No external dependencies on any module, place this in the core.
Increase is about 1800 byte for xfrm_input.o.
The beet helpers get added to internal header, as they can be reused
from xfrm_output.c in the next patch (kernel contains several
copies of them in the xfrm{4,6}_mode_beet.c files).
Before:
   text    data     bss     dec filename
   5578     176    2364    8118 net/xfrm/xfrm_input.o
   1180      64       0    1244 net/ipv4/xfrm4_mode_beet.o
    171      40       0     211 net/ipv4/xfrm4_mode_transport.o
   1163      40       0    1203 net/ipv4/xfrm4_mode_tunnel.o
   1083      52       0    1135 net/ipv6/xfrm6_mode_beet.o
    172      40       0     212 net/ipv6/xfrm6_mode_ro.o
    172      40       0     212 net/ipv6/xfrm6_mode_transport.o
   1056      40       0    1096 net/ipv6/xfrm6_mode_tunnel.o
After:
   text    data     bss     dec filename
   7373     200    2364    9937 net/xfrm/xfrm_input.o
    587      44       0     631 net/ipv4/xfrm4_mode_beet.o
    171      32       0     203 net/ipv4/xfrm4_mode_transport.o
    649      32       0     681 net/ipv4/xfrm4_mode_tunnel.o
    625      44       0     669 net/ipv6/xfrm6_mode_beet.o
    172      32       0     204 net/ipv6/xfrm6_mode_ro.o
    172      32       0     204 net/ipv6/xfrm6_mode_transport.o
    599      32       0     631 net/ipv6/xfrm6_mode_tunnel.o
v2: pass inner_mode to xfrm_inner_mode_encap_remove to fix
    AF_UNSPEC selector breakage (bisected by Benedict Wong)
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
 int xfrm_unregister_type_offload(const struct xfrm_type_offload *type, unsigned short family);
 
 struct xfrm_mode {
-       /*
-        * Remove encapsulation header.
-        *
-        * The IP header will be moved over the top of the encapsulation
-        * header.
-        *
-        * On entry, the transport header shall point to where the IP header
-        * should be and the network header shall be set to where the IP
-        * header currently is.  skb->data shall point to the start of the
-        * payload.
-        */
-       int (*input2)(struct xfrm_state *x, struct sk_buff *skb);
-
        /*
         * Add encapsulation header.
         *
 
        return 0;
 }
 
-static int xfrm4_beet_input(struct xfrm_state *x, struct sk_buff *skb)
-{
-       struct iphdr *iph;
-       int optlen = 0;
-       int err = -EINVAL;
-
-       if (unlikely(XFRM_MODE_SKB_CB(skb)->protocol == IPPROTO_BEETPH)) {
-               struct ip_beet_phdr *ph;
-               int phlen;
-
-               if (!pskb_may_pull(skb, sizeof(*ph)))
-                       goto out;
-
-               ph = (struct ip_beet_phdr *)skb->data;
-
-               phlen = sizeof(*ph) + ph->padlen;
-               optlen = ph->hdrlen * 8 + (IPV4_BEET_PHMAXLEN - phlen);
-               if (optlen < 0 || optlen & 3 || optlen > 250)
-                       goto out;
-
-               XFRM_MODE_SKB_CB(skb)->protocol = ph->nexthdr;
-
-               if (!pskb_may_pull(skb, phlen))
-                       goto out;
-               __skb_pull(skb, phlen);
-       }
-
-       skb_push(skb, sizeof(*iph));
-       skb_reset_network_header(skb);
-       skb_mac_header_rebuild(skb);
-
-       xfrm4_beet_make_header(skb);
-
-       iph = ip_hdr(skb);
-
-       iph->ihl += optlen / 4;
-       iph->tot_len = htons(skb->len);
-       iph->daddr = x->sel.daddr.a4;
-       iph->saddr = x->sel.saddr.a4;
-       iph->check = 0;
-       iph->check = ip_fast_csum(skb_network_header(skb), iph->ihl);
-       err = 0;
-out:
-       return err;
-}
-
 static struct xfrm_mode xfrm4_beet_mode = {
-       .input2 = xfrm4_beet_input,
        .output2 = xfrm4_beet_output,
        .owner = THIS_MODULE,
        .encap = XFRM_MODE_BEET,
 
 #include <net/ip.h>
 #include <net/xfrm.h>
 
-static inline void ipip_ecn_decapsulate(struct sk_buff *skb)
-{
-       struct iphdr *inner_iph = ipip_hdr(skb);
-
-       if (INET_ECN_is_ce(XFRM_MODE_SKB_CB(skb)->tos))
-               IP_ECN_set_ce(inner_iph);
-}
-
 /* Add encapsulation header.
  *
  * The top IP header will be constructed per RFC 2401.
        return 0;
 }
 
-static int xfrm4_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
-{
-       int err = -EINVAL;
-
-       if (XFRM_MODE_SKB_CB(skb)->protocol != IPPROTO_IPIP)
-               goto out;
-
-       if (!pskb_may_pull(skb, sizeof(struct iphdr)))
-               goto out;
-
-       err = skb_unclone(skb, GFP_ATOMIC);
-       if (err)
-               goto out;
-
-       if (x->props.flags & XFRM_STATE_DECAP_DSCP)
-               ipv4_copy_dscp(XFRM_MODE_SKB_CB(skb)->tos, ipip_hdr(skb));
-       if (!(x->props.flags & XFRM_STATE_NOECN))
-               ipip_ecn_decapsulate(skb);
-
-       skb_reset_network_header(skb);
-       skb_mac_header_rebuild(skb);
-       if (skb->mac_len)
-               eth_hdr(skb)->h_proto = skb->protocol;
-
-       err = 0;
-
-out:
-       return err;
-}
-
 static struct xfrm_mode xfrm4_tunnel_mode = {
-       .input2 = xfrm4_mode_tunnel_input,
        .output2 = xfrm4_mode_tunnel_output,
        .owner = THIS_MODULE,
        .encap = XFRM_MODE_TUNNEL,
 
        top_iph->daddr = *(struct in6_addr *)&x->id.daddr;
        return 0;
 }
-
-static int xfrm6_beet_input(struct xfrm_state *x, struct sk_buff *skb)
-{
-       struct ipv6hdr *ip6h;
-       int size = sizeof(struct ipv6hdr);
-       int err;
-
-       err = skb_cow_head(skb, size + skb->mac_len);
-       if (err)
-               goto out;
-
-       __skb_push(skb, size);
-       skb_reset_network_header(skb);
-       skb_mac_header_rebuild(skb);
-
-       xfrm6_beet_make_header(skb);
-
-       ip6h = ipv6_hdr(skb);
-       ip6h->payload_len = htons(skb->len - size);
-       ip6h->daddr = x->sel.daddr.in6;
-       ip6h->saddr = x->sel.saddr.in6;
-       err = 0;
-out:
-       return err;
-}
-
 static struct xfrm_mode xfrm6_beet_mode = {
-       .input2 = xfrm6_beet_input,
        .output2 = xfrm6_beet_output,
        .owner = THIS_MODULE,
        .encap = XFRM_MODE_BEET,
 
 #include <net/ipv6.h>
 #include <net/xfrm.h>
 
-static inline void ipip6_ecn_decapsulate(struct sk_buff *skb)
-{
-       struct ipv6hdr *inner_iph = ipipv6_hdr(skb);
-
-       if (INET_ECN_is_ce(XFRM_MODE_SKB_CB(skb)->tos))
-               IP6_ECN_set_ce(skb, inner_iph);
-}
-
 /* Add encapsulation header.
  *
  * The top IP header will be constructed per RFC 2401.
        return 0;
 }
 
-#define for_each_input_rcu(head, handler)      \
-       for (handler = rcu_dereference(head);   \
-            handler != NULL;                   \
-            handler = rcu_dereference(handler->next))
-
-
-static int xfrm6_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
-{
-       int err = -EINVAL;
-
-       if (XFRM_MODE_SKB_CB(skb)->protocol != IPPROTO_IPV6)
-               goto out;
-       if (!pskb_may_pull(skb, sizeof(struct ipv6hdr)))
-               goto out;
-
-       err = skb_unclone(skb, GFP_ATOMIC);
-       if (err)
-               goto out;
-
-       if (x->props.flags & XFRM_STATE_DECAP_DSCP)
-               ipv6_copy_dscp(ipv6_get_dsfield(ipv6_hdr(skb)),
-                              ipipv6_hdr(skb));
-       if (!(x->props.flags & XFRM_STATE_NOECN))
-               ipip6_ecn_decapsulate(skb);
-
-       skb_reset_network_header(skb);
-       skb_mac_header_rebuild(skb);
-       if (skb->mac_len)
-               eth_hdr(skb)->h_proto = skb->protocol;
-
-       err = 0;
-
-out:
-       return err;
-}
-
-
 static struct xfrm_mode xfrm6_tunnel_mode = {
-       .input2 = xfrm6_mode_tunnel_input,
        .output2 = xfrm6_mode_tunnel_output,
        .owner = THIS_MODULE,
        .encap = XFRM_MODE_TUNNEL,
 
--- /dev/null
+/* SPDX-License-Identifier: GPL-2.0 */
+#include <linux/ipv6.h>
+#include <net/dsfield.h>
+#include <net/xfrm.h>
+
+#ifndef XFRM_INOUT_H
+#define XFRM_INOUT_H 1
+
+static inline void xfrm6_beet_make_header(struct sk_buff *skb)
+{
+       struct ipv6hdr *iph = ipv6_hdr(skb);
+
+       iph->version = 6;
+
+       memcpy(iph->flow_lbl, XFRM_MODE_SKB_CB(skb)->flow_lbl,
+              sizeof(iph->flow_lbl));
+       iph->nexthdr = XFRM_MODE_SKB_CB(skb)->protocol;
+
+       ipv6_change_dsfield(iph, 0, XFRM_MODE_SKB_CB(skb)->tos);
+       iph->hop_limit = XFRM_MODE_SKB_CB(skb)->ttl;
+}
+
+static inline void xfrm4_beet_make_header(struct sk_buff *skb)
+{
+       struct iphdr *iph = ip_hdr(skb);
+
+       iph->ihl = 5;
+       iph->version = 4;
+
+       iph->protocol = XFRM_MODE_SKB_CB(skb)->protocol;
+       iph->tos = XFRM_MODE_SKB_CB(skb)->tos;
+
+       iph->id = XFRM_MODE_SKB_CB(skb)->id;
+       iph->frag_off = XFRM_MODE_SKB_CB(skb)->frag_off;
+       iph->ttl = XFRM_MODE_SKB_CB(skb)->ttl;
+}
+
+#endif
 
 #include <net/ip_tunnels.h>
 #include <net/ip6_tunnel.h>
 
+#include "xfrm_inout.h"
+
 struct xfrm_trans_tasklet {
        struct tasklet_struct tasklet;
        struct sk_buff_head queue;
 }
 EXPORT_SYMBOL(xfrm_parse_spi);
 
+static int xfrm4_remove_beet_encap(struct xfrm_state *x, struct sk_buff *skb)
+{
+       struct iphdr *iph;
+       int optlen = 0;
+       int err = -EINVAL;
+
+       if (unlikely(XFRM_MODE_SKB_CB(skb)->protocol == IPPROTO_BEETPH)) {
+               struct ip_beet_phdr *ph;
+               int phlen;
+
+               if (!pskb_may_pull(skb, sizeof(*ph)))
+                       goto out;
+
+               ph = (struct ip_beet_phdr *)skb->data;
+
+               phlen = sizeof(*ph) + ph->padlen;
+               optlen = ph->hdrlen * 8 + (IPV4_BEET_PHMAXLEN - phlen);
+               if (optlen < 0 || optlen & 3 || optlen > 250)
+                       goto out;
+
+               XFRM_MODE_SKB_CB(skb)->protocol = ph->nexthdr;
+
+               if (!pskb_may_pull(skb, phlen))
+                       goto out;
+               __skb_pull(skb, phlen);
+       }
+
+       skb_push(skb, sizeof(*iph));
+       skb_reset_network_header(skb);
+       skb_mac_header_rebuild(skb);
+
+       xfrm4_beet_make_header(skb);
+
+       iph = ip_hdr(skb);
+
+       iph->ihl += optlen / 4;
+       iph->tot_len = htons(skb->len);
+       iph->daddr = x->sel.daddr.a4;
+       iph->saddr = x->sel.saddr.a4;
+       iph->check = 0;
+       iph->check = ip_fast_csum(skb_network_header(skb), iph->ihl);
+       err = 0;
+out:
+       return err;
+}
+
+static void ipip_ecn_decapsulate(struct sk_buff *skb)
+{
+       struct iphdr *inner_iph = ipip_hdr(skb);
+
+       if (INET_ECN_is_ce(XFRM_MODE_SKB_CB(skb)->tos))
+               IP_ECN_set_ce(inner_iph);
+}
+
+static int xfrm4_remove_tunnel_encap(struct xfrm_state *x, struct sk_buff *skb)
+{
+       int err = -EINVAL;
+
+       if (XFRM_MODE_SKB_CB(skb)->protocol != IPPROTO_IPIP)
+               goto out;
+
+       if (!pskb_may_pull(skb, sizeof(struct iphdr)))
+               goto out;
+
+       err = skb_unclone(skb, GFP_ATOMIC);
+       if (err)
+               goto out;
+
+       if (x->props.flags & XFRM_STATE_DECAP_DSCP)
+               ipv4_copy_dscp(XFRM_MODE_SKB_CB(skb)->tos, ipip_hdr(skb));
+       if (!(x->props.flags & XFRM_STATE_NOECN))
+               ipip_ecn_decapsulate(skb);
+
+       skb_reset_network_header(skb);
+       skb_mac_header_rebuild(skb);
+       if (skb->mac_len)
+               eth_hdr(skb)->h_proto = skb->protocol;
+
+       err = 0;
+
+out:
+       return err;
+}
+
+static void ipip6_ecn_decapsulate(struct sk_buff *skb)
+{
+       struct ipv6hdr *inner_iph = ipipv6_hdr(skb);
+
+       if (INET_ECN_is_ce(XFRM_MODE_SKB_CB(skb)->tos))
+               IP6_ECN_set_ce(skb, inner_iph);
+}
+
+static int xfrm6_remove_tunnel_encap(struct xfrm_state *x, struct sk_buff *skb)
+{
+       int err = -EINVAL;
+
+       if (XFRM_MODE_SKB_CB(skb)->protocol != IPPROTO_IPV6)
+               goto out;
+       if (!pskb_may_pull(skb, sizeof(struct ipv6hdr)))
+               goto out;
+
+       err = skb_unclone(skb, GFP_ATOMIC);
+       if (err)
+               goto out;
+
+       if (x->props.flags & XFRM_STATE_DECAP_DSCP)
+               ipv6_copy_dscp(ipv6_get_dsfield(ipv6_hdr(skb)),
+                              ipipv6_hdr(skb));
+       if (!(x->props.flags & XFRM_STATE_NOECN))
+               ipip6_ecn_decapsulate(skb);
+
+       skb_reset_network_header(skb);
+       skb_mac_header_rebuild(skb);
+       if (skb->mac_len)
+               eth_hdr(skb)->h_proto = skb->protocol;
+
+       err = 0;
+
+out:
+       return err;
+}
+
+static int xfrm6_remove_beet_encap(struct xfrm_state *x, struct sk_buff *skb)
+{
+       struct ipv6hdr *ip6h;
+       int size = sizeof(struct ipv6hdr);
+       int err;
+
+       err = skb_cow_head(skb, size + skb->mac_len);
+       if (err)
+               goto out;
+
+       __skb_push(skb, size);
+       skb_reset_network_header(skb);
+       skb_mac_header_rebuild(skb);
+
+       xfrm6_beet_make_header(skb);
+
+       ip6h = ipv6_hdr(skb);
+       ip6h->payload_len = htons(skb->len - size);
+       ip6h->daddr = x->sel.daddr.in6;
+       ip6h->saddr = x->sel.saddr.in6;
+       err = 0;
+out:
+       return err;
+}
+
+/* Remove encapsulation header.
+ *
+ * The IP header will be moved over the top of the encapsulation
+ * header.
+ *
+ * On entry, the transport header shall point to where the IP header
+ * should be and the network header shall be set to where the IP
+ * header currently is.  skb->data shall point to the start of the
+ * payload.
+ */
+static int
+xfrm_inner_mode_encap_remove(struct xfrm_state *x,
+                            const struct xfrm_mode *inner_mode,
+                            struct sk_buff *skb)
+{
+       switch (inner_mode->encap) {
+       case XFRM_MODE_BEET:
+               if (inner_mode->family == AF_INET)
+                       return xfrm4_remove_beet_encap(x, skb);
+               if (inner_mode->family == AF_INET6)
+                       return xfrm6_remove_beet_encap(x, skb);
+               break;
+       case XFRM_MODE_TUNNEL:
+               if (inner_mode->family == AF_INET)
+                       return xfrm4_remove_tunnel_encap(x, skb);
+               if (inner_mode->family == AF_INET6)
+                       return xfrm6_remove_tunnel_encap(x, skb);
+               break;
+       }
+
+       WARN_ON_ONCE(1);
+       return -EOPNOTSUPP;
+}
+
 static int xfrm_prepare_input(struct xfrm_state *x, struct sk_buff *skb)
 {
        struct xfrm_mode *inner_mode = x->inner_mode;
        }
 
        skb->protocol = inner_mode->afinfo->eth_proto;
-       return inner_mode->input2(x, skb);
+       return xfrm_inner_mode_encap_remove(x, inner_mode, skb);
 }
 
 /* Remove encapsulation header.