On Sat Jul 4, 2026 at 5:21 AM EDT, Avinash Duduskar wrote: > bpf_fib_lookup() returns the FIB-resolved egress ifindex straight > from the fib result. When the egress is a VLAN device, the returned > ifindex is the VLAN netdev's, which has no XDP xmit handler; XDP > programs that want to forward the frame (e.g. xdp-forward) must > instead target the underlying physical device and push the VLAN tag > themselves. Today the program has no way to learn either the > underlying ifindex or the VLAN tag without maintaining its own > VLAN-to-ifindex map in userspace and refreshing it on netlink > events. > > Add BPF_FIB_LOOKUP_VLAN. When the caller sets this flag and the fib > result is a VLAN device whose immediate parent is a real (non-VLAN) > device in the same network namespace, populate the existing output > fields params->h_vlan_proto and params->h_vlan_TCI from the VLAN > device and replace params->ifindex with the parent's ifindex. > params->h_vlan_TCI carries the VID only, with PCP and DEI bits zero; a > consumer wanting to set egress priority writes PCP itself. > params->smac is the VLAN device's own address, which can differ from > the parent's. > > Only the immediate parent is resolved, via vlan_dev_priv(dev)->real_dev > and not vlan_dev_real_dev(), which walks to the bottom of a stack. When > the immediate parent is not a real device in the same namespace, the > lookup returns BPF_FIB_LKUP_RET_VLAN_FAILURE and leaves params->ifindex > at the input. This covers a stacked VLAN (QinQ), where the immediate > parent is itself a VLAN device and one h_vlan_proto/h_vlan_TCI pair > cannot describe two tags, and a parent in another network namespace (a > VLAN device can be moved while its parent stays), whose ifindex would > be meaningless in the caller's namespace. A program that wants the VLAN > device's own ifindex re-issues the lookup without BPF_FIB_LOOKUP_VLAN, > so the unreducible case stays distinct from a physical egress. That > distinction matters for XDP: a program cannot xmit on a VLAN device, so > a success carrying the VLAN ifindex would make it redirect to a device > with no ndo_xdp_xmit and drop the frame at xdp_do_flush(). The swap and > the vlan fields are written only on the reduce path; other output > fields keep their existing behaviour, so a frag-needed result still > reports the route mtu in params->mtu_result. > > BPF_FIB_LOOKUP_VLAN is only useful to XDP, which cannot redirect to a > VLAN device. A tc program can redirect to the VLAN device directly, so > bpf_skb_fib_lookup() rejects the flag with -EINVAL; bpf_xdp_fib_lookup() > accepts it. When the flag is not set, behaviour is unchanged: > h_vlan_proto and h_vlan_TCI are zeroed and ifindex is left at the FIB > result. > > The new block is compiled only under CONFIG_VLAN_8021Q since > vlan_dev_priv() is not defined otherwise; without that config > is_vlan_dev() is constant false and the flag is accepted but never > acts. That is safe because no VLAN device can exist there, so every > egress is already physical. > > This lets an XDP redirect target the physical device and learn the > tag to push in a single lookup, which xdp-forward's optional VLAN > mode (xdp-project/xdp-tools#504) wants from the kernel side. > > The helper's input semantics are unchanged; the reverse direction > (supplying a tag as lookup input) is added in the following patch. > > Suggested-by: Toke Høiland-Jørgensen <[email protected]> > Reviewed-by: Toke Høiland-Jørgensen <[email protected]> > Acked-by: David Ahern <[email protected]> > Signed-off-by: Avinash Duduskar <[email protected]>
Reviewed-by: Emil Tsalapatis <[email protected]> > --- > include/uapi/linux/bpf.h | 31 ++++++++++++++++++++++++++++++- > net/core/filter.c | 33 +++++++++++++++++++++++++++++---- > tools/include/uapi/linux/bpf.h | 31 ++++++++++++++++++++++++++++++- > 3 files changed, 89 insertions(+), 6 deletions(-) > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h > index 89b36de5fdbb..e00f0392e728 100644 > --- a/include/uapi/linux/bpf.h > +++ b/include/uapi/linux/bpf.h > @@ -3532,6 +3532,29 @@ union bpf_attr { > * Use the mark present in *params*->mark for the fib > lookup. > * This option should not be used with > BPF_FIB_LOOKUP_DIRECT, > * as it only has meaning for full lookups. > + * **BPF_FIB_LOOKUP_VLAN** > + * If the fib lookup resolves to a VLAN device whose > + * parent is a real (non-VLAN) device, set > + * *params*->h_vlan_proto and *params*->h_vlan_TCI from > + * the VLAN device and replace *params*->ifindex with the > + * parent's ifindex. *params*->h_vlan_TCI carries the VID > + * only, with PCP and DEI bits zero; a consumer wanting to > + * set egress priority writes PCP itself. *params*->smac is > + * the VLAN device's own address, which can differ from the > + * parent's. Only the immediate parent is resolved; if it > + * is itself a VLAN device (QinQ) or in another namespace, > + * the egress cannot be reduced to a physical device plus > + * one tag and the lookup returns > + * **BPF_FIB_LKUP_RET_VLAN_FAILURE** with *params*->ifindex > + * left at the input. Re-issue without > + * **BPF_FIB_LOOKUP_VLAN** to obtain the VLAN device's own > + * ifindex. The swap and the vlan fields > + * are written only on success; other output fields keep > + * the helper's existing behaviour, so a frag-needed result > + * still reports the route mtu in *params*->mtu_result. > + * This flag is only valid for XDP programs; tc programs > + * receive -EINVAL since they can redirect to the VLAN > + * device directly. > * > * *ctx* is either **struct xdp_md** for XDP programs or > * **struct sk_buff** tc cls_act programs. > @@ -7327,6 +7350,7 @@ enum { > BPF_FIB_LOOKUP_TBID = (1U << 3), > BPF_FIB_LOOKUP_SRC = (1U << 4), > BPF_FIB_LOOKUP_MARK = (1U << 5), > + BPF_FIB_LOOKUP_VLAN = (1U << 6), > }; > > enum { > @@ -7340,6 +7364,7 @@ enum { > BPF_FIB_LKUP_RET_NO_NEIGH, /* no neighbor entry for nh */ > BPF_FIB_LKUP_RET_FRAG_NEEDED, /* fragmentation required to fwd */ > BPF_FIB_LKUP_RET_NO_SRC_ADDR, /* failed to derive IP src addr */ > + BPF_FIB_LKUP_RET_VLAN_FAILURE, /* VLAN egress, parent unresolvable */ > }; > > struct bpf_fib_lookup { > @@ -7393,7 +7418,11 @@ struct bpf_fib_lookup { > > union { > struct { > - /* output */ > + /* > + * output with BPF_FIB_LOOKUP_VLAN: set from the > + * resolved egress VLAN device (see the flag); zeroed > + * on other successful lookups. > + */ > __be16 h_vlan_proto; > __be16 h_vlan_TCI; > }; > diff --git a/net/core/filter.c b/net/core/filter.c > index 2e96b4b847ce..b5a45485a54b 100644 > --- a/net/core/filter.c > +++ b/net/core/filter.c > @@ -6201,10 +6201,29 @@ static const struct bpf_func_proto > bpf_skb_get_xfrm_state_proto = { > #endif > > #if IS_ENABLED(CONFIG_INET) || IS_ENABLED(CONFIG_IPV6) > -static int bpf_fib_set_fwd_params(struct bpf_fib_lookup *params, u32 mtu) > +static int bpf_fib_set_fwd_params(struct net_device *dev, > + struct bpf_fib_lookup *params, > + u32 flags, u32 mtu, u32 in_ifindex) > { > params->h_vlan_TCI = 0; > params->h_vlan_proto = 0; > + > +#if IS_ENABLED(CONFIG_VLAN_8021Q) > + if ((flags & BPF_FIB_LOOKUP_VLAN) && is_vlan_dev(dev)) { > + struct net_device *real_dev = vlan_dev_priv(dev)->real_dev; > + > + if (!is_vlan_dev(real_dev) && > + net_eq(dev_net(real_dev), dev_net(dev))) { > + params->h_vlan_proto = vlan_dev_vlan_proto(dev); > + params->h_vlan_TCI = htons(vlan_dev_vlan_id(dev)); > + params->ifindex = real_dev->ifindex; > + } else { > + params->ifindex = in_ifindex; > + return BPF_FIB_LKUP_RET_VLAN_FAILURE; > + } > + } > +#endif > + > if (mtu) > params->mtu_result = mtu; /* union with tot_len */ > > @@ -6216,6 +6235,7 @@ static int bpf_fib_set_fwd_params(struct bpf_fib_lookup > *params, u32 mtu) > static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup > *params, > u32 flags, bool check_mtu) > { > + u32 in_ifindex = params->ifindex; > struct neighbour *neigh = NULL; > struct fib_nh_common *nhc; > struct in_device *in_dev; > @@ -6347,7 +6367,7 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct > bpf_fib_lookup *params, > memcpy(params->smac, dev->dev_addr, ETH_ALEN); > > set_fwd_params: > - return bpf_fib_set_fwd_params(params, mtu); > + return bpf_fib_set_fwd_params(dev, params, flags, mtu, in_ifindex); > } > #endif > > @@ -6357,6 +6377,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct > bpf_fib_lookup *params, > { > struct in6_addr *src = (struct in6_addr *) params->ipv6_src; > struct in6_addr *dst = (struct in6_addr *) params->ipv6_dst; > + u32 in_ifindex = params->ifindex; > struct fib6_result res = {}; > struct neighbour *neigh; > struct net_device *dev; > @@ -6486,13 +6507,14 @@ static int bpf_ipv6_fib_lookup(struct net *net, > struct bpf_fib_lookup *params, > memcpy(params->smac, dev->dev_addr, ETH_ALEN); > > set_fwd_params: > - return bpf_fib_set_fwd_params(params, mtu); > + return bpf_fib_set_fwd_params(dev, params, flags, mtu, in_ifindex); > } > #endif > > #define BPF_FIB_LOOKUP_MASK (BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT | > \ > BPF_FIB_LOOKUP_SKIP_NEIGH | BPF_FIB_LOOKUP_TBID | \ > - BPF_FIB_LOOKUP_SRC | BPF_FIB_LOOKUP_MARK) > + BPF_FIB_LOOKUP_SRC | BPF_FIB_LOOKUP_MARK | \ > + BPF_FIB_LOOKUP_VLAN) > > BPF_CALL_4(bpf_xdp_fib_lookup, struct xdp_buff *, ctx, > struct bpf_fib_lookup *, params, int, plen, u32, flags) > @@ -6541,6 +6563,9 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb, > if (flags & ~BPF_FIB_LOOKUP_MASK) > return -EINVAL; > > + if (flags & BPF_FIB_LOOKUP_VLAN) > + return -EINVAL; > + > if (params->tot_len) > check_mtu = true; > > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h > index 89b36de5fdbb..e00f0392e728 100644 > --- a/tools/include/uapi/linux/bpf.h > +++ b/tools/include/uapi/linux/bpf.h > @@ -3532,6 +3532,29 @@ union bpf_attr { > * Use the mark present in *params*->mark for the fib > lookup. > * This option should not be used with > BPF_FIB_LOOKUP_DIRECT, > * as it only has meaning for full lookups. > + * **BPF_FIB_LOOKUP_VLAN** > + * If the fib lookup resolves to a VLAN device whose > + * parent is a real (non-VLAN) device, set > + * *params*->h_vlan_proto and *params*->h_vlan_TCI from > + * the VLAN device and replace *params*->ifindex with the > + * parent's ifindex. *params*->h_vlan_TCI carries the VID > + * only, with PCP and DEI bits zero; a consumer wanting to > + * set egress priority writes PCP itself. *params*->smac is > + * the VLAN device's own address, which can differ from the > + * parent's. Only the immediate parent is resolved; if it > + * is itself a VLAN device (QinQ) or in another namespace, > + * the egress cannot be reduced to a physical device plus > + * one tag and the lookup returns > + * **BPF_FIB_LKUP_RET_VLAN_FAILURE** with *params*->ifindex > + * left at the input. Re-issue without > + * **BPF_FIB_LOOKUP_VLAN** to obtain the VLAN device's own > + * ifindex. The swap and the vlan fields > + * are written only on success; other output fields keep > + * the helper's existing behaviour, so a frag-needed result > + * still reports the route mtu in *params*->mtu_result. > + * This flag is only valid for XDP programs; tc programs > + * receive -EINVAL since they can redirect to the VLAN > + * device directly. > * > * *ctx* is either **struct xdp_md** for XDP programs or > * **struct sk_buff** tc cls_act programs. > @@ -7327,6 +7350,7 @@ enum { > BPF_FIB_LOOKUP_TBID = (1U << 3), > BPF_FIB_LOOKUP_SRC = (1U << 4), > BPF_FIB_LOOKUP_MARK = (1U << 5), > + BPF_FIB_LOOKUP_VLAN = (1U << 6), > }; > > enum { > @@ -7340,6 +7364,7 @@ enum { > BPF_FIB_LKUP_RET_NO_NEIGH, /* no neighbor entry for nh */ > BPF_FIB_LKUP_RET_FRAG_NEEDED, /* fragmentation required to fwd */ > BPF_FIB_LKUP_RET_NO_SRC_ADDR, /* failed to derive IP src addr */ > + BPF_FIB_LKUP_RET_VLAN_FAILURE, /* VLAN egress, parent unresolvable */ > }; > > struct bpf_fib_lookup { > @@ -7393,7 +7418,11 @@ struct bpf_fib_lookup { > > union { > struct { > - /* output */ > + /* > + * output with BPF_FIB_LOOKUP_VLAN: set from the > + * resolved egress VLAN device (see the flag); zeroed > + * on other successful lookups. > + */ > __be16 h_vlan_proto; > __be16 h_vlan_TCI; > };
