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fix: detect virtual inheritance in add_base to prevent pointer offset crash (#6017)

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Virtual inheritance places the base subobject at a dynamic offset, but
load_impl Case 2a uses reinterpret_cast which assumes a fixed offset.
This caused segfaults when dispatching inherited methods through virtual
bases (e.g. SftVirtDerived2::name()).

Add an is_static_downcastable SFINAE trait that detects whether
static_cast<Derived*>(Base*) is valid. When it is not (virtual
inheritance), set multiple_inheritance = true in add_base to force the
implicit_casts path, which correctly adjusts pointers at runtime.

Remove the workaround .def("name", &SftVirtDerived2::name) from
test_smart_ptr.cpp that was papering over the issue.

Made-with: Cursor
This commit is contained in:
Ralf W. Grosse-Kunstleve
2026-03-30 10:49:27 +07:00
committed by GitHub
parent 524d72b36d
commit 83f71d8b82
3 changed files with 18 additions and 4 deletions
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11
include/pybind11/detail/common.h
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@@ -1039,6 +1039,17 @@ struct is_instantiation<Class, Class<Us...>> : std::true_type {};
template <typename T>
using is_shared_ptr = is_instantiation<std::shared_ptr, T>;
/// Detects whether static_cast<Derived*>(Base*) is valid, i.e. the inheritance is non-virtual.
/// Used to detect virtual bases: if this is false, pointer adjustments require the implicit_casts
/// chain rather than reinterpret_cast.
template <typename Base, typename Derived, typename = void>
struct is_static_downcastable : std::false_type {};
template <typename Base, typename Derived>
struct is_static_downcastable<Base,
Derived,
void_t<decltype(static_cast<Derived *>(std::declval<Base *>()))>>
: std::true_type {};
/// Check if T looks like an input iterator
template <typename T, typename = void>
struct is_input_iterator : std::false_type {};

7
include/pybind11/pybind11.h
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@@ -2416,6 +2416,13 @@ public:
rec.add_base(typeid(Base), [](void *src) -> void * {
return static_cast<Base *>(reinterpret_cast<type *>(src));
});
// Virtual inheritance means the base subobject is at a dynamic offset,
// so the reinterpret_cast shortcut in load_impl Case 2a is invalid.
// Force the MI path (implicit_casts) for correct pointer adjustment.
// Detection: static_cast<Derived*>(Base*) is ill-formed for virtual bases.
if PYBIND11_MAYBE_CONSTEXPR (!detail::is_static_downcastable<Base, type>::value) {
rec.multiple_inheritance = true;
}
}
template <typename Base, detail::enable_if_t<!is_base<Base>::value, int> = 0>

4
tests/test_smart_ptr.cpp
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@@ -553,10 +553,6 @@ TEST_SUBMODULE(smart_ptr, m) {
py::class_<SftVirtDerived2, SftVirtDerived, std::shared_ptr<SftVirtDerived2>>(
m, "SftVirtDerived2")
.def(py::init<>(&SftVirtDerived2::create))
// TODO: Remove this once inherited methods work through virtual bases.
// Without it, d2.name() segfaults because pybind11 uses an incorrect
// pointer offset when dispatching through the virtual inheritance chain.
.def("name", &SftVirtDerived2::name)
.def("call_name", &SftVirtDerived2::call_name, py::arg("d2"));
// test_move_only_holder