Re: [PATCH bpf-next V5 4/5] bpf: drop MTU check when doing TC-BPF redirect to ingress

[John Fastabend <john.fastabend@gmail.com>]

Jesper Dangaard Brouer wrote:
> On Fri, 30 Oct 2020 13:36:05 -0700
> John Fastabend <john.fastabend@gmail.com> wrote:
> 
> > 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@redhat.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 964b494b0e8d..bd02ddab8dfe 100644
> > > --- a/include/linux/netdevice.h
> > > +++ b/include/linux/netdevice.h
> > > @@ -3891,11 +3891,38 @@ 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)  
> > 
> > It looks like if check_mtu=false then this is just an interface up check.
> > Can we leave is_skb_forwardable logic alone and just change the spots where
> > this is called with false to something with a name that describes the check,
> > such as is_dev_up(dev). I think it will make this change smaller and the
> > code easier to read. Did I miss something?
> 
> People should realized that this is constructed such, the compiler will
> compile-time remove the actual argument (the const bool check_mtu).
> And this propagates also to ____dev_forward_skb() where the call places
> are also inlined.

The comment was about human reading the code not what gets generated
by the compiler.

> 
> Yes, this (check_mtu=false) is basically an interface up check, but the
> only place it is used directly is in the ndo_get_peer_dev() case, and
> reading the code I find it more readable that is says
> __is_skb_forwardable because this is used as part of a forwarding step,
> and is_dev_up() doesn't convey the intent in this use-case.

OK.

[...]

> 
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index bd4a416bd9ad..71b78b8d443c 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;
> >  }
> 
> I'm replicating two lines from dev_forward_skb(), but I couldn't find a
> way to avoid this, without causing larger code changes (and slower code).
> 

OK looks good to me then.

> 
> 
> > Other than style aspects it looks correct to me.
> > 
> > >  	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 9499a414d67e..445ccf92c149 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -2188,28 +2188,13 @@ static inline void net_timestamp_set(struct sk_buff *skb)
> > >    
> > 
> 
> 
> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
>