Re: process_l3_headers_v6 in test_xdp_noinline.c

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

> On Wed, Apr 24, 2024 at 5:25 AM Jose E. Marchesi
> <jose.marchesi@oracle.com> wrote:
>>
>>
>> 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?
>
> Feel free to send a patch that reduces it to 5 args.
> This test was a copy paste of katran.
> There was no intent to test this 6->5 llvm optimization.

Ok perfect.  Will do.  Thanks!