Commit 5f7d5728 authored by Jesper Dangaard Brouer's avatar Jesper Dangaard Brouer Committed by Daniel Borkmann
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bpf: Drop MTU check when doing TC-BPF redirect to ingress 

The use-case for dropping the MTU check when TC-BPF does redirect to
ingress, is described by Eyal Birger in email[0]. The summary is the
ability to increase packet size (e.g. with IPv6 headers for NAT64) and
ingress redirect packet and let normal netstack fragment packet as needed.

[0] https://lore.kernel.org/netdev/CAHsH6Gug-hsLGHQ6N0wtixdOa85LDZ3HNRHVd0opR=19Qo4W4Q@mail.gmail.com/



V15:
 - missing static for function declaration

V9:
 - Make net_device "up" (IFF_UP) check explicit in skb_do_redirect

V4:
 - Keep net_device "up" (IFF_UP) check.
 - Adjustment to handle bpf_redirect_peer() helper

Signed-off-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/161287790971.790810.11785274340154740591.stgit@firesoul
parent 34b2021c

include/linux/netdevice.h

+30 −2
Original line number Diff line number Diff line
@@ -3931,14 +3931,42 @@ int xdp_umem_query(struct net_device *dev, u16 queue_id); 


int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb);

int dev_forward_skb(struct net_device *dev, struct sk_buff *skb);

int dev_forward_skb_nomtu(struct net_device *dev, struct sk_buff *skb);

bool is_skb_forwardable(const struct net_device *dev,

			const struct sk_buff *skb);



static __always_inline bool __is_skb_forwardable(const struct net_device *dev,

						 const struct sk_buff *skb,

						 const bool check_mtu)

{

	const u32 vlan_hdr_len = 4; /* VLAN_HLEN */

	unsigned int len;



	if (!(dev->flags & IFF_UP))

		return false;



	if (!check_mtu)

		return true;



	len = dev->mtu + dev->hard_header_len + vlan_hdr_len;

	if (skb->len <= len)

		return true;



	/* if TSO is enabled, we don't care about the length as the packet

	 * could be forwarded without being segmented before

	 */

	if (skb_is_gso(skb))

		return true;



	return false;

}



static __always_inline int ____dev_forward_skb(struct net_device *dev,

					       struct sk_buff *skb)

					       struct sk_buff *skb,

					       const bool check_mtu)

{

	if (skb_orphan_frags(skb, GFP_ATOMIC) ||

	    unlikely(!is_skb_forwardable(dev, skb))) {

	    unlikely(!__is_skb_forwardable(dev, skb, check_mtu))) {

		atomic_long_inc(&dev->rx_dropped);

		kfree_skb(skb);

		return NET_RX_DROP;

net/core/dev.c

+14 −18
Original line number Diff line number Diff line
@@ -2194,28 +2194,14 @@ static inline void net_timestamp_set(struct sk_buff *skb) 


bool is_skb_forwardable(const struct net_device *dev, const struct sk_buff *skb)

{

	unsigned int len;



	if (!(dev->flags & IFF_UP))

		return false;



	len = dev->mtu + dev->hard_header_len + VLAN_HLEN;

	if (skb->len <= len)

		return true;



	/* if TSO is enabled, we don't care about the length as the packet

	 * could be forwarded without being segmented before

	 */

	if (skb_is_gso(skb))

		return true;



	return false;

	return __is_skb_forwardable(dev, skb, true);

}

EXPORT_SYMBOL_GPL(is_skb_forwardable);



int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb)

static int __dev_forward_skb2(struct net_device *dev, struct sk_buff *skb,

			      bool check_mtu)

{

	int ret = ____dev_forward_skb(dev, skb);

	int ret = ____dev_forward_skb(dev, skb, check_mtu);



	if (likely(!ret)) {

		skb->protocol = eth_type_trans(skb, dev);
@@ -2224,6 +2210,11 @@ int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb) 


	return ret;

}



int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb)

{

	return __dev_forward_skb2(dev, skb, true);

}

EXPORT_SYMBOL_GPL(__dev_forward_skb);



/**
@@ -2250,6 +2241,11 @@ int dev_forward_skb(struct net_device *dev, struct sk_buff *skb) 
}

EXPORT_SYMBOL_GPL(dev_forward_skb);



int dev_forward_skb_nomtu(struct net_device *dev, struct sk_buff *skb)

{

	return __dev_forward_skb2(dev, skb, false) ?: netif_rx_internal(skb);

}



static inline int deliver_skb(struct sk_buff *skb,

			      struct packet_type *pt_prev,

			      struct net_device *orig_dev)

net/core/filter.c

+3 −3
Original line number Diff line number Diff line
@@ -2083,13 +2083,13 @@ static const struct bpf_func_proto bpf_csum_level_proto = { 


static inline int __bpf_rx_skb(struct net_device *dev, struct sk_buff *skb)

{

	return dev_forward_skb(dev, skb);

	return dev_forward_skb_nomtu(dev, skb);

}



static inline int __bpf_rx_skb_no_mac(struct net_device *dev,

				      struct sk_buff *skb)

{

	int ret = ____dev_forward_skb(dev, skb);

	int ret = ____dev_forward_skb(dev, skb, false);



	if (likely(!ret)) {

		skb->dev = dev;
@@ -2480,7 +2480,7 @@ int skb_do_redirect(struct sk_buff *skb) 
			goto out_drop;

		dev = ops->ndo_get_peer_dev(dev);

		if (unlikely(!dev ||

			     !is_skb_forwardable(dev, skb) ||

			     !(dev->flags & IFF_UP) ||

			     net_eq(net, dev_net(dev))))

			goto out_drop;

		skb->dev = dev;