On Wed, Apr 24, 2024 at 5:25 AM Jose E. Marchesi
<jose.marchesi@xxxxxxxxxx> 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.
