bpf: add bpf_skb_change_proto helper

This patch adds a minimal helper for doing the groundwork of changing
the skb->protocol in a controlled way. Currently supported is v4 to
v6 and vice versa transitions, which allows f.e. for a minimal, static
nat64 implementation where applications in containers that still
require IPv4 can be transparently operated in an IPv6-only environment.
For example, host facing veth of the container can transparently do
the transitions in a programmatic way with the help of clsact qdisc
and cls_bpf.

Idea is to separate concerns for keeping complexity of the helper
lower, which means that the programs utilize bpf_skb_change_proto(),
bpf_skb_store_bytes() and bpf_lX_csum_replace() to get the job done,
instead of doing everything in a single helper (and thus partially
duplicating helper functionality). Also, bpf_skb_change_proto()
shouldn't need to deal with raw packet data as this is done by other
helpers.

bpf_skb_proto_6_to_4() and bpf_skb_proto_4_to_6() unclone the skb to
operate on a private one, push or pop additionally required header
space and migrate the gso/gro meta data from the shared info. We do
mark the gso type as dodgy so that headers are checked and segs
recalculated by the gso/gro engine. The gso_size target is adapted
as well. The flags argument added is currently reserved and can be
used for future extensions.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 58df2da3e9bf..66cd738a937a 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -313,6 +313,20 @@ enum bpf_func_id {
 	 */
 	BPF_FUNC_skb_get_tunnel_opt,
 	BPF_FUNC_skb_set_tunnel_opt,
+
+	/**
+	 * bpf_skb_change_proto(skb, proto, flags)
+	 * Change protocol of the skb. Currently supported is
+	 * v4 -> v6, v6 -> v4 transitions. The helper will also
+	 * resize the skb. eBPF program is expected to fill the
+	 * new headers via skb_store_bytes and lX_csum_replace.
+	 * @skb: pointer to skb
+	 * @proto: new skb->protocol type
+	 * @flags: reserved
+	 * Return: 0 on success or negative error
+	 */
+	BPF_FUNC_skb_change_proto,
+
 	__BPF_FUNC_MAX_ID,
 };