我正在为好玩而编写的 VM 中实现一个简单的 JIT 编译器(主要是为了了解有关语言设计的更多信息)并且我遇到了一些奇怪的行为,也许有人可以告诉我原因。
首先,我为 C 和 C++ 定义了一个 JIT“原型”:
#ifdef __cplusplus
typedef void* (*_JIT_METHOD) (...);
#else
typedef (*_JIT_METHOD) ();
#endif
我有一个compile()
函数可以将内容编译到 ASM 中并将其粘贴到内存中的某个位置:
void* compile (void* something)
{
// grab some memory
unsigned char* buffer = (unsigned char*) malloc (1024);
// xor eax, eax
// inc eax
// inc eax
// inc eax
// ret -> eax should be 3
/* WORKS!
buffer[0] = 0x67;
buffer[1] = 0x31;
buffer[2] = 0xC0;
buffer[3] = 0x67;
buffer[4] = 0x40;
buffer[5] = 0x67;
buffer[6] = 0x40;
buffer[7] = 0x67;
buffer[8] = 0x40;
buffer[9] = 0xC3; */
// xor eax, eax
// mov eax, 9
// ret 4 -> eax should be 9
/* WORKS!
buffer[0] = 0x67;
buffer[1] = 0x31;
buffer[2] = 0xC0;
buffer[3] = 0x67;
buffer[4] = 0xB8;
buffer[5] = 0x09;
buffer[6] = 0x00;
buffer[7] = 0x00;
buffer[8] = 0x00;
buffer[9] = 0xC3; */
// push ebp
// mov ebp, esp
// mov eax, [ebp + 6] ; wtf? shouldn't this be [ebp + 8]!?
// mov esp, ebp
// pop ebp
// ret -> eax should be the first value sent to the function
/* WORKS! */
buffer[0] = 0x66;
buffer[1] = 0x55;
buffer[2] = 0x66;
buffer[3] = 0x89;
buffer[4] = 0xE5;
buffer[5] = 0x66;
buffer[6] = 0x66;
buffer[7] = 0x8B;
buffer[8] = 0x45;
buffer[9] = 0x06;
buffer[10] = 0x66;
buffer[11] = 0x89;
buffer[12] = 0xEC;
buffer[13] = 0x66;
buffer[14] = 0x5D;
buffer[15] = 0xC3;
// mov eax, 5
// add eax, ecx
// ret -> eax should be 50
/* WORKS!
buffer[0] = 0x67;
buffer[1] = 0xB8;
buffer[2] = 0x05;
buffer[3] = 0x00;
buffer[4] = 0x00;
buffer[5] = 0x00;
buffer[6] = 0x66;
buffer[7] = 0x01;
buffer[8] = 0xC8;
buffer[9] = 0xC3; */
return buffer;
}
最后,我有了程序的主要部分:
int main (int argc, char **args)
{
DWORD oldProtect = (DWORD) NULL;
int i = 667, j = 1, k = 5, l = 0;
// generate some arbitrary function
_JIT_METHOD someFunc = (_JIT_METHOD) compile(NULL);
// windows only
#if defined _WIN64 || defined _WIN32
// set memory permissions and flush CPU code cache
VirtualProtect(someFunc,1024,PAGE_EXECUTE_READWRITE, &oldProtect);
FlushInstructionCache(GetCurrentProcess(), someFunc, 1024);
#endif
// this asm just for some debugging/testing purposes
__asm mov ecx, i
// run compiled function (from wherever *someFunc is pointing to)
l = (int)someFunc(i, k);
// did it work?
printf("result: %d", l);
free (someFunc);
_getch();
return 0;
}
如您所见,该compile()
函数有几个测试我运行,以确保我得到预期的结果,几乎一切正常,但我有一个问题......
在大多数教程或文档资源中,要获取传递的函数的第一个值(在整数的情况下),您需要执行[ebp+8]
第二个[ebp+12]
,依此类推。出于某种原因,我必须这样做[ebp+6]
等等[ebp+10]
。谁能告诉我为什么?