我需要从内存中的一个小代码段逐个读取指令，并找出我在内存中拥有的指令的大小。
以下是一个原始反汇编代码示例，用于解释我的问题：
(gdb) disas /r 0x400281,+8
Dump of assembler code from 0x400281 to 0x400289:
0x0000000000400281: 48 89 c7 movq %rax, %rdi
0x0000000000400284: b0 00 movb $0, %al
0x0000000000400286: e8 f2 48 00 00 callq 0x10001f30a
End of assembler dump.
我知道第一条指令的内存地址（在这个例子中为p = 0x0000000000400281），并且可以从p读取每个内存地址。问题是，我无法确定*(p + offset)的值是否是操作码，并且我知道每个操作码的大小信息不是固定的。
那么，我能否获得每个汇编指令的大小？或者我能否知道我读取的值是操作码还是信息？
/* (C) Copyright 2012-2014 Semantic Designs, Inc.
You may freely use this code provided you retain this copyright message
*/
typedef unsigned int natural;
natural InstructionLength(BYTE* pc)
{ // returns length of instruction at PC
natural length=0;
natural opcode, opcode2;
natural modrm;
natural sib;
BYTE* p=pc;
while (true)
{ // scan across prefix bytes
opcode=*p++;
switch (opcode)
{ case 0x64: case 0x65: // FS: GS: prefixes
case 0x36: // SS: prefix
case 0x66: case 0x67: // operand size overrides
case 0xF0: case 0xF2: // LOCK, REPNE prefixes
length++;
break;
case 0x2E: // CS: prefix, used as HNT prefix on jumps
case 0x3E: // DS: prefix, used as HT prefix on jumps
length++;
// goto process relative jmp // tighter check possible here
break;
default:
goto process_instruction_body;
}
}
process_instruction_body:
switch(opcode) // switch on main opcode
{
// ONE BYTE OPCODE, move to next opcode without remark
case 0x27: case 0x2F:
case 0x37: case 0x3F:
case 0x40: case 0x41: case 0x42: case 0x43: case 0x44: case 0x45: case 0x46: case 0x47:
case 0x48: case 0x49: case 0x4A: case 0x4B: case 0x4C: case 0x4D: case 0x4E: case 0x4F:
case 0x50: case 0x51: case 0x52: case 0x53: case 0x54: case 0x55: case 0x56: case 0x57:
case 0x58: case 0x59: case 0x5A: case 0x5B: case 0x5C: case 0x5D: case 0x5E: case 0x5F:
case 0x90: // nop
case 0x91: case 0x92: case 0x93: case 0x94: case 0x95: case 0x96: case 0x97: // xchg
case 0x98: case 0x99:
case 0x9C: case 0x9D: case 0x9E: case 0x9F:
case 0xA4: case 0xA5: case 0xA6: case 0xA7: case 0xAA: case 0xAB: // string operators
case 0xAC: case 0xAD: case 0xAE: case 0xAF:
/* case 0xC3: // RET handled elsewhere */
case 0xC9:
case 0xCC: // int3
case 0xF5: case 0xF8: case 0xF9: case 0xFC: case 0xFD:
return length+1; // include opcode
case 0xC3: // RET
if (*p++ != 0xCC)
return length+1;
if (*p++ != 0xCC)
return length+2;
if (*p++ == 0xCC
&& *p++ == 0xCC)
return length+5;
goto error;
// TWO BYTE INSTRUCTION
case 0x04: case 0x0C: case 0x14: case 0x1C: case 0x24: case 0x2C: case 0x34: case 0x3C:
case 0x6A:
case 0xB0: case 0xB1: case 0xB2: case 0xB3: case 0xB4: case 0xB5: case 0xB6: case 0xB7:
case 0xC2:
return length+2;
// TWO BYTE RELATIVE BRANCH
case 0x70: case 0x71: case 0x72: case 0x73: case 0x74: case 0x75: case 0x76: case 0x77:
case 0x78: case 0x79: case 0x7A: case 0x7B: case 0x7C: case 0x7D: case 0x7E: case 0x7F:
case 0xE0: case 0xE1: case 0xE2: case 0xE3: case 0xEB:
return length+2;
// THREE BYTE INSTRUCTION (NONE!)
// FIVE BYTE INSTRUCTION:
case 0x05: case 0x0D: case 0x15: case 0x1D:
case 0x25: case 0x2D: case 0x35: case 0x3D:
case 0x68:
case 0xA9:
case 0xB8: case 0xB9: case 0xBA: case 0xBB: case 0xBC: case 0xBD: case 0xBE: case 0xBF:
return length+5;
// FIVE BYTE RELATIVE CALL
case 0xE8:
return length+5;
// FIVE BYTE RELATIVE BRANCH
case 0xE9:
if (p[4]==0xCC)
return length+6; // <jmp near ptr ... int 3>
return length+5; // plain <jmp near ptr>
// FIVE BYTE DIRECT ADDRESS
case 0xA1: case 0xA2: case 0xA3: // MOV AL,AX,EAX moffset...
return length+5;
break;
// ModR/M with no immediate operand
case 0x00: case 0x01: case 0x02: case 0x03: case 0x08: case 0x09: case 0x0A: case 0x0B:
case 0x10: case 0x11: case 0x12: case 0x13: case 0x18: case 0x19: case 0x1A: case 0x1B:
case 0x20: case 0x21: case 0x22: case 0x23: case 0x28: case 0x29: case 0x2A: case 0x2B:
case 0x30: case 0x31: case 0x32: case 0x33: case 0x38: case 0x39: case 0x3A: case 0x3B:
case 0x84: case 0x85: case 0x86: case 0x87: case 0x88: case 0x89: case 0x8A: case 0x8B: case 0x8D: case 0x8F:
case 0xD1: case 0xD2: case 0xD3:
case 0xFE: case 0xFF: // misinterprets JMP far and CALL far, not worth fixing
length++; // count opcode
goto modrm;
// ModR/M with immediate 8 bit value
case 0x80: case 0x82: case 0x83:
case 0xC0: case 0xC1:
case 0xC6: // with r=0?
length+=2; // count opcode and immediate byte
goto modrm;
// ModR/M with immediate 32 bit value
case 0x81:
case 0xC7: // with r=0?
length+=5; // count opcode and immediate byte
goto modrm;
case 0x9B: // FSTSW AX = 9B DF E0
if (*p++==0xDF)
{ if (*p++==0xE0)
return length+3;
printf("InstructionLength: Unimplemented 0x9B tertiary opcode %2x at %x\n",*p,p);
goto error;
}
else { printf("InstructionLength: Unimplemented 0x9B secondary opcode %2x at %x\n",*p,p);
goto error;
}
case 0xD9: // various FP instructions
modrm=*p++;
length++; // account for FP prefix
switch (modrm)
{ case 0xC9: case 0xD0:
case 0xE0: case 0xE1: case 0xE4: case 0xE5:
case 0xE8: case 0xE9: case 0xEA: case 0xEB: case 0xEC: case 0xED: case 0xEE:
case 0xF8: case 0xF9: case 0xFA: case 0xFB: case 0xFC: case 0xFD: case 0xFE: case 0xFF:
return length+1;
default: // r bits matter if not one of the above specific opcodes
switch((modrm&0x38)>>3)
{ case 0: goto modrm_fetched; // fld
case 1: return length+1; // fxch
case 2: goto modrm_fetched; // fst
case 3: goto modrm_fetched; // fstp
case 4: goto modrm_fetched; // fldenv
case 5: goto modrm_fetched; // fldcw
case 6: goto modrm_fetched; // fnstenv
case 7: goto modrm_fetched; // fnstcw
}
goto error; // unrecognized 2nd byte
}
case 0xDB: // various FP instructions
modrm=*p++;
length++; // account for FP prefix
switch (modrm)
{ case 0xE3:
return length+1;
default: // r bits matter if not one of the above specific opcodes
#if 0
switch((modrm&0x38)>>3)
{ case 0: goto modrm_fetched; // fld
case 1: return length+1; // fxch
case 2: goto modrm_fetched; // fst
case 3: goto modrm_fetched; // fstp
case 4: goto modrm_fetched; // fldenv
case 5: goto modrm_fetched; // fldcw
case 6: goto modrm_fetched; // fnstenv
case 7: goto modrm_fetched; // fnstcw
}
#endif
goto error; // unrecognized 2nd byte
}
case 0xDD: // various FP instructions
modrm=*p++;
length++; // account for FP prefix
switch (modrm)
{ case 0xE1: case 0xE9:
return length+1;
default: // r bits matter if not one of the above specific opcodes
switch((modrm&0x38)>>3)
{ case 0: goto modrm_fetched; // fld
// case 1: return length+1; // fisttp
case 2: goto modrm_fetched; // fst
case 3: goto modrm_fetched; // fstp
case 4: return length+1; // frstor
case 5: return length+1; // fucomp
case 6: goto modrm_fetched; // fnsav
case 7: goto modrm_fetched; // fnstsw
}
goto error; // unrecognized 2nd byte
}
case 0xF3: // funny prefix REPE
opcode2=*p++; // get second opcode byte
switch (opcode2)
{ case 0x90: // == PAUSE
case 0xA4: case 0xA5: case 0xA6: case 0xA7: case 0xAA: case 0xAB: // string operators
return length+2;
case 0xC3: // (REP) RET
if (*p++ != 0xCC)
return length+2; // only (REP) RET
if (*p++ != 0xCC)
goto error;
if (*p++ == 0xCC)
return length+5; // (REP) RET CLONE IS LONG JUMP RELATIVE
goto error;
case 0x66: // operand size override (32->16 bits)
if (*p++ == 0xA5) // "rep movsw"
return length+3;
goto error;
default: goto error;
}
case 0xF6: // funny subblock of opcodes
modrm=*p++;
if ((modrm & 0x20) == 0)
length++; // 8 bit immediate operand
goto modrm_fetched;
case 0xF7: // funny subblock of opcodes
modrm=*p++;
if ((modrm & 0x30) == 0)
length+=4; // 32 bit immediate operand
goto modrm_fetched;
// Intel's special prefix opcode
case 0x0F:
length+=2; // add one for special prefix, and one for following opcode
opcode2=*p++;
switch(opcode2)
{ case 0x31: // RDTSC
return length;
// CMOVxx
case 0x40: case 0x41: case 0x42: case 0x43: case 0x44: case 0x45: case 0x46: case 0x47:
case 0x48: case 0x49: case 0x4A: case 0x4B: case 0x4C: case 0x4D: case 0x4E: case 0x4F:
goto modrm;
// JC relative 32 bits
case 0x80: case 0x81: case 0x82: case 0x83: case 0x84: case 0x85: case 0x86: case 0x87:
case 0x88: case 0x89: case 0x8A: case 0x8B: case 0x8C: case 0x8D: case 0x8E: case 0x8F:
return length+4; // account for subopcode and displacement
// SETxx rm32
case 0x90: case 0x91: case 0x92: case 0x93: case 0x94: case 0x95: case 0x96: case 0x97:
case 0x98: case 0x99: case 0x9A: case 0x9B: case 0x9C: case 0x9D: case 0x9E: case 0x9F:
goto modrm;
case 0xA2: // CPUID
return length+2;
case 0xAE: // LFENCE, SFENCE, MFENCE
opcode2=*p++;
switch (opcode2)
{ case 0xE8: // LFENCE
case 0xF0: // MFENCE
case 0xF8: // SFENCE
return length+1;
default:
printf("InstructionLength: Unimplemented 0x0F, 0xAE tertiary opcode in clone %2x at %x\n",opcode2,p-1);
goto error;
}
case 0xAF: // imul
case 0xB0: // cmpxchg 8 bits
goto error;
case 0xB1: // cmpxchg 32 bits
case 0xB6: case 0xB7: // movzx
case 0xBC: /* bsf */ case 0xBD: // bsr
// case 0xBE: case 0xBF: // movsx
case 0xC1: // xadd
case 0xC7: // cmpxchg8b
goto modrm;
default:
printf("InstructionLength: Unimplemented 0x0F secondary opcode in clone %2x at %x\n",opcode,p-1);
goto error;
} // switch
// ALL THE THE REST OF THE INSTRUCTIONS; these are instructions that runtime system shouldn't ever use
default:
/* case 0x26: case 0x36: // ES: SS: prefixes
case 0x9A:
case 0xC8: case 0xCA: case 0xCB: case 0xCD: case 0xCE: case 0xCF:
case 0xD6: case 0xD7:
case 0xE4: case 0xE5: case 0xE6: case 0xE7: case 0xEA: case 0xEB: case 0xEC: case 0xED: case 0xEF:
case 0xF4: case 0xFA: case 0xFB:
*/
printf("InstructionLength: Unexpected opcode %2x\n",opcode);
goto error;
}
modrm:
modrm=*p++;
modrm_fetched:
if (trace_clone_checking)
printf("InstructionLength: ModR/M byte %x %2x\n",pc,modrm);
if (modrm >= 0xC0)
return length+1; // account for modrm opcode
else
{ /* memory access */
if ((modrm & 0x7) == 0x04)
{ /* instruction with SIB byte */
length++; // account for SIB byte
sib=*p++; // fetch the sib byte
if ((sib & 0x7) == 0x05)
{ if ((modrm & 0xC0) == 0x40)
return length+1+1; // account for MOD + byte displacment
else return length+1+4; // account for MOD + dword displacement
}
}
switch(modrm & 0xC0)
{ case 0x0:
if ( (modrm & 0x07) == 0x05)
return length+5; // 4 byte displacement
else return length+1; // zero length offset
case 0x80:
return length+5; // 4 byte offset
default:
return length+2; // one byte offset
}
}
error:
{ printf("InstructionLength: unhandled opcode at %8x with opcode %2x\n",pc,opcode);
}
return 0; // can't actually execute this
}
总之，使用LLVM库（正如评论中有人提到的）可能是更好/更容易的选择，尽管如果你的内容有限，它可能比你需要的要大得多。
objdump使用，参见https://www.gnu.org/software/binutils/。 - Peter CordesIntel有一款名为XED的库，可以处理x86/x86_64指令：https://github.com/intelxed/xed，同时也是在x86和x86_64模式下处理英特尔机器码的唯一正确方式。Intel使用它（并且它曾是Pin的一部分）：https://software.intel.com/en-us/articles/xed-x86-encoder-decoder-software-library
https://software.intel.com/sites/landingpage/pintool/docs/67254/Xed/html/main.html XED用户指南（2014年） https://software.intel.com/sites/landingpage/pintool/docs/56759/Xed/html/main.html XED2用户指南（2011年）
xed_decode函数将为您提供有关指令的所有信息：https://intelxed.github.io/ref-manual/group__DEC.html
https://intelxed.github.io/ref-manual/group__DEC.html#ga9a27c2bb97caf98a6024567b261d0652
xed_ild_decode 只会解码其长度的指令：
https://intelxed.github.io/ref-manual/group__DEC.html#ga4bef6152f61997a47c4e0fe4327a3254
要从由XED_DLL_EXPORT xed_error_enum_t xed_ild_decode ( xed_decoded_inst_t * xedd, const xed_uint8_t * itext, const unsigned int bytes )This function just does instruction length decoding. It does not return a fully decoded instruction.
Parameters
- xedd the decoded instruction of type xed_decoded_inst_t . Mode/state sent in via xedd; See the xed_state_t .
- itext the pointer to the array of instruction text bytes
- bytes the length of the itext input array. 1 to 15 bytes, anything more is ignored.
Returns:
xed_error_enum_t indiciating success (XED_ERROR_NONE) or failure. Only two failure codes are valid for this function: XED_ERROR_BUFFER_TOO_SHORT and XED_ERROR_GENERAL_ERROR. In general this function cannot tell if the instruction is valid or not. For valid instructions, XED can figure out if enough bytes were provided to decode the instruction. If not enough were provided, XED returns XED_ERROR_BUFFER_TOO_SHORT. From this function, the XED_ERROR_GENERAL_ERROR is an indication that XED could not decode the instruction's length because the instruction was so invalid that even its length may across implementations.
xed_ild_decode填充的xedd结构中获取长度，请使用xed_decoded_inst_get_length: https://intelxed.github.io/ref-manual/group__DEC.html#gad1051f7b86c94d5670f684a6ea79fcdf。
static XED_INLINE xed_uint_t xed_decoded_inst_get_length ( const xed_decoded_inst_t * p )Return the length of the decoded instruction in bytes.
示例代码（由英特尔2016年发布的“Apache许可证，版本2.0”）：https://github.com/intelxed/xed/blob/master/examples/xed-ex-ild.c
#include "xed/xed-interface.h"
#include <stdio.h>
int main()
{
xed_bool_t long_mode = 1;
xed_decoded_inst_t xedd;
xed_state_t dstate;
unsigned char itext[15] = { 0xf2, 0x2e, 0x4f, 0x0F, 0x85, 0x99,
0x00, 0x00, 0x00 };
xed_tables_init(); // one time per process
if (long_mode)
dstate.mmode=XED_MACHINE_MODE_LONG_64;
else
dstate.mmode=XED_MACHINE_MODE_LEGACY_32;
xed_decoded_inst_zero_set_mode(&xedd, &dstate);
xed_ild_decode(&xedd, itext, XED_MAX_INSTRUCTION_BYTES);
printf("length = %u\n",xed_decoded_inst_get_length(&xedd));
return 0;
}
有一个小型的反汇编库叫做udis86：http://udis86.sourceforge.net/。
该库很小并且有良好的文档。如果您通过ud_set_syntax设置翻译器为NULL，那么函数ud_disassemble只会解码指令并返回字节数。