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Date: Thu, 17 Dec 2020 15:46:55 +0100
From: Jesper Dangaard Brouer <brouer@...hat.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org,
Daniel Borkmann <borkmann@...earbox.net>,
Alexei Starovoitov <alexei.starovoitov@...il.com>,
maze@...gle.com, lmb@...udflare.com, shaun@...era.io,
Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
John Fastabend <john.fastabend@...il.com>,
Jakub Kicinski <kuba@...nel.org>, eyal.birger@...il.com,
colrack@...il.com, brouer@...hat.com
Subject: Re: [PATCH bpf-next V8 5/8] bpf: drop MTU check when doing TC-BPF
redirect to ingress
On Thu, 3 Dec 2020 00:43:36 +0100
Daniel Borkmann <daniel@...earbox.net> wrote:
> On 11/27/20 7:06 PM, Jesper Dangaard Brouer wrote:
> > 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/
> >
> > V4:
> > - Keep net_device "up" (IFF_UP) check.
> > - Adjustment to handle bpf_redirect_peer() helper
> >
> > Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> > ---
> > include/linux/netdevice.h | 31 +++++++++++++++++++++++++++++--
> > net/core/dev.c | 19 ++-----------------
> > net/core/filter.c | 14 +++++++++++---
> > 3 files changed, 42 insertions(+), 22 deletions(-)
> >
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index 7ce648a564f7..4a854e09e918 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -3917,11 +3917,38 @@ int dev_forward_skb(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;
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 60d325bda0d7..6ceb6412ee97 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -2189,28 +2189,13 @@ 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);
>
> Only user of is_skb_forwardable() that is left after this patch is bridge, maybe
> the whole thing should be moved into the header?
Well, yes, maybe... I just felt it belongs in another patchset.
> > 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 d6125cfc49c3..4673afe59533 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);
>
> Why netif_rx() and not netif_rx_internal() as in dev_forward_skb() originally?
> One extra call otherwise.
This is because the function below calls netif_rx(), which is just
outside patch-diff-window. Thus, it looked wrong/strange to call
netif_rx_internal(), but sure I can use netif_rx_internal() instead.
>
> > + }
> > +
> > + 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;
> > @@ -2480,7 +2488,7 @@ 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) ||
> > + !__is_skb_forwardable(dev, skb, false) ||
>
> If we only use __is_skb_forwardable() with false directly here, maybe then
> lets just have the !(dev->flags & IFF_UP) test here instead..
Sure, let do that.
> > net_eq(net, dev_net(dev))))
> > goto out_drop;
> > skb->dev = dev;
> >
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
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