Remove buggy IPv6 support from hs_get_extend_info_from_lspecs()

The previous version of this function has the following issues:
* it doesn't choose between IPv4 and IPv6 addresses correctly, and
* it doesn't fall back to a 3-hop path when the address for a direct
  connection is unreachable.
But we can't fix these things in a bugfix release.

Instead, treat IPv6 addresses like any other unrecognised link specifier
and ignore them. If there is no IPv4 address, return NULL.

This supports v3 hidden services on IPv4, dual-stack, and IPv6, and
v3 single onion services on IPv4 only.

Part of 23820, bugfix on 0.3.2.1-alpha.

src/or/hs_common.c

@@ -1638,24 +1638,22 @@ hs_pick_hsdir(smartlist_t *responsible_dirs, const char *req_key_str)
 
 /* From a list of link specifier, an onion key and if we are requesting a
  * direct connection (ex: single onion service), return a newly allocated
- * extend_info_t object. This function checks the firewall policies and if we
- * are allowed to extend to the chosen address.
+ * extend_info_t object. This function always returns an extend info with
+ * an IPv4 address, or NULL.
  *
- *  if either IPv4 or legacy ID is missing, error.
- *  if not direct_conn, IPv4 is prefered.
- *  if direct_conn, IPv6 is prefered if we have one available.
- *  if firewall does not allow the chosen address, error.
- *
- * Return NULL if we can fulfill the conditions. */
+ * It performs the following checks:
+ *  if either IPv4 or legacy ID is missing, return NULL.
+ *  if direct_conn, and we can't reach the IPv4 address, return NULL.
+ */
 extend_info_t *
 hs_get_extend_info_from_lspecs(const smartlist_t *lspecs,
                                const curve25519_public_key_t *onion_key,
                                int direct_conn)
 {
-  int have_v4 = 0, have_v6 = 0, have_legacy_id = 0, have_ed25519_id = 0;
+  int have_v4 = 0, have_legacy_id = 0, have_ed25519_id = 0;
   char legacy_id[DIGEST_LEN] = {0};
-  uint16_t port_v4 = 0, port_v6 = 0, port = 0;
-  tor_addr_t addr_v4, addr_v6, *addr = NULL;
+  uint16_t port_v4 = 0;
+  tor_addr_t addr_v4;
   ed25519_public_key_t ed25519_pk;
   extend_info_t *info = NULL;
 
@@ -1671,14 +1669,6 @@ hs_get_extend_info_from_lspecs(const smartlist_t *lspecs,
       port_v4 = link_specifier_get_un_ipv4_port(ls);
       have_v4 = 1;
       break;
-    case LS_IPV6:
-      /* Skip if we already seen a v6. */
-      if (have_v6) continue;
-      tor_addr_from_ipv6_bytes(&addr_v6,
-          (const char *) link_specifier_getconstarray_un_ipv6_addr(ls));
-      port_v6 = link_specifier_get_un_ipv6_port(ls);
-      have_v6 = 1;
-      break;
     case LS_LEGACY_ID:
       /* Make sure we do have enough bytes for the legacy ID. */
       if (link_specifier_getlen_un_legacy_id(ls) < sizeof(legacy_id)) {
@@ -1700,55 +1690,45 @@ hs_get_extend_info_from_lspecs(const smartlist_t *lspecs,
     }
   } SMARTLIST_FOREACH_END(ls);
 
-  /* IPv4 and legacy ID are mandatory. */
+  /* Legacy ID is mandatory, and we require IPv4. */
   if (!have_v4 || !have_legacy_id) {
     goto done;
   }
-  /* By default, we pick IPv4 but this might change to v6 if certain
-   * conditions are met. */
-  addr = &addr_v4; port = port_v4;
 
-  /* If we are NOT in a direct connection, we'll use our Guard and a 3-hop
-   * circuit so we can't extend in IPv6. And at this point, we do have an IPv4
-   * address available so go to validation. */
+  /* We know we have IPv4, because we just checked. */
   if (!direct_conn) {
+    /* All clients can extend to any IPv4 via a 3-hop path. */
     goto validate;
-  }
-
-  /* From this point on, we have a request for a direct connection to the
-   * rendezvous point so make sure we can actually connect through our
-   * firewall. We'll prefer IPv6. */
-
-  /* IPv6 test. */
-  if (have_v6 &&
-      fascist_firewall_allows_address_addr(&addr_v6, port_v6,
-                                           FIREWALL_OR_CONNECTION, 1, 1)) {
-    /* Direct connection and we can reach it in IPv6 so go for it. */
-    addr = &addr_v6; port = port_v6;
-    goto validate;
-  }
-  /* IPv4 test and we are sure we have a v4 because of the check above. */
-  if (fascist_firewall_allows_address_addr(&addr_v4, port_v4,
-                                           FIREWALL_OR_CONNECTION, 0, 0)) {
+  } else if (direct_conn &&
+             fascist_firewall_allows_address_addr(&addr_v4, port_v4,
+                                                  FIREWALL_OR_CONNECTION,
+                                                  0, 0)) {
     /* Direct connection and we can reach it in IPv4 so go for it. */
-    addr = &addr_v4; port = port_v4;
     goto validate;
+
+    /* We will add support for falling back to a 3-hop path in a later
+     * release. */
+  } else {
+    /* If we can't reach IPv4, return NULL. */
+    goto done;
   }
 
+  /* We will add support for IPv6 in a later release. */
+
  validate:
   /* We'll validate now that the address we've picked isn't a private one. If
    * it is, are we allowing to extend to private address? */
-  if (!extend_info_addr_is_allowed(addr)) {
-    log_warn(LD_REND, "Requested address is private and it is not "
-                      "allowed to extend to it: %s:%u",
-             fmt_addr(&addr_v4), port_v4);
+  if (!extend_info_addr_is_allowed(&addr_v4)) {
+    log_fn(LOG_PROTOCOL_WARN, LD_REND,
+           "Requested address is private and we are not allowed to extend to "
+           "it: %s:%u", fmt_addr(&addr_v4), port_v4);
     goto done;
   }
 
   /* We do have everything for which we think we can connect successfully. */
   info = extend_info_new(NULL, legacy_id,
                          (have_ed25519_id) ? &ed25519_pk : NULL, NULL,
-                         onion_key, addr, port);
+                         onion_key, &addr_v4, port_v4);
  done:
   return info;
 }