[libcxx-commits] [libcxx] b3a1994 - [libc++] Properly implement array cookies in the ARM ABI (#160182)
via libcxx-commits
libcxx-commits at lists.llvm.org
Thu Oct 16 20:41:02 PDT 2025
Author: Louis Dionne
Date: 2025-10-16T23:40:58-04:00
New Revision: b3a199469c9d54fb3bdabf6dff5677b401f8fef6
URL: https://github.com/llvm/llvm-project/commit/b3a199469c9d54fb3bdabf6dff5677b401f8fef6
DIFF: https://github.com/llvm/llvm-project/commit/b3a199469c9d54fb3bdabf6dff5677b401f8fef6.diff
LOG: [libc++] Properly implement array cookies in the ARM ABI (#160182)
When we implemented array cookie support for hardening std::unique_ptr,
the implementation was only done for the Itanium ABI. I did not
initially realize that ARM was using a different ABI for array cookies,
so unique_ptr should not have been hardened on ARM.
However, we were also incorrectly setting the ABI-detection macro: we
were pretending to be using a vanilla Itanium ABI when in reality the
(similar but different) ARM ABI was in use. As a result, unique_ptr was
using the wrong representation for array cookies on ARM, which
fortunately only mattered in the case of overaligned types.
This patch fixes that.
rdar://160852193
Added:
Modified:
libcxx/include/__configuration/abi.h
libcxx/include/__memory/array_cookie.h
libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/assert.subscript.pass.cpp
Removed:
################################################################################
diff --git a/libcxx/include/__configuration/abi.h b/libcxx/include/__configuration/abi.h
index 2d33b9c03090b..c9936df30ff7f 100644
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@@ -30,8 +30,20 @@
#elif _LIBCPP_ABI_FORCE_MICROSOFT
# define _LIBCPP_ABI_MICROSOFT
#else
+// Windows uses the Microsoft ABI
# if defined(_WIN32) && defined(_MSC_VER)
# define _LIBCPP_ABI_MICROSOFT
+
+// 32-bit ARM uses the Itanium ABI with a few
diff erences (array cookies, etc),
+// and so does 64-bit ARM on Apple platforms.
+# elif defined(__arm__) || (defined(__APPLE__) && defined(__aarch64__))
+# define _LIBCPP_ABI_ITANIUM_WITH_ARM_DIFFERENCES
+
+// Non-Apple 64-bit ARM uses the vanilla Itanium ABI
+# elif defined(__aarch64__)
+# define _LIBCPP_ABI_ITANIUM
+
+// We assume that other architectures also use the vanilla Itanium ABI too
# else
# define _LIBCPP_ABI_ITANIUM
# endif
diff --git a/libcxx/include/__memory/array_cookie.h b/libcxx/include/__memory/array_cookie.h
index 806a9e99ecafe..be59f365aa80c 100644
--- a/libcxx/include/__memory/array_cookie.h
+++ b/libcxx/include/__memory/array_cookie.h
@@ -13,6 +13,7 @@
#include <__config>
#include <__configuration/abi.h>
#include <__cstddef/size_t.h>
+#include <__memory/addressof.h>
#include <__type_traits/integral_constant.h>
#include <__type_traits/is_trivially_destructible.h>
#include <__type_traits/negation.h>
@@ -26,14 +27,15 @@ _LIBCPP_BEGIN_NAMESPACE_STD
// Trait representing whether a type requires an array cookie at the start of its allocation when
// allocated as `new T[n]` and deallocated as `delete[] array`.
//
-// Under the Itanium C++ ABI [1], we know that an array cookie is available unless `T` is trivially
-// destructible and the call to `operator delete[]` is not a sized operator delete. Under ABIs other
-// than the Itanium ABI, we assume there are no array cookies.
+// Under the Itanium C++ ABI [1] and the ARM ABI which derives from it, we know that an array cookie is available
+// unless `T` is trivially destructible and the call to `operator delete[]` is not a sized operator delete. Under
+// other ABIs, we assume there are no array cookies.
//
// [1]: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#array-cookies
-#ifdef _LIBCPP_ABI_ITANIUM
+#if defined(_LIBCPP_ABI_ITANIUM) || defined(_LIBCPP_ABI_ITANIUM_WITH_ARM_DIFFERENCES)
// TODO: Use a builtin instead
-// TODO: We should factor in the choice of the usual deallocation function in this determination.
+// TODO: We should factor in the choice of the usual deallocation function in this determination:
+// a cookie may be available in more cases but we ignore those for now.
template <class _Tp>
struct __has_array_cookie : _Not<is_trivially_destructible<_Tp> > {};
#else
@@ -41,13 +43,79 @@ template <class _Tp>
struct __has_array_cookie : false_type {};
#endif
+struct __itanium_array_cookie {
+ size_t __element_count;
+};
+
+template <class _Tp>
+struct [[__gnu__::__aligned__(_LIBCPP_ALIGNOF(_Tp))]] __arm_array_cookie {
+ size_t __element_size;
+ size_t __element_count;
+};
+
+// Return the element count in the array cookie located before the given pointer.
+//
+// In the Itanium ABI [1]
+// ----------------------
+// The element count is stored immediately before the first element of the array. If the preferred alignment
+// of array elements (which is
diff erent from the ABI alignment) is more than that of size_t, additional
+// padding bytes exist before the array cookie. Assuming array elements of size and alignment 16 bytes, that
+// gives us the following layout:
+//
+// |ooooooooxxxxxxxxaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbccccccccccccccccdddddddddddddddd|
+// ^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// | ^^^^^^^^ |
+// | | array elements
+// padding |
+// element count
+//
+//
+// In the Itanium ABI with ARM
diff erences [2]
+// -------------------------------------------
+// The array cookie is stored at the very start of the allocation and it has the following form:
+//
+// struct array_cookie {
+// std::size_t element_size; // element_size != 0
+// std::size_t element_count;
+// };
+//
+// Assuming elements of size and alignment 32 bytes, this gives us the following layout:
+//
+// |xxxxxxxxXXXXXXXXooooooooooooooooaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb|
+// ^^^^^^^^ ^^^^^^^^^^^^^^^^
+// | ^^^^^^^^ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// element size | padding |
+// element count array elements
+//
+// We must be careful to take into account the alignment of the array cookie, which may result in padding
+// bytes between the element count and the first element of the array. Note that for ARM, the compiler
+// aligns the array cookie using the ABI alignment, not the preferred alignment of array elements.
+//
+// [1]: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#array-cookies
+// [2]: https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Handle-C++-
diff erences
template <class _Tp>
// Avoid failures when -fsanitize-address-poison-custom-array-cookie is enabled
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_SANITIZE("address") size_t __get_array_cookie(_Tp const* __ptr) {
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_SANITIZE("address") size_t __get_array_cookie([[__maybe_unused__]] _Tp const* __ptr) {
static_assert(
__has_array_cookie<_Tp>::value, "Trying to access the array cookie of a type that is not guaranteed to have one");
- size_t const* __cookie = reinterpret_cast<size_t const*>(__ptr) - 1; // TODO: Use a builtin instead
- return *__cookie;
+
+#if defined(_LIBCPP_ABI_ITANIUM)
+ using _ArrayCookie = __itanium_array_cookie;
+#elif defined(_LIBCPP_ABI_ITANIUM_WITH_ARM_DIFFERENCES)
+ using _ArrayCookie = __arm_array_cookie<_Tp>;
+#else
+ static_assert(false, "The array cookie layout is unknown on this ABI");
+ struct _ArrayCookie { // dummy definition required to make the function parse
+ size_t element_count;
+ };
+#endif
+
+ char const* __array_cookie_start = reinterpret_cast<char const*>(__ptr) - sizeof(_ArrayCookie);
+ _ArrayCookie __cookie;
+ // This is necessary to avoid violating strict aliasing. It's valid because _ArrayCookie is an
+ // implicit lifetime type.
+ __builtin_memcpy(std::addressof(__cookie), __array_cookie_start, sizeof(_ArrayCookie));
+ return __cookie.__element_count;
}
_LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/assert.subscript.pass.cpp b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/assert.subscript.pass.cpp
index b7cc12350027b..f7390ef5eb5d2 100644
--- a/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/assert.subscript.pass.cpp
+++ b/libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.observers/assert.subscript.pass.cpp
@@ -58,15 +58,18 @@ void test() {
{
{
std::unique_ptr<WithCookie[]> ptr(new WithCookie[5]);
+ assert(&ptr[1] == ptr.get() + 1); // ensure no assertion
TEST_LIBCPP_ASSERT_FAILURE(ptr[6], "unique_ptr<T[]>::operator[](index): index out of range");
}
{
std::unique_ptr<WithCookie[]> ptr = std::make_unique<WithCookie[]>(5);
+ assert(&ptr[1] == ptr.get() + 1); // ensure no assertion
TEST_LIBCPP_ASSERT_FAILURE(ptr[6], "unique_ptr<T[]>::operator[](index): index out of range");
}
#if TEST_STD_VER >= 20
{
std::unique_ptr<WithCookie[]> ptr = std::make_unique_for_overwrite<WithCookie[]>(5);
+ assert(&ptr[1] == ptr.get() + 1); // ensure no assertion
TEST_LIBCPP_ASSERT_FAILURE(ptr[6] = WithCookie(), "unique_ptr<T[]>::operator[](index): index out of range");
}
#endif
@@ -82,11 +85,13 @@ void test() {
{
{
std::unique_ptr<NoCookie[]> ptr = std::make_unique<NoCookie[]>(5);
+ assert(&ptr[1] == ptr.get() + 1); // ensure no assertion
TEST_LIBCPP_ASSERT_FAILURE(ptr[6], "unique_ptr<T[]>::operator[](index): index out of range");
}
# if TEST_STD_VER >= 20
{
std::unique_ptr<NoCookie[]> ptr = std::make_unique_for_overwrite<NoCookie[]>(5);
+ assert(&ptr[1] == ptr.get() + 1); // ensure no assertion
TEST_LIBCPP_ASSERT_FAILURE(ptr[6] = NoCookie(), "unique_ptr<T[]>::operator[](index): index out of range");
}
# endif
@@ -101,6 +106,7 @@ void test() {
{
std::unique_ptr<T[]> ptr = std::make_unique<T[]>(5);
std::unique_ptr<T[]> other(std::move(ptr));
+ assert(&other[1] == other.get() + 1); // ensure no assertion
TEST_LIBCPP_ASSERT_FAILURE(other[6], "unique_ptr<T[]>::operator[](index): index out of range");
}
@@ -109,6 +115,7 @@ void test() {
std::unique_ptr<T[]> ptr = std::make_unique<T[]>(5);
std::unique_ptr<T[]> other;
other = std::move(ptr);
+ assert(&other[1] == other.get() + 1); // ensure no assertion
TEST_LIBCPP_ASSERT_FAILURE(other[6], "unique_ptr<T[]>::operator[](index): index out of range");
}
@@ -116,6 +123,7 @@ void test() {
{
std::unique_ptr<T[]> ptr = std::make_unique<T[]>(5);
std::unique_ptr<T[], MyDeleter> other(std::move(ptr));
+ assert(&other[1] == other.get() + 1); // ensure no assertion
TEST_LIBCPP_ASSERT_FAILURE(other[6], "unique_ptr<T[]>::operator[](index): index out of range");
}
@@ -124,6 +132,7 @@ void test() {
std::unique_ptr<T[]> ptr = std::make_unique<T[]>(5);
std::unique_ptr<T[], MyDeleter> other;
other = std::move(ptr);
+ assert(&other[1] == other.get() + 1); // ensure no assertion
TEST_LIBCPP_ASSERT_FAILURE(other[6], "unique_ptr<T[]>::operator[](index): index out of range");
}
});
@@ -144,6 +153,34 @@ struct WithCookie {
char padding[Size];
};
+template <std::size_t Size>
+struct alignas(128) OveralignedNoCookie {
+ char padding[Size];
+};
+
+template <std::size_t Size>
+struct alignas(128) OveralignedWithCookie {
+ OveralignedWithCookie() = default;
+ OveralignedWithCookie(OveralignedWithCookie const&) {}
+ OveralignedWithCookie& operator=(OveralignedWithCookie const&) { return *this; }
+ ~OveralignedWithCookie() {}
+ char padding[Size];
+};
+
+// These types have a
diff erent ABI alignment (alignof) and preferred alignment (__alignof) on some platforms.
+// Make sure things work with these types because array cookies can be sensitive to preferred alignment on some
+// platforms.
+struct WithCookiePreferredAlignment {
+ WithCookiePreferredAlignment() = default;
+ WithCookiePreferredAlignment(WithCookiePreferredAlignment const&) {}
+ WithCookiePreferredAlignment& operator=(WithCookiePreferredAlignment const&) { return *this; }
+ ~WithCookiePreferredAlignment() {}
+ long double data;
+};
+struct NoCookiePreferredAlignment {
+ long double data;
+};
+
int main(int, char**) {
test<WithCookie<1>, NoCookie<1>>();
test<WithCookie<2>, NoCookie<2>>();
@@ -153,7 +190,18 @@ int main(int, char**) {
test<WithCookie<16>, NoCookie<16>>();
test<WithCookie<32>, NoCookie<32>>();
test<WithCookie<256>, NoCookie<256>>();
+
+ test<OveralignedWithCookie<1>, OveralignedNoCookie<1>>();
+ test<OveralignedWithCookie<2>, OveralignedNoCookie<2>>();
+ test<OveralignedWithCookie<3>, OveralignedNoCookie<3>>();
+ test<OveralignedWithCookie<4>, OveralignedNoCookie<4>>();
+ test<OveralignedWithCookie<8>, OveralignedNoCookie<8>>();
+ test<OveralignedWithCookie<16>, OveralignedNoCookie<16>>();
+ test<OveralignedWithCookie<32>, OveralignedNoCookie<32>>();
+ test<OveralignedWithCookie<256>, OveralignedNoCookie<256>>();
+
test<std::string, int>();
+ test<WithCookiePreferredAlignment, NoCookiePreferredAlignment>();
return 0;
}
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