samples/bpf/xdp2skb_meta_kern.c

   1 /* SPDX-License-Identifier: GPL-2.0
   2  * Copyright (c) 2018 Jesper Dangaard Brouer, Red Hat Inc.
   3  *
   4  * Example howto transfer info from XDP to SKB, e.g. skb->mark
   5  * -----------------------------------------------------------
   6  * This uses the XDP data_meta infrastructure, and is a cooperation
   7  * between two bpf-programs (1) XDP and (2) clsact at TC-ingress hook.
   8  *
   9  * Notice: This example does not use the BPF C-loader (bpf_load.c),
  10  * but instead rely on the iproute2 TC tool for loading BPF-objects.
  11  */
  12 #include <uapi/linux/bpf.h>
  13 #include <uapi/linux/pkt_cls.h>
  14
  15 #include "bpf_helpers.h"
  16
  17 /*
  18  * This struct is stored in the XDP 'data_meta' area, which is located
  19  * just in-front-of the raw packet payload data.  The meaning is
  20  * specific to these two BPF programs that use it as a communication
  21  * channel.  XDP adjust/increase the area via a bpf-helper, and TC use
  22  * boundary checks to see if data have been provided.
  23  *
  24  * The struct must be 4 byte aligned, which here is enforced by the
  25  * struct __attribute__((aligned(4))).
  26  */
  27 struct meta_info {
  28         __u32 mark;
  29 } __attribute__((aligned(4)));
  30
  31 SEC("xdp_mark")
  32 int _xdp_mark(struct xdp_md *ctx)
  33 {
  34         struct meta_info *meta;
  35         void *data, *data_end;
  36         int ret;
  37
  38         /* Reserve space in-front of data pointer for our meta info.
  39          * (Notice drivers not supporting data_meta will fail here!)
  40          */
  41         ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(*meta));
  42         if (ret < 0)
  43                 return XDP_ABORTED;
  44
  45         /* Notice: Kernel-side verifier requires that loading of
  46          * ctx->data MUST happen _after_ helper bpf_xdp_adjust_meta(),
  47          * as pkt-data pointers are invalidated.  Helpers that require
  48          * this are determined/marked by bpf_helper_changes_pkt_data()
  49          */
  50         data = (void *)(unsigned long)ctx->data;
  51
  52         /* Check data_meta have room for meta_info struct */
  53         meta = (void *)(unsigned long)ctx->data_meta;
  54         if (meta + 1 > data)
  55                 return XDP_ABORTED;
  56
  57         meta->mark = 42;
  58
  59         return XDP_PASS;
  60 }
  61
  62 SEC("tc_mark")
  63 int _tc_mark(struct __sk_buff *ctx)
  64 {
  65         void *data      = (void *)(unsigned long)ctx->data;
  66         void *data_end  = (void *)(unsigned long)ctx->data_end;
  67         void *data_meta = (void *)(unsigned long)ctx->data_meta;
  68         struct meta_info *meta = data_meta;
  69
  70         /* Check XDP gave us some data_meta */
  71         if (meta + 1 > data) {
  72                 ctx->mark = 41;
  73                  /* Skip "accept" if no data_meta is avail */
  74                 return TC_ACT_OK;
  75         }
  76
  77         /* Hint: See func tc_cls_act_is_valid_access() for BPF_WRITE access */
  78         ctx->mark = meta->mark; /* Transfer XDP-mark to SKB-mark */
  79
  80         return TC_ACT_OK;
  81 }
  82
  83 /* Manually attaching these programs:
  84 export DEV=ixgbe2
  85 export FILE=xdp2skb_meta_kern.o
  86
  87 # via TC command
  88 tc qdisc del dev $DEV clsact 2> /dev/null
  89 tc qdisc add dev $DEV clsact
  90 tc filter  add dev $DEV ingress prio 1 handle 1 bpf da obj $FILE sec tc_mark
  91 tc filter show dev $DEV ingress
  92
  93 # XDP via IP command:
  94 ip link set dev $DEV xdp off
  95 ip link set dev $DEV xdp obj $FILE sec xdp_mark
  96
  97 # Use iptable to "see" if SKBs are marked
  98 iptables -I INPUT -p icmp -m mark --mark 41  # == 0x29
  99 iptables -I INPUT -p icmp -m mark --mark 42  # == 0x2a
 100
 101 # Hint: catch XDP_ABORTED errors via
 102 perf record -e xdp:*
 103 perf script
 104
 105 */