you for your explanation. It is very informative. And thanks for
the review as well.

Shigeru

>>
>>>>+			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
>>>>
>>>
>