Re: [PATCH v3 net-next v3 2/6] net: add support for segmenting TCP fraglist GSO packets

[Paolo Abeni <pabeni@redhat.com>]

On Fri, 2024-04-26 at 11:39 +0200, Felix Fietkau wrote:
> On 26.04.24 10:28, Paolo Abeni wrote:
> > On Fri, 2024-04-26 at 08:51 +0200, Felix Fietkau wrote:
> > > Preparation for adding TCP fraglist GRO support. It expects packets to be
> > > combined in a similar way as UDP fraglist GSO packets.
> > > For IPv4 packets, NAT is handled in the same way as UDP fraglist GSO.
> > > 
> > > Signed-off-by: Felix Fietkau <nbd@nbd.name>
> > > ---
> > >  net/ipv4/tcp_offload.c   | 65 ++++++++++++++++++++++++++++++++++++++++
> > >  net/ipv6/tcpv6_offload.c |  3 ++
> > >  2 files changed, 68 insertions(+)
> > > 
> > > diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> > > index fab0973f995b..c493e95e09a5 100644
> > > --- a/net/ipv4/tcp_offload.c
> > > +++ b/net/ipv4/tcp_offload.c
> > > @@ -28,6 +28,68 @@ static void tcp_gso_tstamp(struct sk_buff *skb, unsigned int ts_seq,
> > >  	}
> > >  }
> > >  
> > > +static void __tcpv4_gso_segment_csum(struct sk_buff *seg,
> > > +				     __be32 *oldip, __be32 *newip,
> > > +				     __be16 *oldport, __be16 *newport)
> > > +{
> > > +	struct tcphdr *th;
> > > +	struct iphdr *iph;
> > > +
> > > +	if (*oldip == *newip && *oldport == *newport)
> > > +		return;
> > > +
> > > +	th = tcp_hdr(seg);
> > > +	iph = ip_hdr(seg);
> > > +
> > > +	inet_proto_csum_replace4(&th->check, seg, *oldip, *newip, true);
> > > +	inet_proto_csum_replace2(&th->check, seg, *oldport, *newport, false);
> > > +	*oldport = *newport;
> > > +
> > > +	csum_replace4(&iph->check, *oldip, *newip);
> > > +	*oldip = *newip;
> > > +}
> > > +
> > > +static struct sk_buff *__tcpv4_gso_segment_list_csum(struct sk_buff *segs)
> > > +{
> > > +	struct sk_buff *seg;
> > > +	struct tcphdr *th, *th2;
> > > +	struct iphdr *iph, *iph2;
> > > +
> > > +	seg = segs;
> > > +	th = tcp_hdr(seg);
> > > +	iph = ip_hdr(seg);
> > > +	th2 = tcp_hdr(seg->next);
> > > +	iph2 = ip_hdr(seg->next);
> > > +
> > > +	if (!(*(u32 *)&th->source ^ *(u32 *)&th2->source) &&
> > > +	    iph->daddr == iph2->daddr && iph->saddr == iph2->saddr)
> > > +		return segs;
> > 
> > As mentioned in previous revisions, I think a problem with this
> > approach is that the stack could make other changes to the TCP header
> > after the GRO stage, that are unnoticed here and could cause csum
> > corruption, if the egress device does not recompute the packet csum.
> 
> On segmentation, each packet keeps its original TCP header and csum. If 
> the stack makes changes, they apply to the first packet only. I don't 
> see how we could get csum corruption.

You are right. I did not take in account that such changes (to the
first skb) are not reflected to the frag_list at segmentation time. The
end result could be different from what the user/admin is expecting,
but at least should not impact drops.

Side note: alike UDP, this is not supporting IPv6 NAT...

Thanks,

Paolo