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I need to read instructions one-by-one from a small code segment in memory and I have to find out the size of the instructions which I have in memory.
The following is just a example of raw disassembled code to explain my problem:
(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.
I know the memory address of the first instruction (p = 0x0000000000400281 in this case) and I can read every memory address from p. The problem is that I cannot know if the value of *(p + offset) is the opcode or not and I know that the size information for every opcode is not fixed.
So, can I get the size of every assembly instruction? Or can I know if the value that I read is opcode or information?
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@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\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
}
Decoding instructions is not that complicated. However, because the Intel family of processors are CISC, it makes the task rather daunting.
First of all, you should not write it in assembler, because it's going to take you a year or two, but maybe you have the time to do that. Since you only need to scan the code, not print out the results, you can do the work much faster than an actual disassembler would do. That being said you'll bump in the same main problems.
First of all, the manuals are there:
http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html?iid=tech_vt_tech+64-32_manuals
I suggest this one:
http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf
Then, all you have to do is read one byte and understand it. You have a table on page 770 that shows you the encoding from the op-code to the instruction.
So for example, 0x33 represents an XOR with Gb,Ev as parameters. G means General register defined in the following ModR/M. Then the b is the size (byte). The E means that you have a ModR/M after that one byte (same byte for G and E). So you will have to read that one byte to determine the addressing mode and from that you can determine the register (Can be ignored) and the address size. The address (Ev) may be another register (then no extra byte), it could be immediate data (1, 2, 4, 8 bytes) or it could be an address (again 1, 2, 4, 8 bytes). Pretty simple, right? Note that ALL instructions use the exact same ModR/M so you have to implement that just once. Also the order in which bytes are added after the instruction code is always exactly the same.
Before the address or immediate (if I'm correct) comes the extra Mod for 64 bit instructions. That one defines additional modes and support for the extended registers. All of that is described in detail in the document I mentioned earlier.
More or less, you need your parser to understand the ModR/M, SIB, prefixes, and voilà. It's not that complicated. Then the first byte tells you the instruction (first 2 bytes if the first byte is 0x0F...)
Some instructions also support prefixes to tweak the size of the operands and other similar things. As far as I know, only the 0x66 (op size) and 0x67 (addr size) have an effect on the size of the address and immediate data. The other prefixes will not affect the number of bytes used by the instruction so you can simply ignore them (well count them, but no need to know what follows).
All of that said, using the LLVM library (As someone mentioned in the comments) is probably a better/easier option, although it may be much bigger than what you'd need if your stuff is limited.
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