diff options
Diffstat (limited to 'src/x86.c')
| -rw-r--r-- | src/x86.c | 40 |
1 files changed, 24 insertions, 16 deletions
@@ -17,23 +17,31 @@ #include "intdefs.h" #include "x86.h" -static int mrm(uchar b, int addrlen) { - // I won't lie: I don't *entirely* understand this particular logic. I - // largely based it on some public domain code I found on the internet. - // See: https://github.com/Nomade040/length-disassembler/blob/e8b34546/ldisasm.cpp#L14 - // Bonus route credit goes to Bill for spotting some bugs in prior versions. - if (addrlen == 4 || b & 0xC0) { - int sib = addrlen == 4 && b < 0xC0 && (b & 7) == 4; - switch (b & 0xC0) { +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 ((b & 7) != 5) return 1 + sib; + 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 && b == 0x26) return 3; - return 1; // NOTE: include the mrm itself in the byte count + if (addrlen == 2 && *p == 0x26) return 3; + return 1; // note: include the mrm itself in the byte count } int x86_len(const void *insn_) { @@ -58,15 +66,15 @@ P: X86_SEG_PREFIXES(CASES) X86_OPS_1BYTE_I8(CASES) operandlen = 1; X86_OPS_1BYTE_IW(CASES) return pfxlen + 1 + operandlen; X86_OPS_1BYTE_I16(CASES) return pfxlen + 3; - X86_OPS_1BYTE_MRM(CASES) return pfxlen + 1 + mrm(insn[1], addrlen); + 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 + mrm(insn[1], addrlen); + 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 + 2 + operandlen + mrm(insn[1], addrlen); + return pfxlen + 2 + operandlen + mrmsib(insn + 1, addrlen); case X86_2BYTE: ++insn; goto b2; } return -1; @@ -76,9 +84,9 @@ b2: switch (*insn) { 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 + mrm(insn[1], addrlen); + X86_OPS_2BYTE_MRM(CASES) return pfxlen + 2 + mrmsib(insn + 1, addrlen); X86_OPS_2BYTE_MRM_I8(CASES) operandlen = 1; - return pfxlen + 2 + operandlen + mrm(insn[1], addrlen); + return pfxlen + 2 + operandlen + mrmsib(insn + 1, addrlen); } return -1; |