Re: [PATCH net-next] net: skip offload for NETIF_F_IPV6_CSUM if ipv6 header contains extension

["Benoît Monin" <benoit.monin@gmx.fr>]

07/10/2024 Willem de Bruijn wrote :
> Benoît Monin wrote:
> > Devices with NETIF_F_IP_CSUM capability can checksum TCP and UDP over
> > IPv4 with an IP header that may contains options; whereas devices with
> > NETIF_F_IPV6_CSUM capability can only checksum TCP and UDP over IPv6 if
> > the IP header does not contains extension.
> 
> Are both these statements universally true across devices?
> 
> I can believe for NETIF_F_IP_CSUM that this is the definition, and
> that devices that cannot handle options must fix it up indivually in
> ndo_features_check.
> 
> And same for NETIF_F_IPV6_CSUM with extension headers.
> 
> But it would be good to see where this is asserted in the code, or
> examples of drivers that have to perform such actions.
> 
I was referring to the documentation in skbuff.h that describes 
NETIF_F_IP_CSUM and NETIF_F_IPV6_CSUM.

For NETIF_F_IPV6_CSUM, at least fsl_dpa and r8169 expect 
ipv6_hdr(skb)->nexthdr to be IPPROTO_{TCP,UDP} to compute the correct 
checksum for IPv6.

I posted more details about the problem I am trying to fix with this 
patch in the following thread: 
https://lore.kernel.org/netdev/26548921.1r3eYUQgxm@benoit.monin/T/#u

> > Enforce that in skb_csum_hwoffload_help by checking the network header
> > length in the case where the IP header version is 6. We cannot simply
> > rely on the network header length since the IPv4 header can from 20 to
> > 60 bytes whereas the IPv6 header must be 40 bytes. So we check the
> > version field which is common to IPv4 and IPv6 headers.
> > 
> > This fixes checksumming errors seen with ip6_tunnel and fou6
> > encapsulation, for example with GRE-in-UDP over IPv6:
> > * fou6 adds a UDP header with a partial checksum if the inner packet
> > does not contains a valid checksum.
> 
> Where in the code is this conditional on the inner packet csum?
> 
This is done by udp6_set_csum, which called by fou6_build_udp.

> > * ip6_tunnel adds an IPv6 header with a destination option extension
> > header if encap_limit is non-zero (the default value is 4).
> 
> 
> If this is a fix, we'll need to target net and best effort find a
> suitable fixes tag.
>  
I guess the particular problem I have found is present since the merge 
of fou6 in 4.7, but it might not be the only code path to create an 
IPv6 packet with an extension header and a partial checksum.

> > Signed-off-by: Benoît Monin <benoit.monin@gmx.fr>
> > ---
> >  net/core/dev.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index ea5fbcd133ae..199831d86ec1 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -3639,6 +3639,9 @@ int skb_csum_hwoffload_help(struct sk_buff *skb,
> >  		return 0;
> > 
> >  	if (features & (NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM)) {
> > +		if (ip_hdr(skb)->version == 6 &&
> > +		    skb_network_header_len(skb) != sizeof(struct ipv6hdr))
> > +			goto sw_checksum;
> >  		switch (skb->csum_offset) {
> >  		case offsetof(struct tcphdr, check):
> >  		case offsetof(struct udphdr, check):
> > @@ -3646,6 +3649,7 @@ int skb_csum_hwoffload_help(struct sk_buff *skb,
> >  		}
> >  	}
> > 
> > +sw_checksum:
> >  	return skb_checksum_help(skb);
> >  }
> >  EXPORT_SYMBOL(skb_csum_hwoffload_help);
> 

-- 
Benoît