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/*
* Copyright © 2024 Michael Smith <mikesmiffy128@gmail.com>
*
* Permission to use, copy, modify, and/or distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED “AS IS” AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
* REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
* AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
* INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
* LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
* OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
* PERFORMANCE OF THIS SOFTWARE.
*/
#include "intdefs.h"
#include "x86.h"
static int mrmsib(const uchar *p, int addrlen) {
// I won't lie: I thought I almost understood this, but after Bill walked me
// through correcting a bunch of wrong cases I now realise that I don't
// really understand it at all. If it helps, I used this as a reference:
// https://github.com/Nomade040/length-disassembler/blob/e8b34546/ldisasm.cpp#L14
// But it's confusingly-written enough that the code I wrote before didn't
// work, so with any luck nobody will need to refer to it again and this is
// actually correct now. Fingers crossed.
if (addrlen == 4 || *p & 0xC0) {
int sib = addrlen == 4 && *p < 0xC0 && (*p & 7) == 4;
switch (*p & 0xC0) {
// disp8
case 0x40: return 2 + sib;
// disp16/32
case 0:
if ((*p & 7) != 5) {
// disp8/32 via SIB
if (sib && (p[1] & 7) == 5) return *p & 0x40 ? 3 : 6;
return 1 + sib;
}
case 0x80: return 1 + addrlen + sib;
}
}
if (addrlen == 2 && (*p & 0xC7) == 0x06) return 3;
return 1; // note: include the mrm itself in the byte count
}
int x86_len(const void *insn_) {
#define CASES(name, _) case name:
const uchar *insn = insn_;
int pfxlen = 0, addrlen = 4, operandlen = 4;
p: switch (*insn) {
case X86_PFX_ADSZ: addrlen = 2; goto P; // bit dumb sorry
case X86_PFX_OPSZ: operandlen = 2;
P: X86_SEG_PREFIXES(CASES)
case X86_PFX_LOCK: case X86_PFX_REPN: case X86_PFX_REP:
// instruction can only be 15 bytes. this could go over, oh well,
// just don't want to loop for 8 million years
if (++pfxlen == 14) return -1;
++insn;
goto p;
}
switch (*insn) {
X86_OPS_1BYTE_NO(CASES) return pfxlen + 1;
X86_OPS_1BYTE_I8(CASES) operandlen = 1;
X86_OPS_1BYTE_IW(CASES) return pfxlen + 1 + operandlen;
X86_OPS_1BYTE_IWI(CASES) return pfxlen + 1 + addrlen;
X86_OPS_1BYTE_I16(CASES) return pfxlen + 3;
X86_OPS_1BYTE_MRM(CASES) return pfxlen + 1 + mrmsib(insn + 1, addrlen);
X86_OPS_1BYTE_MRM_I8(CASES) operandlen = 1;
X86_OPS_1BYTE_MRM_IW(CASES)
return pfxlen + 1 + operandlen + mrmsib(insn + 1, addrlen);
case X86_ENTER: return pfxlen + 4;
case X86_CRAZY8: operandlen = 1;
case X86_CRAZYW:
if ((insn[1] & 0x38) >= 0x10) operandlen = 0;
return pfxlen + 1 + operandlen + mrmsib(insn + 1, addrlen);
case X86_2BYTE: ++insn; goto b2;
}
return -1;
b2: switch (*insn) {
// we don't support any 3 byte ops for now, implement if ever needed...
case X86_3BYTE1: case X86_3BYTE2: case X86_3DNOW: return -1;
X86_OPS_2BYTE_NO(CASES) return pfxlen + 2;
X86_OPS_2BYTE_IW(CASES) return pfxlen + 2 + operandlen;
X86_OPS_2BYTE_MRM(CASES) return pfxlen + 2 + mrmsib(insn + 1, addrlen);
X86_OPS_2BYTE_MRM_I8(CASES) operandlen = 1;
return pfxlen + 2 + operandlen + mrmsib(insn + 1, addrlen);
}
return -1;
#undef CASES
}
// vi: sw=4 ts=4 noet tw=80 cc=80
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