FEATURE: use a safer eval code, with limited arithmetic capabilities.

ardupilot_methodic_configurator/safe_eval.py

@@ -0,0 +1,93 @@
+"""
+This file is part of Ardupilot methodic configurator. https://github.com/ArduPilot/MethodicConfigurator.
+
+SPDX-FileCopyrightText: 2024 Amilcar do Carmo Lucas <amilcar.lucas@iav.de>
+
+SPDX-License-Identifier: GPL-3.0-or-later
+"""
+
+import ast
+import logging
+import math
+import operator
+from typing import Callable, Union, cast
+
+logger = logging.getLogger(__name__)
+
+# Type aliases
+Number = Union[int, float]
+MathFunc = Callable[..., Number]
+BinOperator = Callable[[Number, Number], Number]
+UnOperator = Callable[[Number], Number]
+
+
+def safe_eval(s: str) -> Number:
+    def checkmath(x: str, *args: Number) -> Number:
+        if x not in [x for x in dir(math) if "__" not in x]:
+            msg = f"Unknown func {x}()"
+            raise SyntaxError(msg)
+        fun = cast("MathFunc", getattr(math, x))
+        try:
+            return fun(*args)
+        except TypeError as e:
+            msg = f"Invalid arguments for {x}(): {e!s}"
+            raise SyntaxError(msg) from e
+
+    bin_ops: dict[type[ast.operator], BinOperator] = {
+        ast.Add: operator.add,
+        ast.Sub: operator.sub,
+        ast.Mult: operator.mul,
+        ast.Div: operator.truediv,
+        ast.Mod: operator.mod,
+        ast.Pow: operator.pow,
+        ast.Call: checkmath,
+        ast.BinOp: ast.BinOp,
+    }
+
+    un_ops: dict[type[ast.UnaryOp], UnOperator] = {
+        ast.USub: operator.neg,
+        ast.UAdd: operator.pos,
+        ast.UnaryOp: ast.UnaryOp,
+    }
+
+    tree = ast.parse(s, mode="eval")
+
+    def _eval(node: ast.AST) -> Number:
+        if isinstance(node, ast.Expression):
+            logger.debug("Expr")
+            return _eval(node.body)
+        if isinstance(node, ast.Constant):
+            logger.info("Const")
+            return cast("Number", node.value)
+        if isinstance(node, ast.Name):
+            # Handle math constants like pi, e, etc.
+            logger.info("MathConst")
+            if hasattr(math, node.id):
+                return cast("Number", getattr(math, node.id))
+            msg = f"Unknown constant: {node.id}"
+            raise SyntaxError(msg)
+        if isinstance(node, ast.BinOp):
+            logger.debug("BinOp")
+            left = _eval(node.left)
+            right = _eval(node.right)
+            if type(node.op) not in bin_ops:
+                msg = f"Unsupported operator: {type(node.op)}"
+                raise SyntaxError(msg)
+            return bin_ops[type(node.op)](left, right)
+        if isinstance(node, ast.UnaryOp):
+            logger.debug("UpOp")
+            operand = _eval(node.operand)
+            if type(node.op) not in un_ops:
+                msg = f"Unsupported operator: {type(node.op)}"
+                raise SyntaxError(msg)
+            return un_ops[type(node.op)](operand)
+        if isinstance(node, ast.Call):
+            if not isinstance(node.func, ast.Name):
+                msg = "Only direct math function calls allowed"
+                raise SyntaxError(msg)
+            args = [_eval(x) for x in node.args]
+            return checkmath(node.func.id, *args)
+        msg = f"Bad syntax, {type(node)}"
+        raise SyntaxError(msg)
+
+    return _eval(tree)

tests/test_safe_eval.py

@@ -0,0 +1,127 @@
+#!/usr/bin/env python3
+"""
+Tests for safe_eval.py.
+
+This file is part of Ardupilot methodic configurator. https://github.com/ArduPilot/MethodicConfigurator
+
+SPDX-FileCopyrightText: 2024 Amilcar do Carmo Lucas <amilcar.lucas@iav.de>
+
+SPDX-License-Identifier: GPL-3.0-or-later
+"""
+
+import math
+
+import pytest
+
+from ardupilot_methodic_configurator.safe_eval import safe_eval
+
+
+def test_basic_arithmetic() -> None:
+    """Test basic arithmetic operations."""
+    assert safe_eval("1+1") == 2
+    assert safe_eval("1+-5") == -4
+    assert safe_eval("-1") == -1
+    assert safe_eval("-+1") == -1
+    assert safe_eval("(100*10)+6") == 1006
+    assert safe_eval("100*(10+6)") == 1600
+    assert safe_eval("2**4") == 16
+    assert pytest.approx(safe_eval("1.2345 * 10")) == 12.345
+
+
+def test_math_functions() -> None:
+    """Test mathematical functions."""
+    assert safe_eval("sqrt(16)+1") == 5
+    assert safe_eval("sin(0)") == 0
+    assert safe_eval("cos(0)") == 1
+    assert safe_eval("tan(0)") == 0
+    assert safe_eval("log(1)") == 0
+    assert safe_eval("exp(0)") == 1
+    assert safe_eval("pi") == math.pi
+
+
+def test_complex_expressions() -> None:
+    """Test more complex mathematical expressions."""
+    assert safe_eval("2 * (3 + 4)") == 14
+    assert safe_eval("2 ** 3 * 4") == 32
+    assert safe_eval("sqrt(16) + sqrt(9)") == 7
+    assert safe_eval("sin(pi/2)") == 1
+
+
+def test_error_cases() -> None:
+    """Test error conditions."""
+    with pytest.raises(SyntaxError):
+        safe_eval("1 + ")  # Incomplete expression
+
+    with pytest.raises(SyntaxError):
+        safe_eval("unknown_func(10)")  # Unknown function
+
+    with pytest.raises(SyntaxError):
+        safe_eval("1 = 1")  # Invalid operator
+
+    with pytest.raises(SyntaxError):
+        safe_eval("import os")  # Attempted import
+
+
+def test_nested_expressions() -> None:
+    """Test nested mathematical expressions."""
+    assert safe_eval("sqrt(pow(3,2) + pow(4,2))") == 5  # Pythagorean theorem
+    assert safe_eval("log(exp(1))") == 1
+    assert safe_eval("sin(pi/6)**2 + cos(pi/6)**2") == pytest.approx(1)
+
+
+def test_division_by_zero() -> None:
+    """Test division by zero handling."""
+    with pytest.raises(ZeroDivisionError):
+        safe_eval("1/0")
+    with pytest.raises(ZeroDivisionError):
+        safe_eval("10 % 0")
+
+
+def test_invalid_math_functions() -> None:
+    """Test invalid math function calls."""
+    with pytest.raises(SyntaxError, match=r".*takes exactly one argument.*"):
+        safe_eval("sin()")  # Missing argument
+    with pytest.raises(SyntaxError, match=r".*takes exactly one argument.*"):
+        safe_eval("sin(1,2)")  # Too many arguments
+    with pytest.raises(ValueError, match=r"math domain error"):
+        safe_eval("sqrt(-1)")  # Domain error
+    with pytest.raises(ValueError, match=r"math domain error"):
+        safe_eval("log(-1)")  # Range error
+    with pytest.raises(SyntaxError, match=r"Unknown func.*"):
+        safe_eval("unknown(1)")  # Unknown function
+
+
+def test_security() -> None:
+    """Test against code injection attempts."""
+    with pytest.raises(SyntaxError):
+        safe_eval("__import__('os').system('ls')")
+    with pytest.raises(SyntaxError):
+        safe_eval("open('/etc/passwd')")
+    with pytest.raises(SyntaxError):
+        safe_eval("eval('1+1')")
+
+
+def test_operator_precedence() -> None:
+    """Test operator precedence rules."""
+    assert safe_eval("2 + 3 * 4") == 14
+    assert safe_eval("(2 + 3) * 4") == 20
+    assert safe_eval("-2 ** 2") == -4  # Exponentiation before negation
+    assert safe_eval("-(2 ** 2)") == -4
+
+
+def test_float_precision() -> None:
+    """Test floating point precision handling."""
+    assert pytest.approx(safe_eval("0.1 + 0.2")) == 0.3
+    assert pytest.approx(safe_eval("sin(pi/2)")) == 1.0
+    assert pytest.approx(safe_eval("cos(pi)")) == -1.0
+    assert pytest.approx(safe_eval("exp(log(2.718281828))")) == math.e
+
+
+def test_math_constants() -> None:
+    """Test mathematical constants."""
+    assert safe_eval("pi") == math.pi
+    assert safe_eval("e") == math.e
+    assert safe_eval("tau") == math.tau
+    assert safe_eval("inf") == math.inf
+    with pytest.raises(SyntaxError):
+        safe_eval("not_a_constant")