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[libc++] Upstream ptrauth support in libc++ and libc++abi #84573

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Apr 3, 2024
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[libc++] Upstream ptrauth support in libc++ and libc++abi #84573

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14 changes: 13 additions & 1 deletion libcxx/include/typeinfo
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Original file line number Diff line number Diff line change
Expand Up @@ -276,7 +276,19 @@ struct __type_info_implementations {
__impl;
};

class _LIBCPP_EXPORTED_FROM_ABI type_info {
# if defined(__arm64__) && __has_cpp_attribute(clang::ptrauth_vtable_pointer)
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This seems like configuration that might be better suited for a config header?

type_info is weird magic though, so I understand if you disagree.

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Separately, to support ELF platforms that don't define __arm64__ we ended up rewriting the feature check as:

+#if __has_feature(ptrauth_calls) &&                          \
+    (__has_feature(ptrauth_vtable_address_discrimination) || \
+     __has_feature(ptrauth_vtable_type_discrimination)) &&   \
+    __has_cpp_attribute(clang::ptrauth_vtable_pointer)

# if __has_feature(ptrauth_type_info_discriminated_vtable_pointer)
# define _LIBCPP_TYPE_INFO_VTABLE_POINTER_AUTH \
[[clang::ptrauth_vtable_pointer(process_independent, address_discrimination, type_discrimination)]]
# else
# define _LIBCPP_TYPE_INFO_VTABLE_POINTER_AUTH \
[[clang::ptrauth_vtable_pointer(process_independent, no_address_discrimination, no_extra_discrimination)]]
# endif
# else
# define _LIBCPP_TYPE_INFO_VTABLE_POINTER_AUTH
# endif

class _LIBCPP_EXPORTED_FROM_ABI _LIBCPP_TYPE_INFO_VTABLE_POINTER_AUTH type_info {
type_info& operator=(const type_info&);
type_info(const type_info&);

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12 changes: 12 additions & 0 deletions libcxx/src/include/overridable_function.h
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Expand Up @@ -13,6 +13,10 @@
#include <__config>
#include <cstdint>

#if defined(__arm64e__) && __has_feature(ptrauth_calls)
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# include <ptrauth.h>
#endif

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif
Expand Down Expand Up @@ -81,6 +85,14 @@ _LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) no
uintptr_t __end = reinterpret_cast<uintptr_t>(&__lcxx_override_end);
uintptr_t __ptr = reinterpret_cast<uintptr_t>(__fptr);

#if defined(__arm64e__) && __has_feature(ptrauth_calls)
// We must pass a void* to ptrauth_strip since it only accepts a pointer type. Also, in particular,
// we must NOT pass a function pointer, otherwise we will strip the function pointer, and then attempt
// to authenticate and re-sign it when casting it to a uintptr_t again, which will fail because we just
// stripped the function pointer. See rdar://122927845.
__ptr = reinterpret_cast<uintptr_t>(ptrauth_strip(reinterpret_cast<void*>(__ptr), ptrauth_key_function_pointer));
#endif

// Finally, the function was overridden if it falls outside of the section's bounds.
return __ptr < __start || __ptr > __end;
}
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19 changes: 19 additions & 0 deletions libcxxabi/src/private_typeinfo.cpp
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Expand Up @@ -51,6 +51,21 @@
#include <atomic>
#endif

#if __has_feature(ptrauth_calls)
#include <ptrauth.h>
#endif

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template<typename T>
static inline
T *
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get_vtable(T *vtable) {
#if __has_feature(ptrauth_calls)
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Could you please give this a more descriptive name than get_vtable, because we already have the vtable in most cases when we call this so it's a touch confusing.

Maybe a name that ties it to ptrauth?

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Hmmm, it may be possible for __builtin_get_vtable() to be used as instead, but I'm not sure what the considerations were when this code was written (or if it predates that builtin?)

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The dynamic_cast machinery had to strip but __builtin_get_vtable always authenticates, so as-is this probably needs to be a manual ptrauth_strip yeah

vtable = ptrauth_strip(vtable, ptrauth_key_cxx_vtable_pointer);
#endif
return vtable;
}

static inline
bool
is_equal(const std::type_info* x, const std::type_info* y, bool use_strcmp)
Expand Down Expand Up @@ -103,6 +118,7 @@ void dyn_cast_get_derived_info(derived_object_info* info, const void* static_ptr
info->dynamic_type = *(reinterpret_cast<const __class_type_info* const*>(ptr_to_ti_proxy));
#else
void **vtable = *static_cast<void ** const *>(static_ptr);
vtable = get_vtable(vtable);
info->offset_to_derived = reinterpret_cast<ptrdiff_t>(vtable[-2]);
info->dynamic_ptr = static_cast<const char*>(static_ptr) + info->offset_to_derived;
info->dynamic_type = static_cast<const __class_type_info*>(vtable[-1]);
Expand Down Expand Up @@ -561,6 +577,7 @@ __base_class_type_info::has_unambiguous_public_base(__dynamic_cast_info* info,
offset_to_base = __offset_flags >> __offset_shift;
if (is_virtual) {
const char* vtable = *static_cast<const char* const*>(adjustedPtr);
vtable = get_vtable(vtable);
offset_to_base = update_offset_to_base(vtable, offset_to_base);
}
} else if (!is_virtual) {
Expand Down Expand Up @@ -1501,6 +1518,7 @@ __base_class_type_info::search_above_dst(__dynamic_cast_info* info,
if (__offset_flags & __virtual_mask)
{
const char* vtable = *static_cast<const char*const*>(current_ptr);
vtable = get_vtable(vtable);
offset_to_base = update_offset_to_base(vtable, offset_to_base);
}
__base_type->search_above_dst(info, dst_ptr,
Expand All @@ -1521,6 +1539,7 @@ __base_class_type_info::search_below_dst(__dynamic_cast_info* info,
if (__offset_flags & __virtual_mask)
{
const char* vtable = *static_cast<const char*const*>(current_ptr);
vtable = get_vtable(vtable);
offset_to_base = update_offset_to_base(vtable, offset_to_base);
}
__base_type->search_below_dst(info,
Expand Down
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