Harden --locals against trashed memory

When attempting to produce a repr for local variables, attempt to cope
better with trashed memory:

- Use "<invalid object at 0x1234>" as the repr of a `PyObject*` pointing
  directly at invalid memory, or one whose `ob_type` points at invalid
  memory.
- Use "<list object at 0x1234>" as, for instance, the repr of a list containing an
  invalid `ob_items` pointer. More generally, use this for any object
  where we can determine the type, but generating the repr according to
  that type's rules fails due to accessing unmapped memory.

Test this with some ctypes abuse.

Signed-off-by: Matt Wozniski <[email protected]>

news/194.bugfix.rst

@@ -0,0 +1 @@
+Heap corruption could cause PyStack to fail to generate a stack when ``--locals`` mode was used. This has been fixed by falling back to a reasonable default when attempting to format the repr of a local variable causes a dereference of an invalid pointer.

src/pystack/_pystack/pytypes.cpp

@@ -503,7 +503,7 @@ std::string
 GenericObject::toString(ssize_t max_size) const
 {
     std::stringstream os;
-    os << "<" << std::hex << std::showbase << d_classname << " at " << d_addr << ">";
+    os << "<" << std::hex << d_classname << " at 0x" << d_addr << ">";
     std::string object_str = os.str();
     return limitOutput(object_str, max_size);
 }
@@ -521,19 +521,39 @@ NoneObject::toString([[maybe_unused]] ssize_t max_size) const
 
 Object::Object(const std::shared_ptr<const AbstractProcessManager>& manager, remote_addr_t addr)
 : d_addr(addr)
+, d_type_addr(0)
+, d_classname()
+, d_flags()
 , d_manager(manager)
 {
     LOG(DEBUG) << std::hex << std::showbase << "Copying PyObject data from address " << addr;
 
     PyObject obj;
-    manager->copyObjectFromProcess(d_addr, &obj);
+    try {
+        manager->copyObjectFromProcess(d_addr, &obj);
+    } catch (RemoteMemCopyError& ex) {
+        LOG(WARNING) << std::hex << std::showbase << "Failed to read PyObject data from address "
+                     << d_addr;
+        d_classname = "invalid object";
+        return;
+    }
 
     PyTypeObject cls;
     LOG(DEBUG) << std::hex << std::showbase << "Copying typeobject from address " << obj.ob_type;
     d_type_addr = reinterpret_cast<remote_addr_t>(obj.ob_type);
-    manager->copyMemoryFromProcess((remote_addr_t)obj.ob_type, manager->offsets().py_type.size, &cls);
+    try {
+        manager->copyMemoryFromProcess(
+                (remote_addr_t)obj.ob_type,
+                manager->offsets().py_type.size,
+                &cls);
 
-    d_flags = manager->getField(cls, &py_type_v::o_tp_flags);
+        d_flags = manager->getField(cls, &py_type_v::o_tp_flags);
+    } catch (RemoteMemCopyError& ex) {
+        LOG(WARNING) << std::hex << std::showbase << "Failed to read typeobject from address "
+                     << obj.ob_type;
+        d_classname = "invalid object";
+        return;
+    }
 
     remote_addr_t name_addr = manager->getField(cls, &py_type_v::o_tp_name);
     try {
@@ -577,24 +597,33 @@ Object::toString(ssize_t max_size) const
     if (max_size <= 5) {
         return ELLIPSIS;
     }
-    std::stringstream os;
-    std::visit(
-            overloaded{
-                    [&](const auto& arg) { os << arg.toString(max_size); },
-                    [&](const bool arg) { os << arg; },
-                    [&](const long arg) { os << arg; },
-                    [&](const double arg) { os << arg; },
-                    [&](const std::string& arg) {
-                        std::string truncated = arg;
-                        if (static_cast<size_t>(max_size) < arg.size()) {
-                            truncated = arg.substr(0, max_size - 3) + "...";
-                        }
-                        os << truncated;
-                    },
-            },
-            toConcreteObject());
+    try {
+        std::stringstream os;
+        std::visit(
+                overloaded{
+                        [&](const auto& arg) { os << arg.toString(max_size); },
+                        [&](const bool arg) { os << arg; },
+                        [&](const long arg) { os << arg; },
+                        [&](const double arg) { os << arg; },
+                        [&](const std::string& arg) {
+                            std::string truncated = arg;
+                            if (static_cast<size_t>(max_size) < arg.size()) {
+                                truncated = arg.substr(0, max_size - 3) + "...";
+                            }
+                            os << truncated;
+                        },
+                },
+                toConcreteObject());
+
+        return os.str();
+    } catch (RemoteMemCopyError& ex) {
+        LOG(WARNING) << std::hex << std::showbase << "Failed to create a repr for object of type "
+                     << d_classname << " at address " << d_addr;
 
-    return os.str();
+        std::stringstream os;
+        os << std::hex << std::showbase << "<" << d_classname << " object at " << d_addr << ">";
+        return os.str();
+    }
 }
 
 long

tests/integration/test_local_variables.py

@@ -558,3 +558,71 @@ def second_func(the_argument):
 
     second_func_frame = find_frame(frames, "second_func")
     assert "the_argument" not in second_func_frame.arguments
+
+
+@ALL_PYTHONS
+@ALL_SOURCES
+def test_trashed_locals(generate_threads, python, tmpdir):
+    # GIVEN
+    _, python_executable = python
+
+    program_template = """
+import ctypes
+import sys
+import time
+
+class ListObject(ctypes.Structure):
+    _fields_ = [
+        ("ob_refcnt", ctypes.c_ssize_t),
+        ("ob_type", ctypes.c_void_p),
+        ("ob_size", ctypes.c_ssize_t),
+        ("ob_item", ctypes.c_void_p),
+    ]
+
+class TupleObject(ctypes.Structure):
+    _fields_ = [
+        ("ob_refcnt", ctypes.c_ssize_t),
+        ("ob_type", ctypes.c_void_p),
+        ("ob_size", ctypes.c_ssize_t),
+        ("ob_item0", ctypes.c_void_p),
+        ("ob_item1", ctypes.c_void_p),
+    ]
+
+def main():
+    bad_type = (1, 2, 3)
+    bad_elem = (4, 5, 6)
+    nullelem = (7, 8, 9)
+    bad_list = [0, 1, 2]
+
+    TupleObject.from_address(id(bad_type)).ob_type = 0xded
+    TupleObject.from_address(id(bad_elem)).ob_item1 = 0xbad
+    TupleObject.from_address(id(nullelem)).ob_item1 = 0x0
+    ListObject.from_address(id(bad_list)).ob_item = 0x0
+
+    fifo = sys.argv[1]
+    with open(sys.argv[1], "w") as fifo:
+        fifo.write("ready")
+    time.sleep(1000)
+
+main()
+    """
+
+    script = tmpdir / "the_script.py"
+    script.write_text(program_template, encoding="utf-8")
+
+    # WHEN
+    threads = generate_threads(python_executable, script, tmpdir, locals=True)
+
+    # THEN
+
+    assert len(threads) == 1
+    (thread,) = threads
+
+    frames = list(thread.frames)
+    assert (len(frames)) == 2
+
+    main = find_frame(frames, "main")
+    assert re.match(r"^<invalid object at 0x[0-9a-f]{4,}>$", main.locals["bad_type"])
+    assert main.locals["bad_elem"] == "(4, <invalid object at 0xbad>, 6)"
+    assert main.locals["nullelem"] == "(7, <invalid object at 0x0>, 9)"
+    assert re.match(r"^<list object at 0x[0-9a-f]{4,}>$", main.locals["bad_list"])