The Engine reads and disassembles the instrumented binary basic block per basic block. In case a conditional branch terminates a basic block, the execution result of this basic block is needed to determine the next basic block to execute. This makes the case for a per basic block processing and execution.
Every basic block is first patched to solve two main problems:
- The basic block will be executed at a different location and thus every usage of the Program Counter, either directly as an operand or indirectly when using relative memory addressing, needs to be patched to make the code relocatable.
- Control Flow Control:
- Branching instructions should not be directly executed as this would result in the execution escaping from the instrumentation process. Thus the resulting target of a branching instruction needs to be computed without being taken.
Once a basic block has been patched, the instrumentations requested by the user code are applied. Both the patching and the instrumentation are expressed in an Embedded Domain Specific Language  called PatchDSL which is executed by the engine.
The resulting instrumented basic block is then handed over to the ExecBlockManager which handles a cache of basic blocks placed inside execution units called ExecBlock. The ExecBlockManager is tasked with finding memory space inside an ExecBlock to place the instrumented basic block and also retrieving cached basic blocks.
An ExecBlock manages on the guest side two memory pages: one for the code, the code block, and one for the data, the data block. The ExecBlock also handles the resolution of the relocation of the patched code before assembling it in the code block.
The instrumentation of the code allows to make callbacks to the user code directly from the instrumented binary through the ExecBlock. These callbacks allow to inspect and modify the state of execution of the guest on the host side at every point in time.
The figure below presents the life of an instruction and summarizes the main steps and classes involved along the way. This is intended to give an overview of what the internals do.
An instruction exists in three different representations inside QBDI:
- Raw bytes of machine code in memory.
- LLVM machine code representation. The instruction is only partially disassembled but still provides a list of operands. One interested in more details regarding this representation should refer to the official LLVM documentation and experiment with llvm-mc -show-inst.
- QBDI representation of a relocatable MCInst. It consists in an MCInst and relocation information.
There is another important class:
the patch and the instrumentation of a single instruction in the form of a list of
QBDI::RelocatableInst. It is the smallest unit of code which can be assembled inside
QBDI::ExecBlock as patching or instrumentation code cannot be split in parts
without problematic side effects.
The assembly and disassembly steps are directly handled by LLVM for us. The Engine takes care of
the patching and instrumentation using a programmable list of
QBDI::InstrRule. More details on those rules can be found in the PatchDSL
chapter. Relocation is handled directly in the