@AlexisWilke's response is right: this is messy. He provides the right insights and references to do the work, too.
I have done this work in C. The code follows; this is used in production contexts.
Caveats: It does a good part of the traditional x86 instruction set, but not all, in particular none of the instructions involving the vector register sets. And it contains decoding for a few "virtual" instructions that we happen to use in our code. I don't think extending this to x86-64 would be difficult, but it would get messier. Lastly, this is lifted directly, but I don't make any guarantees this will compile out-of-the box.
/* (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
",*p,p);
goto error;
}
else { printf("InstructionLength: Unimplemented 0x9B secondary opcode %2x at %x
",*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
",opcode2,p-1);
goto error;
}
case 0xAF: // imul
case 0xB0: // cmpxchg 8 bits
goto error;
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