ipv6: do compat setsockopt for MCAST_MSFILTER directly

similar to the ipv4 counterpart of that patch - the same
trick used to align the tail array properly.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

net/ipv6/ipv6_sockglue.c

@@ -980,9 +980,55 @@ int compat_ipv6_setsockopt(struct sock *sk, int level, int optname,
 	case MCAST_LEAVE_SOURCE_GROUP:
 	case MCAST_BLOCK_SOURCE:
 	case MCAST_UNBLOCK_SOURCE:
-	case MCAST_MSFILTER:
 		return compat_mc_setsockopt(sk, level, optname, optval, optlen,
 			ipv6_setsockopt);
+	case MCAST_MSFILTER:
+	{
+		const int size0 = offsetof(struct compat_group_filter, gf_slist);
+		struct compat_group_filter *gf32;
+		void *p;
+		int n;
+
+		if (optlen < size0)
+			return -EINVAL;
+		if (optlen > sysctl_optmem_max - 4)
+			return -ENOBUFS;
+
+		p = kmalloc(optlen + 4, GFP_KERNEL);
+		if (!p)
+			return -ENOMEM;
+
+		gf32 = p + 4; /* we want ->gf_group and ->gf_slist aligned */
+		if (copy_from_user(gf32, optval, optlen)) {
+			err = -EFAULT;
+			goto mc_msf_out;
+		}
+
+		n = gf32->gf_numsrc;
+		/* numsrc >= (4G-140)/128 overflow in 32 bits */
+		if (n >= 0x1ffffffU ||
+		    n > sysctl_mld_max_msf) {
+			err = -ENOBUFS;
+			goto mc_msf_out;
+		}
+		if (offsetof(struct compat_group_filter, gf_slist[n]) > optlen) {
+			err = -EINVAL;
+			goto mc_msf_out;
+		}
+
+		rtnl_lock();
+		lock_sock(sk);
+		err = ip6_mc_msfilter(sk, &(struct group_filter){
+				.gf_interface = gf32->gf_interface,
+				.gf_group = gf32->gf_group,
+				.gf_fmode = gf32->gf_fmode,
+				.gf_numsrc = gf32->gf_numsrc}, gf32->gf_slist);
+		release_sock(sk);
+		rtnl_unlock();
+mc_msf_out:
+		kfree(p);
+		return err;
+	}
 	}
 
 	err = do_ipv6_setsockopt(sk, level, optname, optval, optlen);