Overview

SLEIGH provides a framework for emulating the processors which have a specification written for them. The key classes in this framework are:

Key Classes

The MemoryState object holds the representation of registers and memory during emulation. It understands the address spaces defined in the SLEIGH specification and how data is encoded in these spaces. It also knows any register names defined by the specification, so these can be used to set or query the state of these registers naturally.

The emulation framework can be tailored to a particular environment by creating breakpoint objects, which derive off the BreakCallBack interface. These can be used to create callbacks during emulation that have full access to the memory state and the emulator, so any action can be accomplished. The breakpoint callbacks can be designed to either augment or replace the instruction at a particular address, or the callback can be used to implement the action of a user-defined pcode op. The BreakCallBack objects are managed by the BreakTable object, which takes care of invoking the callback at the appropriate time.

The Emulate object serves as a basic execution engine. Its main method is Emulate::executeCurrentOp() which executes a single pcode operation on the memory state. Methods exist for querying and setting the current execution address and examining the pcode op being executed.

The main implementation of the Emulate interface is the EmulatePcodeCache object. It uses SLEIGH to translate machine instructions as they are executed. The currently executing instruction is translated into a cached sequence of pcode operations. Additional methods allow this entire sequence to be inspected, and there is another stepping function which allows the emulator to be stepped through an entire machine instruction at a time. The single pcode stepping methods are of course still available and the two methods can be used together without conflict.

Building a Memory State

Assuming the SLEIGH Translate object and the LoadImage object have already been built (see The Basic SLEIGH Interface), the only required step left before instantiating an emulator is to create a MemoryState object. The MemoryState object can be instantiated simply by passing the constructor the Translate object, but before it will work properly, you need to register individual MemoryBank objects with it, for each address space that might get used by the emulator.

A MemoryBank is a representation of data stored in a single address space There are some choices for the type of MemoryBank associated with an address space. A MemoryImage is a read-only memory bank that gets its data from a LoadImage. In order to make this writeable, or to create a writeable memory bank which starts with its bytes initialized to zero, you can use a MemoryHashOverlay or a MemoryPageOverlay.

A MemoryHashOverlay overlays some other memory bank, such as a MemoryImage. If you read from a location that hasn't been written to directly before, you get the data in the underlying memory bank. But if you write to this overlay, the value is stored in a hash table, and subsequent reads will return this value. Internally, the hashtable stores values in a preferred wordsize only on aligned addresses, but this is irrelevant to the interface. Unaligned requests are split up and handled transparently.

A MemoryPageOverlay overlays another memory bank as well. But it implements writes to the bank by caching memory pages. Any write creates an aligned page to hold the new data. The class takes care of loading and filling in pages as needed.

Here is an example of instantiating a MemoryState and registering memory banks for a ram space which is initialized with the load image. The ram space is implemented with the MemoryPageOverlay, and the register space and the temporary space are implemented using the MemoryHashOverlay.

 void setupMemoryState(Translate &trans,LoadImage &loader) {
   // Set up memory state object
   MemoryImage loadmemory(trans.getDefaultCodeSpace(),8,4096,&loader);
   MemoryPageOverlay ramstate(trans.getDefaultCodeSpace(),8,4096,&loadmemory);
   MemoryHashOverlay registerstate(trans.getSpaceByName("register"),8,4096,4096,(MemoryBank *)0);
   MemoryHashOverlay tmpstate(trans.getUniqueSpace(),8,4096,4096,(MemoryBank *)0);
   MemoryState memstate(&trans);       // Instantiate the memory state object
   memstate.setMemoryBank(&ramstate);
   memstate.setMemoryBank(&registerstate);
   memstate.setMemoryBank(&tmpstate);
}

All the memory bank constructors need a preferred wordsize, which is most relevant to the hashtable implementation, and a page size, which is most relevant to the page implementation. The hash overlays need an additional initializer specifying how big the hashtable should be. The null pointers passed in, in place of a real memory bank, indicate that the memory bank is initialized with all zeroes. Once the memory banks are instantiated, they are registered with the memory state via the MemoryState::setMemoryBank() method.

Breakpoints

In order to provide behavior within the emulator beyond just what the core instruction emulation provides, the framework supports breakpoint classes. A breakpoint is created by deriving a class from the BreakCallBack class and overriding either BreakCallBack::addressCallback() or BreakCallBack::pcodeCallback(). Here is an example of a breakpoint that implements a standard C library puts call an the x86 architecture. When the breakpoint is invoked, a call to puts has just been made, so the stack pointer is pointing to the return address and the next 4 bytes on the stack are a pointer to the string being passed in.

class PutsCallBack : public BreakCallBack {
public:
  virtual bool addressCallback(const Address &addr);
};
bool PutsCallBack::addressCallback(const Address &addr)
{
  MemoryState *mem = emulate->getMemoryState();
  uint1 buffer[256];
  uint4 esp = mem->getValue("ESP");
  AddrSpace *ram = mem->getTranslate()->getSpaceByName("ram");
  uint4 param1 = mem->getValue(ram,esp+4,4);
  mem->getChunk(buffer,ram,param1,255);
  cout << (char *)&buffer << endl;
  uint4 returnaddr = mem->getValue(ram,esp,4);
  mem->setValue("ESP",esp+8);
  emulate->setExecuteAddress(Address(ram,returnaddr));
  return true;                        // This replaces the indicated instruction
}

Notice that the callback retrieves the value of the stack pointer by name. Using this value, the string pointer is retrieved, then the data for the actual string is retrieved. After dumping the string to standard out, the return address is recovered and the return instruction is emulated by explicitly setting the next execution address to be the return value.

Running the Emulator

Here is an example of instantiating an EmulatePcodeCache object. A breakpoint is also instantiated and registered with the BreakTable.

...
Sleigh trans(&loader,&context);    // Instantiate the translator
...
MemoryState memstate(&trans);      // Instantiate the memory state
...
BreakTableCallBack breaktable(&trans);  // Instantiate a breakpoint table
EmulatePcodeCache emulator(&trans,&memstate,&breaktable);  // Instantiate the emulator
// Set up the initial stack pointer
memstate.setValue("ESP",0xbffffffc);
emulator.setExecuteAddress(Address(trans.getDefaultCodeSpace(),0x1D00114));  // Initial execution address
PutsCallBack putscallback;
breaktable.registerAddressCallback(Address(trans.getDefaultCodeSpace(),0x1D00130),&putscallback);
AssemblyRaw assememit;
for(;;) {
  Address addr = emulator.getExecuteAddress();
  trans.printAssembly(assememit,addr);
  emulator.executeInstruction();
}

Notice how the initial stack pointer and initial execute address is set up. The breakpoint is registered with the BreakTable, giving it a specific address. The executeInstruction method is called inside the loop, to actually run the emulator. Notice that a disassembly of each instruction is printed after each step of the emulator.

Other information can be examined from within this execution loop or in other tailored breakpoints. In particular, the Emulate::getCurrentOp() method can be used to retrieve the an instance of the currently executing pcode operation. From this starting point, you can examine the low-level objects:

PcodeOpRaw and
VarnodeData