netdev - RE: [PATCH v2 1/1] net: gro: fix misuse of CB in udp socket lookup

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Date: Fri, 21 Jul 2023 09:52:05 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Richard Gobert' <richardbgobert@...il.com>, "davem@...emloft.net"
	<davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>,
	"willemdebruijn.kernel@...il.com" <willemdebruijn.kernel@...il.com>,
	"dsahern@...nel.org" <dsahern@...nel.org>, "tom@...bertland.com"
	<tom@...bertland.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"gal@...dia.com" <gal@...dia.com>
Subject: RE: [PATCH v2 1/1] net: gro: fix misuse of CB in udp socket lookup

From: Richard Gobert
> Sent: 20 July 2023 17:26
> 
> This patch fixes a misuse of IP{6}CB(skb) in GRO, while calling to
> `udp6_lib_lookup2` when handling udp tunnels. `udp6_lib_lookup2` fetch the
> device from CB. The fix changes it to fetch the device from `skb->dev`.
> l3mdev case requires special attention since it has a master and a slave
> device.
> 
...
> +/* This function is the alternative to 'inet_iif' and 'inet_sdif'
> + * functions in case we can not rely on fields of IPCB.
> + *
> + * The caller must verify skb_valid_dst(skb) is false and skb->dev is initialized.
> + * The caller must hold the RCU read lock.
> + */
> +inline void udp4_get_iif_sdif(const struct sk_buff *skb, int *iif, int *sdif)
> +{
> +	*iif = inet_iif(skb) ?: skb->dev->ifindex;
> +	*sdif = 0;
> +
> +#if IS_ENABLED(CONFIG_NET_L3_MASTER_DEV)
> +	if (netif_is_l3_slave(skb->dev)) {
> +		struct net_device *master = netdev_master_upper_dev_get_rcu(skb->dev);
> +
> +		*sdif = *iif;
> +		*iif = master ? master->ifindex : 0;
> +	}
> +#endif
> +}

You need to make that a 'static inline' in the .h file.
Otherwise the code generated will be horrid.

It would be much better to use the return value - say for 'iif'
then have:
{
	iif = inet_iif(skb) ?: skb->dev->ifindex;

if IS_ENABLED(CONFIG_NET_L3_MASTER_DEV)
	if (netif_is_l3_slave(skb->dev)) {
		struct net_device *master = netdev_master_upper_dev_get_rcu(skb->dev);

		*sdif = iif;
		return master ? master->ifindex : 0;
	}
#endif
	*sdif = 0;
	return iif;
}

The compiler might generate that is inlined, not inlined
it is definitely better.

	David

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