slirp: Make RA build more flexible

Do not hardcode the RA size at all, use a pl_size variable which
accounts the accumulated size, and fill rip->ip_pl at the end.

This will allow to make some blocks optional.
Signed-off-by: NSamuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>

slirp/ip6_icmp.c

@@ -143,17 +143,10 @@ void ndp_send_ra(Slirp *slirp)
     /* Build IPv6 packet */
     struct mbuf *t = m_get(slirp);
     struct ip6 *rip = mtod(t, struct ip6 *);
+    size_t pl_size = 0;
     rip->ip_src = (struct in6_addr)LINKLOCAL_ADDR;
     rip->ip_dst = (struct in6_addr)ALLNODES_MULTICAST;
     rip->ip_nh = IPPROTO_ICMPV6;
-    rip->ip_pl = htons(ICMP6_NDP_RA_MINLEN
-                        + NDPOPT_LINKLAYER_LEN
-                        + NDPOPT_PREFIXINFO_LEN
-#ifndef _WIN32
-                        + NDPOPT_RDNSS_LEN
-#endif
-                        );
-    t->m_len = sizeof(struct ip6) + ntohs(rip->ip_pl);
 
     /* Build ICMPv6 packet */
     t->m_data += sizeof(struct ip6);
@@ -171,6 +164,7 @@ void ndp_send_ra(Slirp *slirp)
     ricmp->icmp6_nra.reach_time = htonl(NDP_AdvReachableTime);
     ricmp->icmp6_nra.retrans_time = htonl(NDP_AdvRetransTime);
     t->m_data += ICMP6_NDP_RA_MINLEN;
+    pl_size += ICMP6_NDP_RA_MINLEN;
 
     /* Source link-layer address (NDP option) */
     struct ndpopt *opt = mtod(t, struct ndpopt *);
@@ -178,6 +172,7 @@ void ndp_send_ra(Slirp *slirp)
     opt->ndpopt_len = NDPOPT_LINKLAYER_LEN / 8;
     in6_compute_ethaddr(rip->ip_src, opt->ndpopt_linklayer);
     t->m_data += NDPOPT_LINKLAYER_LEN;
+    pl_size += NDPOPT_LINKLAYER_LEN;
 
     /* Prefix information (NDP option) */
     struct ndpopt *opt2 = mtod(t, struct ndpopt *);
@@ -192,6 +187,7 @@ void ndp_send_ra(Slirp *slirp)
     opt2->ndpopt_prefixinfo.reserved2 = 0;
     opt2->ndpopt_prefixinfo.prefix = slirp->vprefix_addr6;
     t->m_data += NDPOPT_PREFIXINFO_LEN;
+    pl_size += NDPOPT_PREFIXINFO_LEN;
 
 #ifndef _WIN32
     /* Prefix information (NDP option) */
@@ -203,16 +199,14 @@ void ndp_send_ra(Slirp *slirp)
     opt3->ndpopt_rdnss.lifetime = htonl(2 * NDP_MaxRtrAdvInterval);
     opt3->ndpopt_rdnss.addr = slirp->vnameserver_addr6;
     t->m_data += NDPOPT_RDNSS_LEN;
+    pl_size += NDPOPT_RDNSS_LEN;
 #endif
 
+    rip->ip_pl = htons(pl_size);
+    t->m_data -= sizeof(struct ip6) + pl_size;
+    t->m_len = sizeof(struct ip6) + pl_size;
+
     /* ICMPv6 Checksum */
-#ifndef _WIN32
-    t->m_data -= NDPOPT_RDNSS_LEN;
-#endif
-    t->m_data -= NDPOPT_PREFIXINFO_LEN;
-    t->m_data -= NDPOPT_LINKLAYER_LEN;
-    t->m_data -= ICMP6_NDP_RA_MINLEN;
-    t->m_data -= sizeof(struct ip6);
     ricmp->icmp6_cksum = ip6_cksum(t);
 
     ip6_output(NULL, t, 0);