linux-user: netlink: Add emulation of IP_MULTICAST_IF

Add IP_MULTICAST_IF and share the code with IP_ADD_MEMBERSHIP / IP_DROP_MEMBERSHIP.
Sharing the code makes sense, because the manpage of ip(7) says:

IP_MULTICAST_IF (since Linux 1.2)
      Set the local device for a multicast socket.  The argument
      for setsockopt(2) is an ip_mreqn or (since Linux 3.5)
      ip_mreq structure similar to IP_ADD_MEMBERSHIP, or an
      in_addr structure.  (The kernel determines which structure
      is being passed based on the size passed in optlen.)  For
      getsockopt(2), the argument is an in_addr structure.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>

Author: hdeller

linux-user/syscall.c

@@ -2130,16 +2130,23 @@ static abi_long do_setsockopt(int sockfd, int level, int optname,
             }
             ret = get_errno(setsockopt(sockfd, level, optname, &val, sizeof(val)));
             break;
+        case IP_MULTICAST_IF:
         case IP_ADD_MEMBERSHIP:
         case IP_DROP_MEMBERSHIP:
         {
             struct ip_mreqn ip_mreq;
             struct target_ip_mreqn *target_smreqn;
+            int min_size;
 
             QEMU_BUILD_BUG_ON(sizeof(struct ip_mreq) !=
                               sizeof(struct target_ip_mreq));
 
-            if (optlen < sizeof (struct target_ip_mreq) ||
+            if (optname == IP_MULTICAST_IF) {
+                min_size = sizeof(struct in_addr);
+            } else {
+                min_size = sizeof(struct target_ip_mreq);
+            }
+            if (optlen < min_size ||
                 optlen > sizeof (struct target_ip_mreqn)) {
                 return -TARGET_EINVAL;
             }
@@ -2149,13 +2156,14 @@ static abi_long do_setsockopt(int sockfd, int level, int optname,
                 return -TARGET_EFAULT;
             }
             ip_mreq.imr_multiaddr.s_addr = target_smreqn->imr_multiaddr.s_addr;
-            ip_mreq.imr_address.s_addr = target_smreqn->imr_address.s_addr;
-            if (optlen == sizeof(struct target_ip_mreqn)) {
-                ip_mreq.imr_ifindex = tswapal(target_smreqn->imr_ifindex);
-                optlen = sizeof(struct ip_mreqn);
+            if (optlen >= sizeof(struct target_ip_mreq)) {
+                ip_mreq.imr_address.s_addr = target_smreqn->imr_address.s_addr;
+                if (optlen >= sizeof(struct target_ip_mreqn)) {
+                    __put_user(target_smreqn->imr_ifindex, &ip_mreq.imr_ifindex);
+                    optlen = sizeof(struct ip_mreqn);
+                }
             }
             unlock_user(target_smreqn, optval_addr, 0);
-
             ret = get_errno(setsockopt(sockfd, level, optname, &ip_mreq, optlen));
             break;
         }