R2RDump Unwind code decoding logic missed the extra word or dword in the UWOP_ALLOC_LARGE case.

When we write the UnwindInfo for stack space allocation, depending on the size, we write an extra WORD/DWORD to the UnwindInfo stream as follow:

runtime/src/coreclr/src/jit/unwindamd64.cpp

Lines 278 to 316 in 0b16471

    
           void Compiler::unwindAllocStackWindows(unsigned size) 
        
           { 
        
               assert(compGeneratingProlog); 
        
               FuncInfoDsc* func = funCurrentFunc(); 
        
               assert(func->unwindHeader.Version == 1);            // Can't call this before unwindBegProlog 
        
               assert(func->unwindHeader.CountOfUnwindCodes == 0); // Can't call this after unwindReserve 
        
               assert(size % 8 == 0);                              // Stack size is *always* 8 byte aligned 
        
               UNWIND_CODE* code; 
        
               if (size <= 128) 
        
               { 
        
                   assert(func->unwindCodeSlot > sizeof(UNWIND_CODE)); 
        
                   code           = (UNWIND_CODE*)&func->unwindCodes[func->unwindCodeSlot -= sizeof(UNWIND_CODE)]; 
        
                   code->UnwindOp = UWOP_ALLOC_SMALL; 
        
                   code->OpInfo   = (size - 8) / 8; 
        
               } 
        
               else if (size <= 0x7FFF8) 
        
               { 
        
                   assert(func->unwindCodeSlot > (sizeof(UNWIND_CODE) + sizeof(USHORT))); 
        
                   USHORT* codedSize = (USHORT*)&func->unwindCodes[func->unwindCodeSlot -= sizeof(USHORT)]; 
        
                   *codedSize        = (USHORT)(size / 8); 
        
                   code              = (UNWIND_CODE*)&func->unwindCodes[func->unwindCodeSlot -= sizeof(UNWIND_CODE)]; 
        
                   code->UnwindOp    = UWOP_ALLOC_LARGE; 
        
                   code->OpInfo      = 0; 
        
               } 
        
               else 
        
               { 
        
                   assert(func->unwindCodeSlot > (sizeof(UNWIND_CODE) + sizeof(ULONG))); 
        
                   ULONG* codedSize = (ULONG*)&func->unwindCodes[func->unwindCodeSlot -= sizeof(ULONG)]; 
        
                   *codedSize       = size; 
        
                   code             = (UNWIND_CODE*)&func->unwindCodes[func->unwindCodeSlot -= sizeof(UNWIND_CODE)]; 
        
                   code->UnwindOp   = UWOP_ALLOC_LARGE; 
        
                   code->OpInfo     = 1; 
        
               } 
        
               unsigned int cbProlog = unwindGetCurrentOffset(func); 
        
               noway_assert((BYTE)cbProlog == cbProlog); 
        
               code->CodeOffset = (BYTE)cbProlog; 
        
           }

Notice how line 298 and line 307 moves the unwindCodeSlot further than just the usual sizeof(UNWIND_CODE).

However, the similar logic is missing from the decoder, here is the parsing code:

runtime/src/coreclr/src/tools/aot/ILCompiler.Reflection.ReadyToRun/Amd64/UnwindInfo.cs

Lines 63 to 79 in 0b16471

    
           public UnwindCode(byte[] image, int index, ref int offset) 
        
           { 
        
               Index = index; 
        
               int off = offset; 
        
               CodeOffset = NativeReader.ReadByte(image, ref off); 
        
               byte op = NativeReader.ReadByte(image, ref off); 
        
               UnwindOp = (UnwindOpCodes)(op & 15); 
        
               OpInfo = (byte)(op >> 4); 
        
               OffsetLow = CodeOffset; 
        
               OffsetHigh = OpInfo; 
        
               FrameOffset = NativeReader.ReadUInt16(image, ref offset); 
        
               NextFrameOffset = -1; 
        
               IsOpInfo = false;

The size of the extra word/dword happens to be a multiple of 2, therefore these word/dword are misinterpreted as extra unwind opcode. This is just wrong. This explains why we saw more than one unwind opcode having the same code offset in R2RDump.

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R2RDump Unwind code decoding logic missed the extra word or dword in the UWOP_ALLOC_LARGE case. #39940

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	void Compiler::unwindAllocStackWindows(unsigned size)
	{
	assert(compGeneratingProlog);

	FuncInfoDsc* func = funCurrentFunc();

	assert(func->unwindHeader.Version == 1); // Can't call this before unwindBegProlog
	assert(func->unwindHeader.CountOfUnwindCodes == 0); // Can't call this after unwindReserve
	assert(size % 8 == 0); // Stack size is always 8 byte aligned
	UNWIND_CODE* code;
	if (size <= 128)
	{
	assert(func->unwindCodeSlot > sizeof(UNWIND_CODE));
	code = (UNWIND_CODE*)&func->unwindCodes[func->unwindCodeSlot -= sizeof(UNWIND_CODE)];
	code->UnwindOp = UWOP_ALLOC_SMALL;
	code->OpInfo = (size - 8) / 8;
	}
	else if (size <= 0x7FFF8)
	{
	assert(func->unwindCodeSlot > (sizeof(UNWIND_CODE) + sizeof(USHORT)));
	USHORT* codedSize = (USHORT*)&func->unwindCodes[func->unwindCodeSlot -= sizeof(USHORT)];
	*codedSize = (USHORT)(size / 8);
	code = (UNWIND_CODE*)&func->unwindCodes[func->unwindCodeSlot -= sizeof(UNWIND_CODE)];
	code->UnwindOp = UWOP_ALLOC_LARGE;
	code->OpInfo = 0;
	}
	else
	{
	assert(func->unwindCodeSlot > (sizeof(UNWIND_CODE) + sizeof(ULONG)));
	ULONG* codedSize = (ULONG*)&func->unwindCodes[func->unwindCodeSlot -= sizeof(ULONG)];
	*codedSize = size;
	code = (UNWIND_CODE*)&func->unwindCodes[func->unwindCodeSlot -= sizeof(UNWIND_CODE)];
	code->UnwindOp = UWOP_ALLOC_LARGE;
	code->OpInfo = 1;
	}
	unsigned int cbProlog = unwindGetCurrentOffset(func);
	noway_assert((BYTE)cbProlog == cbProlog);
	code->CodeOffset = (BYTE)cbProlog;
	}

	public UnwindCode(byte[] image, int index, ref int offset)
	{
	Index = index;

	int off = offset;
	CodeOffset = NativeReader.ReadByte(image, ref off);
	byte op = NativeReader.ReadByte(image, ref off);
	UnwindOp = (UnwindOpCodes)(op & 15);
	OpInfo = (byte)(op >> 4);

	OffsetLow = CodeOffset;
	OffsetHigh = OpInfo;

	FrameOffset = NativeReader.ReadUInt16(image, ref offset);
	NextFrameOffset = -1;

	IsOpInfo = false;

R2RDump Unwind code decoding logic missed the extra word or dword in the UWOP_ALLOC_LARGE case. #39940

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