#!/usr/bin/env stap

# bpf_xdp_guru.stp - bump IPv6 hop limits for proxied health checks.
#
# A tiny guru-mode XDP packet rewrite demo.  Health-check probes from
# container-proxy sidecars often arrive with IPv6 hop_limit 1; this
# handler finds them and bumps the limit so the reply can traverse one
# more hop.  Requires -g for the store; reads work without it.
# Example run: stap --runtime=bpf -g bpf_xdp_guru.stp -c "ping -6 -t 1 localhost"

global bumped, total

probe xdp {
  eth = @cast($ctx, "xdp_md", "kernel")->data
  proto = @cast(eth, "ethhdr", "kernel")->h_proto
  proto = ((proto & 0xff) << 8) | (proto >> 8 & 0xff)

  if (proto == 34525) {           # IPv6
    ip6 = eth + 14                # ethhdr is fixed-size
    hl = @cast(ip6, "ipv6hdr", "kernel")->hop_limit
    if (hl == 1) {
      @cast(ip6, "ipv6hdr", "kernel")->hop_limit = 63
      bumped++
    }
  }
  total++

  # Forwarding-only policy: never drop on the way through.
  $return = XDP_PASS
}

probe timer.ms(1500) {
  printf("%d packets seen, %d hop-limit bumps\n", total, bumped)
  exit()
}