CVM is a header-only turing-complete virtual machine engine made in pure C and based on x86-64. It's not fully replicating the architecture, but can be used to implement any existing algorithm. It's created as an example of a VM for code protection purposes.
Here's an example of a pow method implementation before and after virtualization. You can find more examples in main.c.
static int pow_normal(int n, int b) {
int r = n;
for (int i = 1; i < b; i++)
r*=n;
return r;
}
static int pow_virt(int n, int b) {
struct vm_context * ctx = vm_makectx();
char code[] = {
// Load params into registers
// mov rax, n
JOINBITS(11, VM_VAL2REG),
VM_MOV,
JOINBITS(0, 0),
ENCODE_QWORD((qword)n),
// mov rbx, b
JOINBITS(11, VM_VAL2REG),
VM_MOV,
JOINBITS(1, 0),
ENCODE_QWORD((qword)b),
// int i = 0;
// mov rdx, 1
JOINBITS(11, VM_VAL2REG),
VM_MOV, JOINBITS(3, 0),
ENCODE_QWORD(1),
// result
// mov rcx, rax
JOINBITS(3, VM_REG2REG),
VM_MOV,
JOINBITS(2, 0),
// loop start
// cmp rdx, rbx
JOINBITS(3, VM_REG2REG),
VM_CMP,
JOINBITS(3, 1),
// jge end_loop if (b >= i) jmp end_loop
// out of scope
JOINBITS(10, VM_REG2REG),
VM_JGE,
ENCODE_QWORD(34), // jump to end_loop
// mul rcx, rax
JOINBITS(3, VM_REG2REG),
VM_MUL,
JOINBITS(2, 0),
// add rdx, 1
JOINBITS(11, VM_VAL2REG),
VM_ADD, JOINBITS(3, 0),
ENCODE_QWORD(1),
// jmp loop_start
JOINBITS(10, VM_VAL2REG),
VM_JMP,
ENCODE_QWORD(-27),
// end_loop
// mov rax, rcx to return in rax
JOINBITS(3, VM_REG2REG),
VM_MOV,
JOINBITS(0, 2)
};
vm_exec(ctx, code, sizeof(code));
qword retval = ctx->rax;
vm_destroyctx(ctx);
return retval;
}
Execution flow (The results are the same)
To use the VM you will have to write the bytecode manually which can be hard and challenging, but remember it will be double as painful to reverse-engineer. Include the header into your projects by simply including the header.
- Add macros to construct instructions easier (aka JMP(), ADD(), etc)
- Make a compiler with label support to write the bytecode easier
- Implement more instructions