diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index f56f3e0..15b1e81 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -1202,6 +1202,15 @@ static inline abi_long target_to_host_cmsg(struct msghdr *msgh,
         space += CMSG_SPACE(len);
         if (space > msgh->msg_controllen) {
             space -= CMSG_SPACE(len);
+            /* This is a QEMU bug, since we allocated the payload
+             * area ourselves (unlike overflow in host-to-target
+             * conversion, which is just the guest giving us a buffer
+             * that's too small). It can't happen for the payload types
+             * we currently support; if it becomes an issue in future
+             * we would need to improve our allocation strategy to
+             * something more intelligent than "twice the size of the
+             * target buffer we're reading from".
+             */
             gemu_log("Host cmsg overflow\n");
             break;
         }
@@ -1267,11 +1276,16 @@ static inline abi_long host_to_target_cmsg(struct target_msghdr *target_msgh,
         void *target_data = TARGET_CMSG_DATA(target_cmsg);
 
         int len = cmsg->cmsg_len - CMSG_ALIGN(sizeof (struct cmsghdr));
+        int tgt_len, tgt_space;
 
-        space += TARGET_CMSG_SPACE(len);
-        if (space > msg_controllen) {
-            space -= TARGET_CMSG_SPACE(len);
-            gemu_log("Target cmsg overflow\n");
+        /* We never copy a half-header but may copy half-data;
+         * this is Linux's behaviour in put_cmsg(). Note that
+         * truncation here is a guest problem (which we report
+         * to the guest via the CTRUNC bit), unlike truncation
+         * in target_to_host_cmsg, which is a QEMU bug.
+         */
+        if (msg_controllen < sizeof(struct cmsghdr)) {
+            target_msgh->msg_flags |= tswap32(MSG_CTRUNC);
             break;
         }
 
@@ -1281,8 +1295,35 @@ static inline abi_long host_to_target_cmsg(struct target_msghdr *target_msgh,
             target_cmsg->cmsg_level = tswap32(cmsg->cmsg_level);
         }
         target_cmsg->cmsg_type = tswap32(cmsg->cmsg_type);
-        target_cmsg->cmsg_len = tswapal(TARGET_CMSG_LEN(len));
 
+        tgt_len = TARGET_CMSG_LEN(len);
+
+        /* Payload types which need a different size of payload on
+         * the target must adjust tgt_len here.
+         */
+        switch (cmsg->cmsg_level) {
+        case SOL_SOCKET:
+            switch (cmsg->cmsg_type) {
+            case SO_TIMESTAMP:
+                tgt_len = sizeof(struct target_timeval);
+                break;
+            default:
+                break;
+            }
+        default:
+            break;
+        }
+
+        if (msg_controllen < tgt_len) {
+            target_msgh->msg_flags |= tswap32(MSG_CTRUNC);
+            tgt_len = msg_controllen;
+        }
+
+        /* We must now copy-and-convert len bytes of payload
+         * into tgt_len bytes of destination space. Bear in mind
+         * that in both source and destination we may be dealing
+         * with a truncated value!
+         */
         switch (cmsg->cmsg_level) {
         case SOL_SOCKET:
             switch (cmsg->cmsg_type) {
@@ -1290,7 +1331,7 @@ static inline abi_long host_to_target_cmsg(struct target_msghdr *target_msgh,
             {
                 int *fd = (int *)data;
                 int *target_fd = (int *)target_data;
-                int i, numfds = len / sizeof(int);
+                int i, numfds = tgt_len / sizeof(int);
 
                 for (i = 0; i < numfds; i++)
                     target_fd[i] = tswap32(fd[i]);
@@ -1302,8 +1343,10 @@ static inline abi_long host_to_target_cmsg(struct target_msghdr *target_msgh,
                 struct target_timeval *target_tv =
                     (struct target_timeval *)target_data;
 
-                if (len != sizeof(struct timeval))
+                if (len != sizeof(struct timeval) ||
+                    tgt_len != sizeof(struct target_timeval)) {
                     goto unimplemented;
+                }
 
                 /* copy struct timeval to target */
                 target_tv->tv_sec = tswapal(tv->tv_sec);
@@ -1330,9 +1373,19 @@ static inline abi_long host_to_target_cmsg(struct target_msghdr *target_msgh,
         unimplemented:
             gemu_log("Unsupported ancillary data: %d/%d\n",
                                         cmsg->cmsg_level, cmsg->cmsg_type);
-            memcpy(target_data, data, len);
+            memcpy(target_data, data, MIN(len, tgt_len));
+            if (tgt_len > len) {
+                memset(target_data + len, 0, tgt_len - len);
+            }
         }
 
+        target_cmsg->cmsg_len = tswapal(tgt_len);
+        tgt_space = TARGET_CMSG_SPACE(tgt_len);
+        if (msg_controllen < tgt_space) {
+            tgt_space = msg_controllen;
+        }
+        msg_controllen -= tgt_space;
+        space += tgt_space;
         cmsg = CMSG_NXTHDR(msgh, cmsg);
         target_cmsg = TARGET_CMSG_NXTHDR(target_msgh, target_cmsg);
     }
