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Further simplified MultiByteNopHandler by using an array of tuples and matching patterns from longest to shortest

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bird_egop
2025-04-14 00:23:58 +03:00
parent 4b549f4b1b
commit 243789892d
1 changed files with 48 additions and 39 deletions
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87
X86Disassembler/X86/Handlers/Nop/MultiByteNopHandler.cs
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@@ -6,27 +6,26 @@ 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()
// NOP variant information (ModR/M byte, memory operand, and expected bytes pattern)
private static readonly (byte ModRm, string MemOperand, byte[] ExpectedBytes)[] NopVariants =
{
// 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
// 8-byte NOP: 0F 1F 84 00 00 00 00 00 (check longest patterns first)
(0x84, "[eax+eax*1]", new byte[] { 0x00, 0x00, 0x00, 0x00, 0x00 }),
// 7-byte NOP: 0F 1F 80 00 00 00 00
{ 0x80, ("[eax]", 4, new byte[] { 0x00, 0x00, 0x00, 0x00 }) },
(0x80, "[eax]", 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 }) }
// 6-byte NOP: 0F 1F 44 00 00 00
(0x44, "[eax+eax*1]", new byte[] { 0x00, 0x00, 0x00 }),
// 5-byte NOP: 0F 1F 44 00 00
(0x44, "[eax+eax*1]", new byte[] { 0x00, 0x00 }),
// 4-byte NOP: 0F 1F 40 00
(0x40, "[eax]", new byte[] { 0x00 }),
// 3-byte NOP: 0F 1F 00
(0x00, "[eax]", Array.Empty<byte>())
};
/// <summary>
@@ -90,42 +89,52 @@ public class MultiByteNopHandler : InstructionHandler
// Determine the size of the operand
string ptrType = hasOperandSizePrefix ? "word ptr" : "dword ptr";
// Read the ModR/M byte to identify the NOP variant
// Read the ModR/M byte but don't advance the position yet
int position = Decoder.GetPosition();
byte modRm = CodeBuffer[position];
Decoder.SetPosition(position + 1); // Skip the ModR/M byte
// Default memory operand if no specific variant is matched
string memOperand = "[eax]";
int bytesToSkip = 1; // Skip at least the ModR/M byte
// 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)
// Try to find a matching NOP variant (we check longest patterns first)
foreach (var (variantModRm, operand, expectedBytes) in NopVariants)
{
memOperand = "[eax+eax*1]";
Decoder.SetPosition(position + 4); // Skip the SIB, displacement, and extra byte
}
// Look up the NOP variant in the dictionary
else if (NopVariants.TryGetValue(modRm, out var variant))
{
// 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++)
// Skip if ModR/M doesn't match
if (variantModRm != modRm)
{
isValidVariant = isValidVariant && CodeBuffer[position + 1 + i] == variant.ExpectedBytes[i];
continue;
}
if (isValidVariant)
// Check if we have enough bytes for this pattern
if (position + expectedBytes.Length >= Length)
{
memOperand = variant.MemOperand;
Decoder.SetPosition(position + variant.BytesToSkip + 1); // +1 for ModR/M byte already skipped
continue;
}
// Check if the expected bytes match
bool isMatch = true;
for (int i = 0; i < expectedBytes.Length; i++)
{
if (position + i + 1 >= Length || CodeBuffer[position + i + 1] != expectedBytes[i])
{
isMatch = false;
break;
}
}
// If we found a match, use it and stop checking
if (isMatch)
{
memOperand = operand;
bytesToSkip = 1 + expectedBytes.Length; // ModR/M byte + additional bytes
break;
}
}
// Skip the bytes we've processed
Decoder.SetPosition(position + bytesToSkip);
// Set the operands with the appropriate size prefix
instruction.Operands = $"{ptrType} {memOperand}";