netdev - Re: [PATCH net-next v6 2/6] net: gro: add p_off param in *_gro

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Message-ID: <66175ca5e3621_2dde6a2947c@willemb.c.googlers.com.notmuch>
Date: Wed, 10 Apr 2024 23:44:37 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Richard Gobert <richardbgobert@...il.com>, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 willemdebruijn.kernel@...il.com, 
 shuah@...nel.org, 
 dsahern@...nel.org, 
 aduyck@...antis.com, 
 netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 linux-kselftest@...r.kernel.org
Cc: Richard Gobert <richardbgobert@...il.com>
Subject: Re: [PATCH net-next v6 2/6] net: gro: add p_off param in
 *_gro_complete

Willem de Bruijn wrote:
> Richard Gobert wrote:
> > Commits a602456 ("udp: Add GRO functions to UDP socket") and 57c67ff ("udp:
> > additional GRO support") introduce incorrect usage of {ip,ipv6}_hdr in the
> > complete phase of gro. The functions always return skb->network_header,
> > which in the case of encapsulated packets at the gro complete phase, is
> > always set to the innermost L3 of the packet. That means that calling
> > {ip,ipv6}_hdr for skbs which completed the GRO receive phase (both in
> > gro_list and *_gro_complete) when parsing an encapsulated packet's _outer_
> > L3/L4 may return an unexpected value.
> > 
> > This incorrect usage leads to a bug in GRO's UDP socket lookup.
> > udp{4,6}_lib_lookup_skb functions use ip_hdr/ipv6_hdr respectively. These
> > *_hdr functions return network_header which will point to the innermost L3,
> > resulting in the wrong offset being used in __udp{4,6}_lib_lookup with
> > encapsulated packets.
> > 
> > To fix this issue p_off param is used in *_gro_complete to pass off the
> > offset of the previous layer.
> 
> What exactly does this mean?
> 
> This patch changes the definition of gro_complete to add a thoff
> alongside the existing "nhoff"..
> 
>     > -     int                     (*gro_complete)(struct sk_buff *skb, int nhoff);
>     > +     int                     (*gro_complete)(struct sk_buff *skb, int nhoff,
>     > +                                             int thoff);
> 
> .. but also fixes up implementations to interpret the existing
> argument as a thoff
> 
>     > -INDIRECT_CALLABLE_SCOPE int tcp4_gro_complete(struct sk_buff *skb, int thoff)
>     > +INDIRECT_CALLABLE_SCOPE int tcp4_gro_complete(struct sk_buff *skb, int nhoff,
>     > +                                           int thoff)
>     >  {
>     > -     const struct iphdr *iph = ip_hdr(skb);
>     > -     struct tcphdr *th = tcp_hdr(skb);
>     > +     const struct iphdr *iph = (const struct iphdr *)(skb->data + nhoff);
>     > +     struct tcphdr *th = (struct tcphdr *)(skb->data + thoff);
> 
> But in some cases the new argument is not nhoff but p_off, e.g.,
> 
>     >  static int geneve_gro_complete(struct sock *sk, struct sk_buff *skb,
>     > -                            int nhoff)
>     > +                            int p_off, int nhoff)
> 
> Really, the argument is the start of the next header, each callback
> just casts to its expected header (ethhdr, tcphdr, etc.)
> 
> The only place where we need to pass an extra argument is in udp,
> because that needs a pointer to the network header right before the
> transport header pointed to by nhoff.
> 
> And only due to possible IPv4 options or IPv6 extension headers, we
> cannot just do
> 
> +        struct udphdr *iph = (struct iphdr *)(skb->data + nhoff - sizeof(*iph));
>          struct udphdr *uh = (struct udphdr *)(skb->data + nhoff);
> 
> I also do not immediately see an a way to avoid all the boilerplate
> of a new argument in every callback. Aside from a per_cpu var -- but
> that is excessive.
> 
> But it can just be left zero in all callsites, except for
> inet_gro_complete/ipv6_gro_complete, which pass in nhoff.

Actually, we can avoid the boilerplate changes that add an extra arg.

By changing the contract between network layer callbacks
(inet_gro_complete/ipv6_gro_complete) and transport layer callbacks
(tcp4_gro_complete et al).

If the first pass their own unmodified offset, nhoff:

        err = INDIRECT_CALL_2(ops->callbacks.gro_complete,
                              tcp4_gro_complete, udp4_gro_complete,
-                             skb, nhoff + sizeof(*iph));
+                             skb, nhoff);

And the latter parse the network header for total_len/payload_len, to
find their original offset.

It's also a bit of a hack. But a lot smaller patch, probably.