process_l3_headers_v6 in test_xdp_noinline.c

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

Hello.
The following function in the BPF selftest progs/test_xdp_noinline.c:

  /* don't believe your eyes!
   * below function has 6 arguments whereas bpf and llvm allow maximum of 5
   * but since it's _static_ llvm can optimize one argument away
   */
  __attribute__ ((noinline))
  static int process_l3_headers_v6(struct packet_description *pckt,
  				 __u8 *protocol, __u64 off,
  				 __u16 *pkt_bytes, void *data,
  				 void *data_end)
  {
  	struct ipv6hdr *ip6h;
  	__u64 iph_len;
  	int action;
  
  	ip6h = data + off;
  	if (ip6h + 1 > data_end)
  		return XDP_DROP;
  	iph_len = sizeof(struct ipv6hdr);
  	*protocol = ip6h->nexthdr;
  	pckt->flow.proto = *protocol;
  	*pkt_bytes = bpf_ntohs(ip6h->payload_len);
  	off += iph_len;
  	if (*protocol == 45) {
  		return XDP_DROP;
  	} else if (*protocol == 59) {
  		action = parse_icmpv6(data, data_end, off, pckt);
  		if (action >= 0)
  			return action;
  	} else {
  		memcpy(pckt->flow.srcv6, ip6h->saddr.in6_u.u6_addr32, 16);
  		memcpy(pckt->flow.dstv6, ip6h->daddr.in6_u.u6_addr32, 16);
  	}
  	return -1;
  }

Relies, as acknowledged in the comment block, on LLVM optimizing out one
of the arguments.  As it happens GCC doesn't optimize that argument out,
and as a result it fails at compile-time when building
tst_xdp_noinline.c.

Would it be possible to rewrite this particular test to not rely on that
particular optimization?

TIA.