nbd: Drop unexpected data for NBD_OPT_LIST

When requesting the list of exports, no data should be sent. If data is
sent, the NBD server should not just inform the client of the invalid
request, but also drop the data.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <1424887718-10800-22-git-send-email-mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

nbd.c

@@ -193,6 +193,26 @@ static ssize_t read_sync(int fd, void *buffer, size_t size)
     return nbd_wr_sync(fd, buffer, size, true);
 }
 
+static ssize_t drop_sync(int fd, size_t size)
+{
+    ssize_t ret, dropped = size;
+    uint8_t *buffer = g_malloc(MIN(65536, size));
+
+    while (size > 0) {
+        ret = read_sync(fd, buffer, MIN(65536, size));
+        if (ret < 0) {
+            g_free(buffer);
+            return ret;
+        }
+
+        assert(ret <= size);
+        size -= ret;
+    }
+
+    g_free(buffer);
+    return dropped;
+}
+
 static ssize_t write_sync(int fd, void *buffer, size_t size)
 {
     int ret;
@@ -303,6 +323,9 @@ static int nbd_handle_list(NBDClient *client, uint32_t length)
 
     csock = client->sock;
     if (length) {
+        if (drop_sync(csock, length) != length) {
+            return -EIO;
+        }
         return nbd_send_rep(csock, NBD_REP_ERR_INVALID, NBD_OPT_LIST);
     }