Feature: constant time overflow checking while computing offsets (#29)

pkg/safemath/safemath.go

@@ -0,0 +1,39 @@
+package safemath
+
+import "math/bits"
+
+// Takes a uint64 and an int16 and outputs their addition as well
+// as the ocurrence of an overflow or underflow.
+//
+// This is a constant-time version of the following function:
+//
+//	func SafeOffset(x uint64, y int16) (res uint64, isOverflow bool) {
+//		res = x + uint64(y)
+//		if y < 0 {
+//			isOverflow = res >= x
+//		} else {
+//			isOverflow = res < x
+//		}
+//		return
+//	}
+//
+// This shows better results because the final bytecode
+// doesn't contain any conditional jump instructions
+// making it easier for a processor to pipeline the function.
+func SafeOffset(x uint64, y int16) (res uint64, isOverflow bool) {
+	enlargedY := uint64(y)
+	// I'll leave proving that this is correct as an exercise for the reader :)
+	res = x + enlargedY
+	// Why does this work?
+	// Let's proceed by cases on the most significant bit of (MSB(x)).
+	// If MSB(x) == 1 and enlargedY < 0 (MSB(enlargedY) == 1) then overflow doesn't happen.
+	// Let's consider the case enlargedY >= 0 (MSB(enlargedY) == 0).
+	// In that case we can only wrap up by going to the begining of uint64 range making the MSB(res) = 0.
+	// This is the second disjunct of the disjunctive formula.
+	//
+	// In the same fashion the case MSB(x) == 0 and MSB(enlargedY) == 1 is reasoned about.
+	//
+	// Finally, we boil everything down to MSBs by rotating and anding with ...000001.
+	isOverflow = bits.RotateLeft64((^x&enlargedY&res)|(x & ^enlargedY & ^res), 1)&0x1 != 0
+	return
+}

pkg/safemath/safemath_test.go

@@ -0,0 +1,35 @@
+package safemath
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestOffsetNeg(t *testing.T) {
+	res, isOverflow := SafeOffset(1215, -3)
+	assert.Equal(t, uint64(1212), res)
+	assert.False(t, isOverflow)
+}
+
+func TestOffsetPos(t *testing.T) {
+	res, isOverflow := SafeOffset(7, 11)
+	assert.Equal(t, uint64(18), res)
+	assert.False(t, isOverflow)
+}
+
+func TestOffsetLeftOverflow(t *testing.T) {
+	_, isOverflow := SafeOffset(4, -10)
+	assert.True(t, isOverflow)
+}
+
+func TestOffsetRightOverflow(t *testing.T) {
+	_, isOverflow := SafeOffset(^uint64(0), 1)
+	assert.True(t, isOverflow)
+}
+
+func TestOffsetRightNoOverflow(t *testing.T) {
+	res, isOverflow := SafeOffset(^uint64(0), -12)
+	assert.Equal(t, uint64(18446744073709551603), res)
+	assert.False(t, isOverflow)
+}

pkg/vm/vm.go

@@ -3,6 +3,7 @@ package vm
 import (
 	"fmt"
 
+	safemath "github.com/NethermindEth/cairo-vm-go/pkg/safemath"
 	mem "github.com/NethermindEth/cairo-vm-go/pkg/vm/memory"
 	f "github.com/consensys/gnark-crypto/ecc/stark-curve/fp"
 )
@@ -156,8 +157,11 @@ func (vm *VirtualMachine) getCellDst(instruction *Instruction) (*mem.Cell, error
 		dstRegister = vm.Context.Fp
 	}
 
-	// todo(rodro): fix this math
-	return vm.MemoryManager.Memory.Peek(executionSegment, dstRegister+uint64(instruction.OffDest))
+	addr, isOverflow := safemath.SafeOffset(dstRegister, instruction.OffDest)
+	if isOverflow {
+		return nil, fmt.Errorf("integer overflow while appying offset: 0x%x %d", dstRegister, instruction.OffDest)
+	}
+	return vm.MemoryManager.Memory.Peek(executionSegment, addr)
 }
 
 func (vm *VirtualMachine) getCellOp0(instruction *Instruction) (*mem.Cell, error) {
@@ -167,9 +171,12 @@ func (vm *VirtualMachine) getCellOp0(instruction *Instruction) (*mem.Cell, error
 	} else {
 		op0Register = vm.Context.Fp
 	}
-	// todo(rodro): fix this math
-	offset := op0Register + uint64(instruction.OffOp0)
-	return vm.MemoryManager.Memory.Peek(executionSegment, offset)
+
+	addr, isOverflow := safemath.SafeOffset(op0Register, instruction.OffOp0)
+	if isOverflow {
+		return nil, fmt.Errorf("integer overflow while appying offset: 0x%x %d", op0Register, instruction.OffOp0)
+	}
+	return vm.MemoryManager.Memory.Peek(executionSegment, addr)
 }
 
 func (vm *VirtualMachine) getCellOp1(instruction *Instruction, op0Cell *mem.Cell) (*mem.Cell, error) {
@@ -189,8 +196,12 @@ func (vm *VirtualMachine) getCellOp1(instruction *Instruction, op0Cell *mem.Cell
 	case ApPlusOffOp1:
 		op1Address = mem.CreateMemoryAddress(executionSegment, vm.Context.Ap)
 	}
-	// todo(rodro): fix this math
-	op1Address.Offset += uint64(instruction.OffOp1)
+
+	addr, isOverflow := safemath.SafeOffset(op1Address.Offset, instruction.OffOp1)
+	if isOverflow {
+		return nil, fmt.Errorf("integer overflow while appying offset: 0x%x %d", op1Address.Offset, instruction.OffOp1)
+	}
+	op1Address.Offset = addr
 
 	return vm.MemoryManager.Memory.PeekFromAddress(op1Address)
 }