fix: avoid MulAcc overwriting constants

frontend/cs/r1cs/api.go

@@ -77,8 +77,16 @@ func (builder *builder[E]) MulAcc(a, b, c frontend.Variable) frontend.Variable {
 	// results fits, _a is mutated without performing a new memalloc
 	builder.mbuf2 = builder.mbuf2[:0]
 	builder.add([]expr.LinearExpression[E]{_a, builder.mbuf1}, false, 0, &builder.mbuf2)
-	_a = _a[:0]
-	if len(builder.mbuf2) <= cap(_a) {
+
+	// if we can add the multiplication term to the accumulator LE (by having sufficient capacity)
+	// then we append directly into _a. However, _a can also be the hardcoded linear expressions corresponding
+	// to zero or one constant. Now, we we would append into those then we would modify the underlying slice
+	// thus modifying the constant themselves. This leads to undefined behaviour.
+	//
+	// So, in addition to only checking the capacity we also check that the underlying slices are different.
+	// to avoid using unsafe.Pointer, we check the address of the first elements.
+	if len(builder.mbuf2) <= cap(_a) && &(_a[0]) != &(builder.cstZero()[0]) && &(_a[0]) != &(builder.cstOne()[0]) {
+		_a = _a[:0]
 		// it fits, no mem alloc
 		_a = append(_a, builder.mbuf2...)
 	} else {

frontend/cs/r1cs/r1cs_test.go

@@ -171,3 +171,36 @@ func TestSubSameNoConstraint(t *testing.T) {
 		t.Fatal("expected 0 constraints")
 	}
 }
+
+type overwriteZeroConstCircuit struct {
+	X frontend.Variable
+	Y frontend.Variable
+}
+
+func (c *overwriteZeroConstCircuit) Define(api frontend.API) error {
+	constVar := api.Mul(0, c.X)               // this create a zero constant
+	constVar = api.MulAcc(constVar, c.X, c.Y) // due to bug the zero constant is overwritten
+	_ = constVar
+	constVar2 := api.Mul(0, c.Y) // this create another zero constant
+	api.AssertIsEqual(constVar2, 0)
+
+	return nil
+}
+
+func TestOverrideZeroConstant(t *testing.T) {
+	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), NewBuilder, &overwriteZeroConstCircuit{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	wit, err := frontend.NewWitness(&overwriteZeroConstCircuit{
+		X: 5,
+		Y: 10,
+	}, ecc.BN254.ScalarField())
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = ccs.Solve(wit)
+	if err != nil {
+		t.Fatal(err)
+	}
+}