Simple Virtual Machine

In computing, Virtual Machine (VM) is a software implementation of either existing or a fictional hardware platform.  VM's are generally divided into two classes - system VM (VM which is capable of running an operating system) and process VM (the one that only can run one executable, roughly saying). Anyway, if you are just interested in the definition of the term, you better go here. 

There are tones of articles dedicated to this matter on the Internet, hundreds of tutorials and explanations. I see no reason to just add another "trivial" article or tutorial to the row. Instead, I think it may be more interesting to see it in action, to have an example of real application. One may say that we are surrounded by those examples - Java, .NET, etc. It is correct, however, I would like to touch a slightly different application of this technology - protect your software/data from being hacked.

Data Protection

Millions of dollars are being spent by software (or content) vendors in an attempt to protect their products from being stolen or used in any other illegal way. There are numerous protection tools and utilities, starting with simple packers/scramblers and ending with complex packages that implement multilevel encryption and virtual machines as well. However, you may disagree, but you wont convince me, an out-of-the-box solution is good until it gains popularity. There is enough evidence for this statement. In my opinion, no one can protect your software better than you. It only depends on how much protected you want it to be.

Although, there are numerous protection methods and techniques, we are going to concentrate on a virtual machine for data coding/decoding. Nothing special, just a trivial XOR method, but, in my opinion, enough to demonstrate the fundamentals.

Design Your VM

While in real life, hardware design precedes its software counterpart, we may let ourselves to do it in reverse order (it is our own VM, after all). Therefore, we will begin with the pseudo executable file format which will be supported by our VM. 

Pseudo Executable File Format

Well, it is a good idea to put a header in the beginning of the file. In order to do so, we have to think what our file is going to contain. The file may be a raw code (remember DOS com files?), but this would not be interesting enough. So, let our file be divided into three sections:

code section - this section would contain code written in our pseudo assembly language (we'll cover it a bit later);

data section - this section would contain all the data needed by our pseudo executable (PE :-) );

export section - this section would contain references to all the elements that we want to make visible to the core program.

Let us define the header as a C structure:

typedef struct _VM_HEADER

{

   unsigned int  version; /* Version of our VM. Will be 0x101 for now */

   unsigned int  codeOffset; /* File offset of the code section */

   unsigned int  codeSize; /* Size of the code section in bytes */

   unsigned int  dataOffset; /* File offset of the data section */

   unsigned int  dataSize; /* Size of the data section in bytes */

   unsigned int  exportOffset; /* File offset of the export section */

   unsigned int  exportSize; /* Size of the export section in bytes */

   unsigned int  requestedStack; /* Required size of stack in 4 bytes blocks */

   unsigned int  fileSize; /* Size of the whole file in bytes */

}VM_HEADER;

Read more: System Programming

QR: 

Posted via email from Jasper-net