target/linux/generic/pending-6.1/680-net-add-TCP-fraglist-GRO-support.patch

   1 From: Felix Fietkau <nbd@nbd.name>
   2 Date: Tue, 23 Apr 2024 11:23:03 +0200
   3 Subject: [PATCH] net: add TCP fraglist GRO support
   4
   5 When forwarding TCP after GRO, software segmentation is very expensive,
   6 especially when the checksum needs to be recalculated.
   7 One case where that's currently unavoidable is when routing packets over
   8 PPPoE. Performance improves significantly when using fraglist GRO
   9 implemented in the same way as for UDP.
  10
  11 Here's a measurement of running 2 TCP streams through a MediaTek MT7622
  12 device (2-core Cortex-A53), which runs NAT with flow offload enabled from
  13 one ethernet port to PPPoE on another ethernet port + cake qdisc set to
  14 1Gbps.
  15
  16 rx-gro-list off: 630 Mbit/s, CPU 35% idle
  17 rx-gro-list on:  770 Mbit/s, CPU 40% idle
  18
  19 Signe-off-by: Felix Fietkau <nbd@nbd.name>
  20 ---
  21
  22 --- a/include/net/gro.h
  23 +++ b/include/net/gro.h
  24 @@ -424,6 +424,7 @@ static inline __wsum ip6_gro_compute_pse
  25  }
  26
  27  int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb);
  28 +int skb_gro_receive_list(struct sk_buff *p, struct sk_buff *skb);
  29
  30  /* Pass the currently batched GRO_NORMAL SKBs up to the stack. */
  31  static inline void gro_normal_list(struct napi_struct *napi)
  32 @@ -446,5 +447,48 @@ static inline void gro_normal_one(struct
  33                 gro_normal_list(napi);
  34  }
  35
  36 +/* This function is the alternative of 'inet_iif' and 'inet_sdif'
  37 + * functions in case we can not rely on fields of IPCB.
  38 + *
  39 + * The caller must verify skb_valid_dst(skb) is false and skb->dev is initialized.
  40 + * The caller must hold the RCU read lock.
  41 + */
  42 +static inline void inet_get_iif_sdif(const struct sk_buff *skb, int *iif, int *sdif)
  43 +{
  44 +       *iif = inet_iif(skb) ?: skb->dev->ifindex;
  45 +       *sdif = 0;
  46 +
  47 +#if IS_ENABLED(CONFIG_NET_L3_MASTER_DEV)
  48 +       if (netif_is_l3_slave(skb->dev)) {
  49 +               struct net_device *master = netdev_master_upper_dev_get_rcu(skb->dev);
  50 +
  51 +               *sdif = *iif;
  52 +               *iif = master ? master->ifindex : 0;
  53 +       }
  54 +#endif
  55 +}
  56 +
  57 +/* This function is the alternative of 'inet6_iif' and 'inet6_sdif'
  58 + * functions in case we can not rely on fields of IP6CB.
  59 + *
  60 + * The caller must verify skb_valid_dst(skb) is false and skb->dev is initialized.
  61 + * The caller must hold the RCU read lock.
  62 + */
  63 +static inline void inet6_get_iif_sdif(const struct sk_buff *skb, int *iif, int *sdif)
  64 +{
  65 +       /* using skb->dev->ifindex because skb_dst(skb) is not initialized */
  66 +       *iif = skb->dev->ifindex;
  67 +       *sdif = 0;
  68 +
  69 +#if IS_ENABLED(CONFIG_NET_L3_MASTER_DEV)
  70 +       if (netif_is_l3_slave(skb->dev)) {
  71 +               struct net_device *master = netdev_master_upper_dev_get_rcu(skb->dev);
  72 +
  73 +               *sdif = *iif;
  74 +               *iif = master ? master->ifindex : 0;
  75 +       }
  76 +#endif
  77 +}
  78 +
  79
  80  #endif /* _NET_IPV6_GRO_H */
  81 --- a/include/net/tcp.h
  82 +++ b/include/net/tcp.h
  83 @@ -2057,7 +2057,10 @@ void tcp_v4_destroy_sock(struct sock *sk
  84
  85  struct sk_buff *tcp_gso_segment(struct sk_buff *skb,
  86                                 netdev_features_t features);
  87 -struct sk_buff *tcp_gro_receive(struct list_head *head, struct sk_buff *skb);
  88 +struct tcphdr *tcp_gro_pull_header(struct sk_buff *skb);
  89 +struct sk_buff *tcp_gro_lookup(struct list_head *head, struct tcphdr *th);
  90 +struct sk_buff *tcp_gro_receive(struct list_head *head, struct sk_buff *skb,
  91 +                               struct tcphdr *th);
  92  INDIRECT_CALLABLE_DECLARE(int tcp4_gro_complete(struct sk_buff *skb, int thoff));
  93  INDIRECT_CALLABLE_DECLARE(struct sk_buff *tcp4_gro_receive(struct list_head *head, struct sk_buff *skb));
  94  INDIRECT_CALLABLE_DECLARE(int tcp6_gro_complete(struct sk_buff *skb, int thoff));
  95 --- a/net/core/gro.c
  96 +++ b/net/core/gro.c
  97 @@ -290,6 +290,33 @@ done:
  98         return 0;
  99  }
 100
 101 +int skb_gro_receive_list(struct sk_buff *p, struct sk_buff *skb)
 102 +{
 103 +       if (unlikely(p->len + skb->len >= 65536))
 104 +               return -E2BIG;
 105 +
 106 +       if (NAPI_GRO_CB(p)->last == p)
 107 +               skb_shinfo(p)->frag_list = skb;
 108 +       else
 109 +               NAPI_GRO_CB(p)->last->next = skb;
 110 +
 111 +       skb_pull(skb, skb_gro_offset(skb));
 112 +
 113 +       NAPI_GRO_CB(p)->last = skb;
 114 +       NAPI_GRO_CB(p)->count++;
 115 +       p->data_len += skb->len;
 116 +
 117 +       /* sk ownership - if any - completely transferred to the aggregated packet */
 118 +       skb->destructor = NULL;
 119 +       skb->sk = NULL;
 120 +       p->truesize += skb->truesize;
 121 +       p->len += skb->len;
 122 +
 123 +       NAPI_GRO_CB(skb)->same_flow = 1;
 124 +
 125 +       return 0;
 126 +}
 127 +
 128
 129  static void napi_gro_complete(struct napi_struct *napi, struct sk_buff *skb)
 130  {
 131 --- a/net/ipv4/tcp_offload.c
 132 +++ b/net/ipv4/tcp_offload.c
 133 @@ -27,6 +27,68 @@ static void tcp_gso_tstamp(struct sk_buf
 134         }
 135  }
 136
 137 +static void __tcpv4_gso_segment_csum(struct sk_buff *seg,
 138 +                                    __be32 *oldip, __be32 *newip,
 139 +                                    __be16 *oldport, __be16 *newport)
 140 +{
 141 +       struct tcphdr *th;
 142 +       struct iphdr *iph;
 143 +
 144 +       if (*oldip == *newip && *oldport == *newport)
 145 +               return;
 146 +
 147 +       th = tcp_hdr(seg);
 148 +       iph = ip_hdr(seg);
 149 +
 150 +       inet_proto_csum_replace4(&th->check, seg, *oldip, *newip, true);
 151 +       inet_proto_csum_replace2(&th->check, seg, *oldport, *newport, false);
 152 +       *oldport = *newport;
 153 +
 154 +       csum_replace4(&iph->check, *oldip, *newip);
 155 +       *oldip = *newip;
 156 +}
 157 +
 158 +static struct sk_buff *__tcpv4_gso_segment_list_csum(struct sk_buff *segs)
 159 +{
 160 +       struct sk_buff *seg;
 161 +       struct tcphdr *th, *th2;
 162 +       struct iphdr *iph, *iph2;
 163 +
 164 +       seg = segs;
 165 +       th = tcp_hdr(seg);
 166 +       iph = ip_hdr(seg);
 167 +       th2 = tcp_hdr(seg->next);
 168 +       iph2 = ip_hdr(seg->next);
 169 +
 170 +       if (!(*(u32 *)&th->source ^ *(u32 *)&th2->source) &&
 171 +           iph->daddr == iph2->daddr && iph->saddr == iph2->saddr)
 172 +               return segs;
 173 +
 174 +       while ((seg = seg->next)) {
 175 +               th2 = tcp_hdr(seg);
 176 +               iph2 = ip_hdr(seg);
 177 +
 178 +               __tcpv4_gso_segment_csum(seg,
 179 +                                        &iph2->saddr, &iph->saddr,
 180 +                                        &th2->source, &th->source);
 181 +               __tcpv4_gso_segment_csum(seg,
 182 +                                        &iph2->daddr, &iph->daddr,
 183 +                                        &th2->dest, &th->dest);
 184 +       }
 185 +
 186 +       return segs;
 187 +}
 188 +
 189 +static struct sk_buff *__tcp4_gso_segment_list(struct sk_buff *skb,
 190 +                                             netdev_features_t features)
 191 +{
 192 +       skb = skb_segment_list(skb, features, skb_mac_header_len(skb));
 193 +       if (IS_ERR(skb))
 194 +               return skb;
 195 +
 196 +       return __tcpv4_gso_segment_list_csum(skb);
 197 +}
 198 +
 199  static struct sk_buff *tcp4_gso_segment(struct sk_buff *skb,
 200                                         netdev_features_t features)
 201  {
 202 @@ -36,6 +98,9 @@ static struct sk_buff *tcp4_gso_segment(
 203         if (!pskb_may_pull(skb, sizeof(struct tcphdr)))
 204                 return ERR_PTR(-EINVAL);
 205
 206 +       if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST)
 207 +               return __tcp4_gso_segment_list(skb, features);
 208 +
 209         if (unlikely(skb->ip_summed != CHECKSUM_PARTIAL)) {
 210                 const struct iphdr *iph = ip_hdr(skb);
 211                 struct tcphdr *th = tcp_hdr(skb);
 212 @@ -177,61 +242,76 @@ out:
 213         return segs;
 214  }
 215
 216 -struct sk_buff *tcp_gro_receive(struct list_head *head, struct sk_buff *skb)
 217 +struct sk_buff *tcp_gro_lookup(struct list_head *head, struct tcphdr *th)
 218  {
 219 -       struct sk_buff *pp = NULL;
 220 +       struct tcphdr *th2;
 221         struct sk_buff *p;
 222 +
 223 +       list_for_each_entry(p, head, list) {
 224 +               if (!NAPI_GRO_CB(p)->same_flow)
 225 +                       continue;
 226 +
 227 +               th2 = tcp_hdr(p);
 228 +               if (*(u32 *)&th->source ^ *(u32 *)&th2->source) {
 229 +                       NAPI_GRO_CB(p)->same_flow = 0;
 230 +                       continue;
 231 +               }
 232 +
 233 +               return p;
 234 +       }
 235 +
 236 +       return NULL;
 237 +}
 238 +
 239 +struct tcphdr *tcp_gro_pull_header(struct sk_buff *skb)
 240 +{
 241 +       unsigned int thlen, hlen, off;
 242         struct tcphdr *th;
 243 -       struct tcphdr *th2;
 244 -       unsigned int len;
 245 -       unsigned int thlen;
 246 -       __be32 flags;
 247 -       unsigned int mss = 1;
 248 -       unsigned int hlen;
 249 -       unsigned int off;
 250 -       int flush = 1;
 251 -       int i;
 252
 253         off = skb_gro_offset(skb);
 254         hlen = off + sizeof(*th);
 255         th = skb_gro_header(skb, hlen, off);
 256         if (unlikely(!th))
 257 -               goto out;
 258 +               return NULL;
 259
 260         thlen = th->doff * 4;
 261         if (thlen < sizeof(*th))
 262 -               goto out;
 263 +               return NULL;
 264
 265         hlen = off + thlen;
 266         if (skb_gro_header_hard(skb, hlen)) {
 267                 th = skb_gro_header_slow(skb, hlen, off);
 268                 if (unlikely(!th))
 269 -                       goto out;
 270 +                       return NULL;
 271         }
 272
 273         skb_gro_pull(skb, thlen);
 274
 275 -       len = skb_gro_len(skb);
 276 -       flags = tcp_flag_word(th);
 277 -
 278 -       list_for_each_entry(p, head, list) {
 279 -               if (!NAPI_GRO_CB(p)->same_flow)
 280 -                       continue;
 281 +       return th;
 282 +}
 283
 284 -               th2 = tcp_hdr(p);
 285 +struct sk_buff *tcp_gro_receive(struct list_head *head, struct sk_buff *skb,
 286 +                               struct tcphdr *th)
 287 +{
 288 +       unsigned int thlen = th->doff * 4;
 289 +       struct sk_buff *pp = NULL;
 290 +       struct sk_buff *p;
 291 +       struct tcphdr *th2;
 292 +       unsigned int len;
 293 +       __be32 flags;
 294 +       unsigned int mss = 1;
 295 +       int flush = 1;
 296 +       int i;
 297
 298 -               if (*(u32 *)&th->source ^ *(u32 *)&th2->source) {
 299 -                       NAPI_GRO_CB(p)->same_flow = 0;
 300 -                       continue;
 301 -               }
 302 +       len = skb_gro_len(skb);
 303 +       flags = tcp_flag_word(th);
 304
 305 -               goto found;
 306 -       }
 307 -       p = NULL;
 308 -       goto out_check_final;
 309 +       p = tcp_gro_lookup(head, th);
 310 +       if (!p)
 311 +               goto out_check_final;
 312
 313 -found:
 314         /* Include the IP ID check below from the inner most IP hdr */
 315 +       th2 = tcp_hdr(p);
 316         flush = NAPI_GRO_CB(p)->flush;
 317         flush |= (__force int)(flags & TCP_FLAG_CWR);
 318         flush |= (__force int)((flags ^ tcp_flag_word(th2)) &
 319 @@ -268,6 +348,19 @@ found:
 320         flush |= p->decrypted ^ skb->decrypted;
 321  #endif
 322
 323 +       if (NAPI_GRO_CB(p)->is_flist) {
 324 +               flush |= (__force int)(flags ^ tcp_flag_word(th2));
 325 +               flush |= skb->ip_summed != p->ip_summed;
 326 +               flush |= skb->csum_level != p->csum_level;
 327 +               flush |= !pskb_may_pull(skb, skb_gro_offset(skb));
 328 +               flush |= NAPI_GRO_CB(p)->count >= 64;
 329 +
 330 +               if (flush || skb_gro_receive_list(p, skb))
 331 +                       mss = 1;
 332 +
 333 +               goto out_check_final;
 334 +       }
 335 +
 336         if (flush || skb_gro_receive(p, skb)) {
 337                 mss = 1;
 338                 goto out_check_final;
 339 @@ -289,7 +382,6 @@ out_check_final:
 340         if (p && (!NAPI_GRO_CB(skb)->same_flow || flush))
 341                 pp = p;
 342
 343 -out:
 344         NAPI_GRO_CB(skb)->flush |= (flush != 0);
 345
 346         return pp;
 347 @@ -315,18 +407,56 @@ int tcp_gro_complete(struct sk_buff *skb
 348  }
 349  EXPORT_SYMBOL(tcp_gro_complete);
 350
 351 +static void tcp4_check_fraglist_gro(struct list_head *head, struct sk_buff *skb,
 352 +                                   struct tcphdr *th)
 353 +{
 354 +       const struct iphdr *iph = skb_gro_network_header(skb);
 355 +       struct net *net = dev_net(skb->dev);
 356 +       struct sk_buff *p;
 357 +       struct sock *sk;
 358 +       int iif, sdif;
 359 +
 360 +       if (!(skb->dev->features & NETIF_F_GRO_FRAGLIST))
 361 +               return;
 362 +
 363 +       p = tcp_gro_lookup(head, th);
 364 +       if (p) {
 365 +               NAPI_GRO_CB(skb)->is_flist = NAPI_GRO_CB(p)->is_flist;
 366 +               return;
 367 +       }
 368 +
 369 +       inet_get_iif_sdif(skb, &iif, &sdif);
 370 +       sk = __inet_lookup_established(net, net->ipv4.tcp_death_row.hashinfo,
 371 +                                      iph->saddr, th->source,
 372 +                                      iph->daddr, ntohs(th->dest),
 373 +                                      iif, sdif);
 374 +       NAPI_GRO_CB(skb)->is_flist = !sk;
 375 +       if (sk)
 376 +               sock_put(sk);
 377 +}
 378 +
 379  INDIRECT_CALLABLE_SCOPE
 380  struct sk_buff *tcp4_gro_receive(struct list_head *head, struct sk_buff *skb)
 381  {
 382 +       struct tcphdr *th;
 383 +
 384         /* Don't bother verifying checksum if we're going to flush anyway. */
 385         if (!NAPI_GRO_CB(skb)->flush &&
 386             skb_gro_checksum_validate(skb, IPPROTO_TCP,
 387 -                                     inet_gro_compute_pseudo)) {
 388 -               NAPI_GRO_CB(skb)->flush = 1;
 389 -               return NULL;
 390 -       }
 391 +                                     inet_gro_compute_pseudo))
 392 +               goto flush;
 393 +
 394 +       th = tcp_gro_pull_header(skb);
 395 +       if (!th)
 396 +               goto flush;
 397
 398 -       return tcp_gro_receive(head, skb);
 399 +       tcp4_check_fraglist_gro(head, skb, th);
 400 +
 401 +       return tcp_gro_receive(head, skb, th);
 402 +
 403 +flush:
 404 +       NAPI_GRO_CB(skb)->flush = 1;
 405 +       return NULL;
 406  }
 407
 408  INDIRECT_CALLABLE_SCOPE int tcp4_gro_complete(struct sk_buff *skb, int thoff)
 409 @@ -334,6 +464,15 @@ INDIRECT_CALLABLE_SCOPE int tcp4_gro_com
 410         const struct iphdr *iph = ip_hdr(skb);
 411         struct tcphdr *th = tcp_hdr(skb);
 412
 413 +       if (NAPI_GRO_CB(skb)->is_flist) {
 414 +               skb_shinfo(skb)->gso_type |= SKB_GSO_FRAGLIST | SKB_GSO_TCPV4;
 415 +               skb_shinfo(skb)->gso_segs = NAPI_GRO_CB(skb)->count;
 416 +
 417 +               __skb_incr_checksum_unnecessary(skb);
 418 +
 419 +               return 0;
 420 +       }
 421 +
 422         th->check = ~tcp_v4_check(skb->len - thoff, iph->saddr,
 423                                   iph->daddr, 0);
 424         skb_shinfo(skb)->gso_type |= SKB_GSO_TCPV4;
 425 --- a/net/ipv4/udp_offload.c
 426 +++ b/net/ipv4/udp_offload.c
 427 @@ -425,33 +425,6 @@ out:
 428         return segs;
 429  }
 430
 431 -static int skb_gro_receive_list(struct sk_buff *p, struct sk_buff *skb)
 432 -{
 433 -       if (unlikely(p->len + skb->len >= 65536))
 434 -               return -E2BIG;
 435 -
 436 -       if (NAPI_GRO_CB(p)->last == p)
 437 -               skb_shinfo(p)->frag_list = skb;
 438 -       else
 439 -               NAPI_GRO_CB(p)->last->next = skb;
 440 -
 441 -       skb_pull(skb, skb_gro_offset(skb));
 442 -
 443 -       NAPI_GRO_CB(p)->last = skb;
 444 -       NAPI_GRO_CB(p)->count++;
 445 -       p->data_len += skb->len;
 446 -
 447 -       /* sk ownership - if any - completely transferred to the aggregated packet */
 448 -       skb->destructor = NULL;
 449 -       skb->sk = NULL;
 450 -       p->truesize += skb->truesize;
 451 -       p->len += skb->len;
 452 -
 453 -       NAPI_GRO_CB(skb)->same_flow = 1;
 454 -
 455 -       return 0;
 456 -}
 457 -
 458
 459  #define UDP_GRO_CNT_MAX 64
 460  static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
 461 --- a/net/ipv6/tcpv6_offload.c
 462 +++ b/net/ipv6/tcpv6_offload.c
 463 @@ -7,24 +7,65 @@
 464   */
 465  #include <linux/indirect_call_wrapper.h>
 466  #include <linux/skbuff.h>
 467 +#include <net/inet6_hashtables.h>
 468  #include <net/gro.h>
 469  #include <net/protocol.h>
 470  #include <net/tcp.h>
 471  #include <net/ip6_checksum.h>
 472  #include "ip6_offload.h"
 473
 474 +static void tcp6_check_fraglist_gro(struct list_head *head, struct sk_buff *skb,
 475 +                                   struct tcphdr *th)
 476 +{
 477 +#if IS_ENABLED(CONFIG_IPV6)
 478 +       const struct ipv6hdr *hdr = skb_gro_network_header(skb);
 479 +       struct net *net = dev_net(skb->dev);
 480 +       struct sk_buff *p;
 481 +       struct sock *sk;
 482 +       int iif, sdif;
 483 +
 484 +       if (!(skb->dev->features & NETIF_F_GRO_FRAGLIST))
 485 +               return;
 486 +
 487 +       p = tcp_gro_lookup(head, th);
 488 +       if (p) {
 489 +               NAPI_GRO_CB(skb)->is_flist = NAPI_GRO_CB(p)->is_flist;
 490 +               return;
 491 +       }
 492 +
 493 +       inet6_get_iif_sdif(skb, &iif, &sdif);
 494 +       sk = __inet6_lookup_established(net, net->ipv4.tcp_death_row.hashinfo,
 495 +                                       &hdr->saddr, th->source,
 496 +                                       &hdr->daddr, ntohs(th->dest),
 497 +                                       iif, sdif);
 498 +       NAPI_GRO_CB(skb)->is_flist = !sk;
 499 +       if (sk)
 500 +               sock_put(sk);
 501 +#endif /* IS_ENABLED(CONFIG_IPV6) */
 502 +}
 503 +
 504  INDIRECT_CALLABLE_SCOPE
 505  struct sk_buff *tcp6_gro_receive(struct list_head *head, struct sk_buff *skb)
 506  {
 507 +       struct tcphdr *th;
 508 +
 509         /* Don't bother verifying checksum if we're going to flush anyway. */
 510         if (!NAPI_GRO_CB(skb)->flush &&
 511             skb_gro_checksum_validate(skb, IPPROTO_TCP,
 512 -                                     ip6_gro_compute_pseudo)) {
 513 -               NAPI_GRO_CB(skb)->flush = 1;
 514 -               return NULL;
 515 -       }
 516 +                                     ip6_gro_compute_pseudo))
 517 +               goto flush;
 518
 519 -       return tcp_gro_receive(head, skb);
 520 +       th = tcp_gro_pull_header(skb);
 521 +       if (!th)
 522 +               goto flush;
 523 +
 524 +       tcp6_check_fraglist_gro(head, skb, th);
 525 +
 526 +       return tcp_gro_receive(head, skb, th);
 527 +
 528 +flush:
 529 +       NAPI_GRO_CB(skb)->flush = 1;
 530 +       return NULL;
 531  }
 532
 533  INDIRECT_CALLABLE_SCOPE int tcp6_gro_complete(struct sk_buff *skb, int thoff)
 534 @@ -32,6 +73,15 @@ INDIRECT_CALLABLE_SCOPE int tcp6_gro_com
 535         const struct ipv6hdr *iph = ipv6_hdr(skb);
 536         struct tcphdr *th = tcp_hdr(skb);
 537
 538 +       if (NAPI_GRO_CB(skb)->is_flist) {
 539 +               skb_shinfo(skb)->gso_type |= SKB_GSO_FRAGLIST | SKB_GSO_TCPV6;
 540 +               skb_shinfo(skb)->gso_segs = NAPI_GRO_CB(skb)->count;
 541 +
 542 +               __skb_incr_checksum_unnecessary(skb);
 543 +
 544 +               return 0;
 545 +       }
 546 +
 547         th->check = ~tcp_v6_check(skb->len - thoff, &iph->saddr,
 548                                   &iph->daddr, 0);
 549         skb_shinfo(skb)->gso_type |= SKB_GSO_TCPV6;
 550 @@ -50,6 +100,9 @@ static struct sk_buff *tcp6_gso_segment(
 551         if (!pskb_may_pull(skb, sizeof(*th)))
 552                 return ERR_PTR(-EINVAL);
 553
 554 +       if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST)
 555 +               return skb_segment_list(skb, features, skb_mac_header_len(skb));
 556 +
 557         if (unlikely(skb->ip_summed != CHECKSUM_PARTIAL)) {
 558                 const struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 559                 struct tcphdr *th = tcp_hdr(skb);