Commit 36ccdf85 authored by Björn Töpel's avatar Björn Töpel Committed by Daniel Borkmann
Browse files

net, xsk: Avoid taking multiple skbuff references 



Commit 642e450b ("xsk: Do not discard packet when NETDEV_TX_BUSY")
addressed the problem that packets were discarded from the Tx AF_XDP
ring, when the driver returned NETDEV_TX_BUSY. Part of the fix was
bumping the skbuff reference count, so that the buffer would not be
freed by dev_direct_xmit(). A reference count larger than one means
that the skbuff is "shared", which is not the case.

If the "shared" skbuff is sent to the generic XDP receive path,
netif_receive_generic_xdp(), and pskb_expand_head() is entered the
BUG_ON(skb_shared(skb)) will trigger.

This patch adds a variant to dev_direct_xmit(), __dev_direct_xmit(),
where a user can select the skbuff free policy. This allows AF_XDP to
avoid bumping the reference count, but still keep the NETDEV_TX_BUSY
behavior.

Fixes: 642e450b ("xsk: Do not discard packet when NETDEV_TX_BUSY")
Reported-by: default avatarYonghong Song <yhs@fb.com>
Signed-off-by: default avatarBjörn Töpel <bjorn.topel@intel.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201123175600.146255-1-bjorn.topel@gmail.com
parent 17864891

include/linux/netdevice.h

+13 −1
Original line number Diff line number Diff line
@@ -2813,9 +2813,21 @@ u16 dev_pick_tx_zero(struct net_device *dev, struct sk_buff *skb, 
		     struct net_device *sb_dev);

u16 dev_pick_tx_cpu_id(struct net_device *dev, struct sk_buff *skb,

		       struct net_device *sb_dev);



int dev_queue_xmit(struct sk_buff *skb);

int dev_queue_xmit_accel(struct sk_buff *skb, struct net_device *sb_dev);

int dev_direct_xmit(struct sk_buff *skb, u16 queue_id);

int __dev_direct_xmit(struct sk_buff *skb, u16 queue_id);



static inline int dev_direct_xmit(struct sk_buff *skb, u16 queue_id)

{

	int ret;



	ret = __dev_direct_xmit(skb, queue_id);

	if (!dev_xmit_complete(ret))

		kfree_skb(skb);

	return ret;

}



int register_netdevice(struct net_device *dev);

void unregister_netdevice_queue(struct net_device *dev, struct list_head *head);

void unregister_netdevice_many(struct list_head *head);

net/core/dev.c

+2 −6
Original line number Diff line number Diff line
@@ -4180,7 +4180,7 @@ int dev_queue_xmit_accel(struct sk_buff *skb, struct net_device *sb_dev) 
}

EXPORT_SYMBOL(dev_queue_xmit_accel);



int dev_direct_xmit(struct sk_buff *skb, u16 queue_id)

int __dev_direct_xmit(struct sk_buff *skb, u16 queue_id)

{

	struct net_device *dev = skb->dev;

	struct sk_buff *orig_skb = skb;
@@ -4210,17 +4210,13 @@ int dev_direct_xmit(struct sk_buff *skb, u16 queue_id) 
	dev_xmit_recursion_dec();



	local_bh_enable();



	if (!dev_xmit_complete(ret))

		kfree_skb(skb);



	return ret;

drop:

	atomic_long_inc(&dev->tx_dropped);

	kfree_skb_list(skb);

	return NET_XMIT_DROP;

}

EXPORT_SYMBOL(dev_direct_xmit);

EXPORT_SYMBOL(__dev_direct_xmit);



/*************************************************************************

 *			Receiver routines

net/xdp/xsk.c

+1 −7
Original line number Diff line number Diff line
@@ -411,11 +411,7 @@ static int xsk_generic_xmit(struct sock *sk) 
		skb_shinfo(skb)->destructor_arg = (void *)(long)desc.addr;

		skb->destructor = xsk_destruct_skb;



		/* Hinder dev_direct_xmit from freeing the packet and

		 * therefore completing it in the destructor

		 */

		refcount_inc(&skb->users);

		err = dev_direct_xmit(skb, xs->queue_id);

		err = __dev_direct_xmit(skb, xs->queue_id);

		if  (err == NETDEV_TX_BUSY) {

			/* Tell user-space to retry the send */

			skb->destructor = sock_wfree;
@@ -429,12 +425,10 @@ static int xsk_generic_xmit(struct sock *sk) 
		/* Ignore NET_XMIT_CN as packet might have been sent */

		if (err == NET_XMIT_DROP) {

			/* SKB completed but not sent */

			kfree_skb(skb);

			err = -EBUSY;

			goto out;

		}



		consume_skb(skb);

		sent_frame = true;

	}

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