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Simplified MultiByteNopHandler by using a dictionary-based approach to replace complex conditional logic

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This commit is contained in:
bird_egop 2025-04-14 00:21:24 +03:00
parent c9901aa9b8
commit 4b549f4b1b
1 changed files with 49 additions and 49 deletions
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98
X86Disassembler/X86/Handlers/Nop/MultiByteNopHandler.cs
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@ -6,6 +6,29 @@ namespace X86Disassembler.X86.Handlers.Nop;
/// </summary>
public class MultiByteNopHandler : InstructionHandler
{
// Dictionary mapping ModR/M byte to NOP variant information (memory operand and additional bytes to skip)
private static readonly Dictionary<byte, (string MemOperand, int BytesToSkip, byte[] ExpectedBytes)> NopVariants = new()
{
// 3-byte NOP: 0F 1F 00
{ 0x00, ("[eax]", 0, Array.Empty<byte>()) },
// 4-byte NOP: 0F 1F 40 00
{ 0x40, ("[eax]", 1, new byte[] { 0x00 }) },
// 5-byte NOP: 0F 1F 44 00 00
{ 0x44, ("[eax+eax*1]", 2, new byte[] { 0x00, 0x00 }) },
// 6-byte NOP: 0F 1F 44 00 00 00
// Same ModR/M as 5-byte but with an extra 0x00 byte
// Handled separately in the code
// 7-byte NOP: 0F 1F 80 00 00 00 00
{ 0x80, ("[eax]", 4, new byte[] { 0x00, 0x00, 0x00, 0x00 }) },
// 8-byte NOP: 0F 1F 84 00 00 00 00 00
{ 0x84, ("[eax+eax*1]", 5, new byte[] { 0x00, 0x00, 0x00, 0x00, 0x00 }) }
};
/// <summary>
/// Initializes a new instance of the MultiByteNopHandler class
/// </summary>
@ -30,16 +53,14 @@ public class MultiByteNopHandler : InstructionHandler
return false;
}
int position = Decoder.GetPosition();
// Check if we have enough bytes to read the second opcode
if (position >= Length)
if (!Decoder.CanReadByte())
{
return false;
}
// Check if the second byte is 0x1F (part of the multi-byte NOP encoding)
byte secondByte = CodeBuffer[position];
byte secondByte = CodeBuffer[Decoder.GetPosition()];
return secondByte == 0x1F;
}
@ -58,7 +79,7 @@ public class MultiByteNopHandler : InstructionHandler
Decoder.ReadByte();
// Check if we have enough bytes to read the ModR/M byte
if (Decoder.GetPosition() >= Length)
if (!Decoder.CanReadByte())
{
return false;
}
@ -74,56 +95,35 @@ public class MultiByteNopHandler : InstructionHandler
byte modRm = CodeBuffer[position];
Decoder.SetPosition(position + 1); // Skip the ModR/M byte
// Determine the operand based on the NOP variant
string memOperand;
// Default memory operand if no specific variant is matched
string memOperand = "[eax]";
// 3-byte NOP: 0F 1F 00
if (modRm == 0x00)
{
memOperand = "[eax]";
}
// 4-byte NOP: 0F 1F 40 00
else if (modRm == 0x40 && position + 1 < Length && CodeBuffer[position + 1] == 0x00)
{
memOperand = "[eax]";
Decoder.SetPosition(position + 2); // Skip the displacement byte
}
// 5-byte NOP: 0F 1F 44 00 00
else if (modRm == 0x44 && position + 2 < Length &&
CodeBuffer[position + 1] == 0x00 && CodeBuffer[position + 2] == 0x00)
{
memOperand = "[eax+eax*1]";
Decoder.SetPosition(position + 3); // Skip the SIB and displacement bytes
}
// 6-byte NOP: 0F 1F 44 00 00 00
else if (modRm == 0x44 && position + 3 < Length &&
CodeBuffer[position + 1] == 0x00 && CodeBuffer[position + 2] == 0x00 &&
CodeBuffer[position + 3] == 0x00)
// Check for the 6-byte NOP special case (0x44 with 3 zero bytes)
if (modRm == 0x44 && position + 3 < Length &&
CodeBuffer[position + 1] == 0x00 &&
CodeBuffer[position + 2] == 0x00 &&
CodeBuffer[position + 3] == 0x00)
{
memOperand = "[eax+eax*1]";
Decoder.SetPosition(position + 4); // Skip the SIB, displacement, and extra byte
}
// 7-byte NOP: 0F 1F 80 00 00 00 00
else if (modRm == 0x80 && position + 4 < Length &&
CodeBuffer[position + 1] == 0x00 && CodeBuffer[position + 2] == 0x00 &&
CodeBuffer[position + 3] == 0x00 && CodeBuffer[position + 4] == 0x00)
// Look up the NOP variant in the dictionary
else if (NopVariants.TryGetValue(modRm, out var variant))
{
memOperand = "[eax]";
Decoder.SetPosition(position + 5); // Skip the displacement bytes
}
// 8-byte NOP: 0F 1F 84 00 00 00 00 00
else if (modRm == 0x84 && position + 5 < Length &&
CodeBuffer[position + 1] == 0x00 && CodeBuffer[position + 2] == 0x00 &&
CodeBuffer[position + 3] == 0x00 && CodeBuffer[position + 4] == 0x00 &&
CodeBuffer[position + 5] == 0x00)
{
memOperand = "[eax+eax*1]";
Decoder.SetPosition(position + 6); // Skip the SIB and displacement bytes
}
// For any other variant, use a generic NOP operand
else
{
memOperand = "[eax]";
// Check if we have enough bytes and if they match the expected pattern
bool isValidVariant = position + variant.BytesToSkip < Length;
// Verify all expected bytes match
for (int i = 0; i < variant.ExpectedBytes.Length && isValidVariant; i++)
{
isValidVariant = isValidVariant && CodeBuffer[position + 1 + i] == variant.ExpectedBytes[i];
}
if (isValidVariant)
{
memOperand = variant.MemOperand;
Decoder.SetPosition(position + variant.BytesToSkip + 1); // +1 for ModR/M byte already skipped
}
}
// Set the operands with the appropriate size prefix