EricWF created this revision.
EricWF added reviewers: mclow.lists, danalbert, rsmith.
EricWF added subscribers: cfe-commits, awi, rsmith.
This patch partially fixes the undefined behavior in `<list>`. The undefined
behavior is caused by casting a pointer to `__node_base __end_` to the derived
type `__node` even though `__end_` is not a sub-object of type `__node`.
See C++14 [expr.static.cast]p11
> A prvalue of type “pointer to cv1 B,” where B is a class type, can be
> converted to a prvalue of type “pointer to cv2 D,” where D is a class derived
> (Clause 10) from B, if a valid standard conversion from “pointer to D” to
> “pointer to B” exists (4.10), cv2 is the same cv-qualification as, or greater
> cv-qualification than, cv1, and B is neither a virtual base class of D nor a
> base class of a virtual base class of D. The null pointer value (4.10) is
> converted to the null pointer value of the destination type. If the prvalue
> of type “pointer to cv1 B” points to a B that is actually a subobject of an
> object of type D, the resulting pointer points to the enclosing object of
> type D. Otherwise, the behavior is undefined.
This patch fixes the problem by adding an intermediary void cast so that the
conversion acts as if it were a reinterpret_cast. A reinterpret cast between
pointer types is not used because this will not work with fancy pointers. This
depends on the wording in:
C++14 [expr.reinterpret.cast]p7
> An object pointer can be explicitly converted to an object pointer of a
> different type. When a prvalue v of object pointer type is converted to the
> object pointer type “pointer to cv T”, the result is static_cast<cv
> T*>(static_cast<cv void*>(v)). Converting a prvalue of type “pointer to T1”
> to the type “pointer to T2” (where T1 and T2 are object types and where the
> alignment requirements of T2 are no stricter than those of T1) and back to
> its original type yields the original pointer value.
However, as @rsmith pointed out, this still has undefined behavior when the
`element_type` stored in the list has a higher alignment requirement than that
of a pointer.
I will post a patch that completely fixes this problem but that likely involves
an ABI break.
http://reviews.llvm.org/D12297
Files:
include/list
Index: include/list
===================================================================
--- include/list
+++ include/list
@@ -205,7 +205,8 @@
_LIBCPP_INLINE_VISIBILITY
pointer __self()
{
- return
static_cast<pointer>(pointer_traits<__base_pointer>::pointer_to(*this));
+ return static_cast<pointer>(static_cast<_VoidPtr>(
+ pointer_traits<__base_pointer>::pointer_to(*this)));
}
};
@@ -543,22 +544,18 @@
iterator end() _NOEXCEPT
{
#if _LIBCPP_DEBUG_LEVEL >= 2
- return iterator(static_cast<__node_pointer>(
- pointer_traits<__node_base_pointer>::pointer_to(__end_)),
this);
+ return iterator(__end_.__self(), this);
#else
- return iterator(static_cast<__node_pointer>(
-
pointer_traits<__node_base_pointer>::pointer_to(__end_)));
+ return iterator(__end_.__self());
#endif
}
_LIBCPP_INLINE_VISIBILITY
const_iterator end() const _NOEXCEPT
{
#if _LIBCPP_DEBUG_LEVEL >= 2
- return const_iterator(static_cast<__node_const_pointer>(
-
pointer_traits<__node_base_pointer>::pointer_to(const_cast<__node_base&>(__end_))),
this);
+ return const_iterator(const_cast<__node_base&>(__end_).__self(), this);
#else
- return const_iterator(static_cast<__node_const_pointer>(
-
pointer_traits<__node_base_pointer>::pointer_to(const_cast<__node_base&>(__end_))));
+ return const_iterator(const_cast<__node_base&>(__end_).__self());
#endif
}
Index: include/list
===================================================================
--- include/list
+++ include/list
@@ -205,7 +205,8 @@
_LIBCPP_INLINE_VISIBILITY
pointer __self()
{
- return static_cast<pointer>(pointer_traits<__base_pointer>::pointer_to(*this));
+ return static_cast<pointer>(static_cast<_VoidPtr>(
+ pointer_traits<__base_pointer>::pointer_to(*this)));
}
};
@@ -543,22 +544,18 @@
iterator end() _NOEXCEPT
{
#if _LIBCPP_DEBUG_LEVEL >= 2
- return iterator(static_cast<__node_pointer>(
- pointer_traits<__node_base_pointer>::pointer_to(__end_)), this);
+ return iterator(__end_.__self(), this);
#else
- return iterator(static_cast<__node_pointer>(
- pointer_traits<__node_base_pointer>::pointer_to(__end_)));
+ return iterator(__end_.__self());
#endif
}
_LIBCPP_INLINE_VISIBILITY
const_iterator end() const _NOEXCEPT
{
#if _LIBCPP_DEBUG_LEVEL >= 2
- return const_iterator(static_cast<__node_const_pointer>(
- pointer_traits<__node_base_pointer>::pointer_to(const_cast<__node_base&>(__end_))), this);
+ return const_iterator(const_cast<__node_base&>(__end_).__self(), this);
#else
- return const_iterator(static_cast<__node_const_pointer>(
- pointer_traits<__node_base_pointer>::pointer_to(const_cast<__node_base&>(__end_))));
+ return const_iterator(const_cast<__node_base&>(__end_).__self());
#endif
}
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