netdev - Re: [PATCH bpf-next 1/2] bpf: Add ipip6 and ip6ip decap support for bpf_skb_adjust

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Date:   Sun, 8 Jan 2023 14:16:20 -0500
From:   Willem de Bruijn <willemb@...gle.com>
To:     Ziyang Xuan <william.xuanziyang@...wei.com>
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, bpf@...r.kernel.org, netdev@...r.kernel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, haoluo@...gle.com,
        jolsa@...nel.org, sdf@...gle.com
Subject: Re: [PATCH bpf-next 1/2] bpf: Add ipip6 and ip6ip decap support for bpf_skb_adjust_room()

On Fri, Jan 6, 2023 at 2:55 PM <sdf@...gle.com> wrote:
>
> On 01/06, Ziyang Xuan wrote:
> > Add ipip6 and ip6ip decap support for bpf_skb_adjust_room().
> > Main use case is for using cls_bpf on ingress hook to decapsulate
> > IPv4 over IPv6 and IPv6 over IPv4 tunnel packets.
>
> CC'd Willem since he has done bpf_skb_adjust_room changes in the past.
> There might be a lot of GRO/GSO context I'm missing.
>
> > Signed-off-by: Ziyang Xuan <william.xuanziyang@...wei.com>
> > ---
> >   net/core/filter.c | 34 ++++++++++++++++++++++++++++++++--
> >   1 file changed, 32 insertions(+), 2 deletions(-)
>
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 929358677183..73982fb4fe2e 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -3495,6 +3495,12 @@ static int bpf_skb_net_grow(struct sk_buff *skb,
> > u32 off, u32 len_diff,
> >   static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
> >                             u64 flags)
> >   {
> > +     union {
> > +             struct iphdr *v4;
> > +             struct ipv6hdr *v6;
> > +             unsigned char *hdr;
> > +     } ip;
> > +     __be16 proto;
> >       int ret;
>
> >       if (unlikely(flags & ~(BPF_F_ADJ_ROOM_FIXED_GSO |
> > @@ -3512,10 +3518,19 @@ static int bpf_skb_net_shrink(struct sk_buff
> > *skb, u32 off, u32 len_diff,
> >       if (unlikely(ret < 0))
> >               return ret;
>
> > +     ip.hdr = skb_inner_network_header(skb);
> > +     if (ip.v4->version == 4)
> > +             proto = htons(ETH_P_IP);
> > +     else
> > +             proto = htons(ETH_P_IPV6);
> > +
> >       ret = bpf_skb_net_hdr_pop(skb, off, len_diff);
> >       if (unlikely(ret < 0))
> >               return ret;
>
> > +     /* Match skb->protocol to new outer l3 protocol */
> > +     skb->protocol = proto;
> > +
> >       if (skb_is_gso(skb)) {
> >               struct skb_shared_info *shinfo = skb_shinfo(skb);
>
> > @@ -3578,10 +3593,14 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *,
> > skb, s32, len_diff,
> >          u32, mode, u64, flags)
> >   {
> >       u32 len_cur, len_diff_abs = abs(len_diff);
> > -     u32 len_min = bpf_skb_net_base_len(skb);
> > -     u32 len_max = BPF_SKB_MAX_LEN;
> > +     u32 len_min, len_max = BPF_SKB_MAX_LEN;
> >       __be16 proto = skb->protocol;
> >       bool shrink = len_diff < 0;
> > +     union {
> > +             struct iphdr *v4;
> > +             struct ipv6hdr *v6;
> > +             unsigned char *hdr;
> > +     } ip;
> >       u32 off;
> >       int ret;
>
> > @@ -3594,6 +3613,9 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *,
> > skb, s32, len_diff,
> >                    proto != htons(ETH_P_IPV6)))
> >               return -ENOTSUPP;
>
> > +     if (unlikely(shrink && !skb->encapsulation))
> > +             return -ENOTSUPP;
> > +

This new restriction might break existing users.

There is no pre-existing requirement that shrink is used solely with
packets encapsulated by the protocol stack.

Indeed, skb->encapsulation is likely not set on packets arriving from
the wire, even if encapsulated. Referring to your comment "Main use
case is for using cls_bpf on ingress hook to decapsulate"

Can a combination of the existing bpf_skb_adjust_room and
bpf_skb_change_proto address your problem?

> >       off = skb_mac_header_len(skb);
> >       switch (mode) {
> >       case BPF_ADJ_ROOM_NET:
> > @@ -3605,6 +3627,14 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *,
> > skb, s32, len_diff,
> >               return -ENOTSUPP;
> >       }
>
> > +     if (shrink) {
> > +             ip.hdr = skb_inner_network_header(skb);
> > +             if (ip.v4->version == 4)
> > +                     len_min = sizeof(struct iphdr);
> > +             else
> > +                     len_min = sizeof(struct ipv6hdr);
> > +     }
> > +
> >       len_cur = skb->len - skb_network_offset(skb);
> >       if ((shrink && (len_diff_abs >= len_cur ||
> >                       len_cur - len_diff_abs < len_min)) ||
> > --
> > 2.25.1
>