Add recomputation check for number representation.

2 files changed, 56 insertions, 1 deletions

diff --git a/libevmasm/ConstantOptimiser.cpp b/libevmasm/ConstantOptimiser.cpp
index a1dfd21c..c1d61b88 100644
--- a/libevmasm/ConstantOptimiser.cpp
+++ b/libevmasm/ConstantOptimiser.cpp
@@ -232,6 +232,54 @@ AssemblyItems ComputeMethod::findRepresentation(u256 const& _value)
 	}
 }
 
+bool ComputeMethod::checkRepresentation(u256 const& _value)
+{
+	// This is a tiny EVM that can only evaluate some instructions.
+	vector<u256> stack;
+	for (AssemblyItem const& item: m_routine)
+	{
+		switch (item.type())
+		{
+		case Operation:
+		{
+			if (stack.size() < size_t(item.arguments()))
+				return false;
+			u256* sp = &stack.back();
+			switch (item.instruction())
+			{
+			case Instruction::MUL:
+				sp[-1] = sp[0] * sp[-1];
+				break;
+			case Instruction::EXP:
+				if (sp[-1] > 0xff)
+					return false;
+				sp[-1] = boost::multiprecision::pow(sp[0], unsigned(sp[-1]));
+				break;
+			case Instruction::ADD:
+				sp[-1] = sp[0] + sp[-1];
+				break;
+			case Instruction::SUB:
+				sp[-1] = sp[0] - sp[-1];
+				break;
+			case Instruction::NOT:
+				sp[0] = ~sp[0];
+				break;
+			default:
+				return false;
+			}
+			stack.resize(stack.size() + item.deposit());
+			break;
+		}
+		case Push:
+			stack.push_back(item.data());
+			break;
+		default:
+			return false;
+		}
+	}
+	return stack.size() == 1 && stack.front() == _value;
+}
+
 bigint ComputeMethod::gasNeeded(AssemblyItems const& _routine)
 {
 	size_t numExps = count(_routine.begin(), _routine.end(), Instruction::EXP);
diff --git a/libevmasm/ConstantOptimiser.h b/libevmasm/ConstantOptimiser.h
index 4f12c49f..8a37b3cb 100644
--- a/libevmasm/ConstantOptimiser.h
+++ b/libevmasm/ConstantOptimiser.h
@@ -21,10 +21,13 @@
 
 #pragma once
 
-#include <vector>
+#include <libevmasm/Exceptions.h>
+
 #include <libdevcore/CommonData.h>
 #include <libdevcore/CommonIO.h>
 
+#include <vector>
+
 namespace dev
 {
 namespace eth
@@ -130,6 +133,8 @@ public:
 		ConstantOptimisationMethod(_params, _value)
 	{
 		m_routine = findRepresentation(m_value);
+		if (!checkRepresentation(m_value))
+			BOOST_THROW_EXCEPTION(AssemblyException());
 	}
 
 	virtual bigint gasNeeded() override { return gasNeeded(m_routine); }
@@ -141,6 +146,8 @@ public:
 protected:
 	/// Tries to recursively find a way to compute @a _value.
 	AssemblyItems findRepresentation(u256 const& _value);
+	/// Recomputes the value from the calculated representation and checks for correctness.
+	bool checkRepresentation(u256 const& _value);
 	bigint gasNeeded(AssemblyItems const& _routine);
 
 	/// Counter for the complexity of optimization, will stop when it reaches zero.