[PATCH bpf-next V3 5/6] bpf: drop MTU check when doing TC-BPF redirect to ingress

Jesper Dangaard Brouer
2020-10-08T14:09:44Z

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/

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
include/linux/netdevice.h | 5 +++--
net/core/dev.c | 2 +-
net/core/filter.c | 12 ++++++++++--
3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 28cfa53daf72..58fb7b4869ba 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3866,10 +3866,11 @@ bool is_skb_forwardable(const struct net_device *dev,
const struct sk_buff *skb);

static __always_inline int ____dev_forward_skb(struct net_device *dev,
- struct sk_buff *skb)
+ struct sk_buff *skb,
+ const bool mtu_check)
{
if (skb_orphan_frags(skb, GFP_ATOMIC) ||
- unlikely(!is_skb_forwardable(dev, skb))) {
+ (mtu_check && unlikely(!is_skb_forwardable(dev, skb)))) {
atomic_long_inc(&dev->rx_dropped);
kfree_skb(skb);
return NET_RX_DROP;
diff --git a/net/core/dev.c b/net/core/dev.c
index b433098896b2..96b455f15872 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2209,7 +2209,7 @@ EXPORT_SYMBOL_GPL(is_skb_forwardable);

int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb)
{
- int ret = ____dev_forward_skb(dev, skb);
+ int ret = ____dev_forward_skb(dev, skb, true);

if (likely(!ret)) {
skb->protocol = eth_type_trans(skb, dev);
diff --git a/net/core/filter.c b/net/core/filter.c
index 5986156e700e..a8e24092e4f5 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2083,13 +2083,21 @@ 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);
+ int ret = ____dev_forward_skb(dev, skb, false);
+
+ if (likely(!ret)) {
+ skb->protocol = eth_type_trans(skb, dev);
+ skb_postpull_rcsum(skb, eth_hdr(skb), ETH_HLEN);
+ ret = netif_rx(skb);
+ }
+
+ return ret;
}

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;