Fix

main/POIFS/FileSystem/NPOIFSFileSystem.cs

@@ -310,8 +310,8 @@ public NPOIFSFileSystem(Stream stream)
                 data.Write(headerBuffer.Buffer);
                 data.Position = headerBuffer.Length;
 
-                //IOUtils.ReadFully(channel, data);
-                data.Position += IOUtils.ReadFully(channel, data.Buffer, data.Position, (int)channel.Length);
+                //IOUtils.ReadFully(channel, data.Buffer);
+                data.Position += IOUtils.ReadFully(channel, data.Buffer, data.Position, (int)maxSize);
                 success = true;
 
                 // Turn it into a DataSource

main/Util/IOUtils.cs

@@ -195,8 +195,8 @@ public static int ReadFully(Stream stream, byte[] b, int off, int len)
             int total = 0;
             while (true)
             {
-                int got = stream.Read(b, off + total, len - total);
-                if (got < 0)
+                int got = stream.Read(b, off + total, len - total - off);
+                if (got <= 0)
                 {
                     return (total == 0) ? -1 : total;
                 }