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fix: fix displacement parsing

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1. correctly return when only displacement in modrm mem adressing
Previously the disassmbler wouldn't stop if only a displacement value
should be used without any base or offset index.

2. Displacement can be negative, so use the signed version where applicable
This commit is contained in:
Marco Thomas
2025-05-14 14:34:47 +09:00
parent c0bb448d79
commit 227f1bd133
3 changed files with 64 additions and 34 deletions
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40
src/disasm.rs
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@@ -2,7 +2,7 @@ use core::fmt;
use std::{fs::File, io::Read, process::exit};
use crate::aout::Aout;
use crate::instructions::{MemoryIndex, ModRmTarget, Operand, Pointer};
use crate::instructions::{Displacement, MemoryIndex, ModRmTarget, Operand, Pointer};
use crate::register::{Register, RegisterId, SegmentRegister};
use crate::{
Args,
@@ -123,7 +123,7 @@ impl Disassembler {
let (mode, reg, rm) = Self::deconstruct_modrm_byte(modrm);
log::debug!(
"0x{:04x} deconstructed into: 0b{:b}, 0b{:b}, 0b{:b}",
"{:#04x} deconstructed into: {:#b}, {:#b}, {:#b}",
modrm,
mode,
reg,
@@ -135,18 +135,26 @@ impl Disassembler {
0b00 => {
if rm == 0b110 {
log::debug!("Additional word during ModRM parsing was read with mod 0.");
displacement = Some(Operand::Word(self.parse_word()));
displacement = Some(Displacement::IWord(self.parse_word() as i16));
return (
ModRmTarget::Memory(MemoryIndex {
base: None,
index: None,
displacement,
}),
reg,
);
} else {
displacement = None;
}
}
0b01 => {
log::debug!("Additional byte during ModRM parsing was read.");
displacement = Some(Operand::Byte(self.parse_byte()))
displacement = Some(Displacement::IByte(self.parse_byte() as i8))
}
0b10 => {
log::debug!("Additional word during ModRM parsing was read.");
displacement = Some(Operand::Word(self.parse_word()));
displacement = Some(Displacement::IWord(self.parse_word() as i16));
}
0b11 => {
log::debug!("ModRM ({:#b}) to/from Register ({:#b})", rm, reg);
@@ -213,9 +221,9 @@ impl Disassembler {
/// Group 1 always have an ModRM target (all modrm bits, without reg) as
/// first and an imm value as second operand (which has to be parsed before
/// call to this function), but is available in both Byte and Word length.
pub fn modrm_reg_to_grp1(reg: u8, target: ModRmTarget, imm: Operand) -> Mnemonic {
pub fn modrm_reg_to_grp1(reg: u8, target: ModRmTarget, imm: Displacement) -> Mnemonic {
match imm {
Operand::Byte(b) => match reg {
Displacement::IByte(b) => match reg {
0b000 => Mnemonic::ADD_Ib(target, b),
0b001 => Mnemonic::OR_Ib(target, b),
0b010 => Mnemonic::ADC_Ib(target, b),
@@ -226,7 +234,7 @@ impl Disassembler {
0b111 => Mnemonic::CMP_Ib(target, b),
_ => panic!("Illegal Group 1 mnemonic"),
},
Operand::Word(w) => match reg {
Displacement::IWord(w) => match reg {
0b000 => Mnemonic::ADD_Iv(target, w),
0b001 => Mnemonic::OR_Iv(target, w),
0b010 => Mnemonic::ADC_Iv(target, w),
@@ -454,25 +462,25 @@ impl Disassembler {
// Group 1
0x80 => {
let (target, reg) = self.parse_modrm_byte(Operand::Byte(0));
let imm = self.parse_byte();
Self::modrm_reg_to_grp1(reg, target, Operand::Byte(imm))
let imm = self.parse_byte() as i8;
Self::modrm_reg_to_grp1(reg, target, Displacement::IByte(imm))
}
0x81 => {
let (target, reg) = self.parse_modrm_byte(Operand::Word(0));
let imm = self.parse_word();
Self::modrm_reg_to_grp1(reg, target, Operand::Word(imm))
let imm = self.parse_word() as i16;
Self::modrm_reg_to_grp1(reg, target, Displacement::IWord(imm))
}
0x82 => {
// same as 0x80
let (target, reg) = self.parse_modrm_byte(Operand::Byte(0));
let imm = self.parse_byte();
Self::modrm_reg_to_grp1(reg, target, Operand::Byte(imm))
let imm = self.parse_byte() as i8;
Self::modrm_reg_to_grp1(reg, target, Displacement::IByte(imm))
}
0x83 => {
// byte extended version
let (target, reg) = self.parse_modrm_byte(Operand::Word(0));
let imm = self.parse_byte();
Self::modrm_reg_to_grp1(reg, target, Operand::Byte(imm))
let imm = self.parse_byte() as i8;
Self::modrm_reg_to_grp1(reg, target, Displacement::IByte(imm))
}
0x84 => modrmb!(self, TEST),