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libstdc++: Clear padding bits in atomic compare_exchange

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This change implements P0528 which requires that padding bits not
participate in atomic compare exchange operations. All arguments to the
generic template are 'sanitized' by the __builtin_clear_padding intrinsic
before they are used in comparisons. This requires that any stores
also sanitize the incoming value.

Co-authored-by: Jakub Jelinek <jakub@redhat.com>
Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

Signed-off-by: Thomas Rodgers <trodgers@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/atomic_base.h (__atomic_impl::__maybe_has_padding):
	New function.
	(__atomic_impl::clear_padding): Likewise.
	(__atomic_impl::__compare_exchange): Likewise.
	(__atomic_impl::compare_exchange_weak): Delegate to
	__compare_exchange.
	(__atomic_impl::compare_exchange_strong): Likewise.
	* include/std/atomic (atomic<T>::atomic(T)): Clear padding when
	possible in a constexpr function.
	(atomic::store): Clear padding.
	(atomic::exchange): Likewise.
	(atomic::compare_exchange_weak): Use __compare_exchange.
	(atomic::compare_exchange_strong): Likewise.
	* testsuite/29_atomics/atomic/compare_exchange_padding.cc: New
	test.
	* testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc:
	New test.
This commit is contained in:
Thomas Rodgers 2022-08-25 12:11:40 +02:00 • committed by Jonathan Wakely
parent d3883dc77b
commit 157236dbd6
4 changed files with 188 additions and 52 deletions
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97
libstdc++-v3/include/bits/atomic_base.h
View file

@ -33,6 +33,7 @@
#pragma GCC system_header
#include <bits/c++config.h>
#include <new> // For placement new
#include <stdint.h>
#include <bits/atomic_lockfree_defines.h>
#include <bits/move.h>
@ -952,19 +953,76 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ return __atomic_fetch_sub(&_M_p, _M_type_size(__d), int(__m)); }
};
/// @endcond
namespace __atomic_impl
{
// Implementation details of atomic padding handling
template<typename _Tp>
constexpr bool
__maybe_has_padding()
{
#if ! __has_builtin(__builtin_clear_padding)
return false;
#elif __has_builtin(__has_unique_object_representations)
return !__has_unique_object_representations(_Tp)
&& !is_same<_Tp, float>::value && !is_same<_Tp, double>::value;
#else
return true;
#endif
}
template<typename _Tp>
_GLIBCXX_ALWAYS_INLINE _Tp*
__clear_padding(_Tp& __val) noexcept
{
auto* __ptr = std::__addressof(__val);
#if __has_builtin(__builtin_clear_padding)
if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>())
__builtin_clear_padding(__ptr);
#endif
return __ptr;
}
// Remove volatile and create a non-deduced context for value arguments.
template<typename _Tp>
using _Val = typename remove_volatile<_Tp>::type;
template<typename _Tp>
_GLIBCXX_ALWAYS_INLINE bool
__compare_exchange(_Tp& __val, _Val<_Tp>& __e, _Val<_Tp>& __i,
bool __weak, memory_order __s, memory_order __f) noexcept
{
__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
using _Vp = _Val<_Tp>;
if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Vp>())
{
// We must not modify __e on success, so cannot clear its padding.
// Copy into a buffer and clear that, then copy back on failure.
alignas(_Vp) unsigned char __buf[sizeof(_Vp)];
_Vp* __exp = ::new((void*)__buf) _Vp(__e);
__atomic_impl::__clear_padding(*__exp);
if (__atomic_compare_exchange(std::__addressof(__val), __exp,
__atomic_impl::__clear_padding(__i),
__weak, int(__s), int(__f)))
return true;
__builtin_memcpy(std::__addressof(__e), __exp, sizeof(_Vp));
return false;
}
else
return __atomic_compare_exchange(std::__addressof(__val),
std::__addressof(__e),
std::__addressof(__i),
__weak, int(__s), int(__f));
}
} // namespace __atomic_impl
#if __cplusplus > 201703L
/// @cond undocumented
// Implementation details of atomic_ref and atomic<floating-point>.
namespace __atomic_impl
{
// Remove volatile and create a non-deduced context for value arguments.
template<typename _Tp>
using _Val = remove_volatile_t<_Tp>;
// As above, but for difference_type arguments.
// Like _Val<T> above, but for difference_type arguments.
template<typename _Tp>
using _Diff = __conditional_t<is_pointer_v<_Tp>, ptrdiff_t, _Val<_Tp>>;
@ -979,7 +1037,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Tp>
_GLIBCXX_ALWAYS_INLINE void
store(_Tp* __ptr, _Val<_Tp> __t, memory_order __m) noexcept
{ __atomic_store(__ptr, std::__addressof(__t), int(__m)); }
{
__atomic_store(__ptr, __atomic_impl::__clear_padding(__t), int(__m));
}
template<typename _Tp>
_GLIBCXX_ALWAYS_INLINE _Val<_Tp>
@ -997,7 +1057,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
auto* __dest = reinterpret_cast<_Val<_Tp>*>(__buf);
__atomic_exchange(__ptr, std::__addressof(__desired), __dest, int(__m));
__atomic_exchange(__ptr, __atomic_impl::__clear_padding(__desired),
__dest, int(__m));
return *__dest;
}
@ -1007,11 +1068,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_Val<_Tp> __desired, memory_order __success,
memory_order __failure) noexcept
{
__glibcxx_assert(__is_valid_cmpexch_failure_order(__failure));
return __atomic_compare_exchange(__ptr, std::__addressof(__expected),
std::__addressof(__desired), true,
int(__success), int(__failure));
return __atomic_impl::__compare_exchange(*__ptr, __expected, __desired,
true, __success, __failure);
}
template<typename _Tp>
@ -1020,11 +1078,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_Val<_Tp> __desired, memory_order __success,
memory_order __failure) noexcept
{
__glibcxx_assert(__is_valid_cmpexch_failure_order(__failure));
return __atomic_compare_exchange(__ptr, std::__addressof(__expected),
std::__addressof(__desired), false,
int(__success), int(__failure));
return __atomic_impl::__compare_exchange(*__ptr, __expected, __desired,
false, __success, __failure);
}
#if __cpp_lib_atomic_wait
@ -1955,9 +2010,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_Tp** _M_ptr;
};
#endif // C++2a
/// @endcond
#endif // C++2a
/// @} group atomics

58
libstdc++-v3/include/std/atomic
View file

@ -230,7 +230,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
atomic& operator=(const atomic&) = delete;
atomic& operator=(const atomic&) volatile = delete;
constexpr atomic(_Tp __i) noexcept : _M_i(__i) { }
constexpr atomic(_Tp __i) noexcept : _M_i(__i)
{
#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>())
__builtin_clear_padding(std::__addressof(_M_i));
#endif
}
operator _Tp() const noexcept
{ return load(); }
@ -270,13 +276,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
void
store(_Tp __i, memory_order __m = memory_order_seq_cst) noexcept
{
__atomic_store(std::__addressof(_M_i), std::__addressof(__i), int(__m));
__atomic_store(std::__addressof(_M_i),
__atomic_impl::__clear_padding(__i),
int(__m));
}
void
store(_Tp __i, memory_order __m = memory_order_seq_cst) volatile noexcept
{
__atomic_store(std::__addressof(_M_i), std::__addressof(__i), int(__m));
__atomic_store(std::__addressof(_M_i),
__atomic_impl::__clear_padding(__i),
int(__m));
}
_Tp
@ -302,7 +312,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
_Tp* __ptr = reinterpret_cast<_Tp*>(__buf);
__atomic_exchange(std::__addressof(_M_i), std::__addressof(__i),
__atomic_exchange(std::__addressof(_M_i),
__atomic_impl::__clear_padding(__i),
__ptr, int(__m));
return *__ptr;
}
@ -313,7 +324,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
_Tp* __ptr = reinterpret_cast<_Tp*>(__buf);
__atomic_exchange(std::__addressof(_M_i), std::__addressof(__i),
__atomic_exchange(std::__addressof(_M_i),
__atomic_impl::__clear_padding(__i),
__ptr, int(__m));
return *__ptr;
}
@ -322,24 +334,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
compare_exchange_weak(_Tp& __e, _Tp __i, memory_order __s,
memory_order __f) noexcept
{
__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
return __atomic_compare_exchange(std::__addressof(_M_i),
std::__addressof(__e),
std::__addressof(__i),
true, int(__s), int(__f));
return __atomic_impl::__compare_exchange(_M_i, __e, __i, true,
__s, __f);
}
bool
compare_exchange_weak(_Tp& __e, _Tp __i, memory_order __s,
memory_order __f) volatile noexcept
{
__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
return __atomic_compare_exchange(std::__addressof(_M_i),
std::__addressof(__e),
std::__addressof(__i),
true, int(__s), int(__f));
return __atomic_impl::__compare_exchange(_M_i, __e, __i, true,
__s, __f);
}
bool
@ -358,24 +362,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
compare_exchange_strong(_Tp& __e, _Tp __i, memory_order __s,
memory_order __f) noexcept
{
__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
return __atomic_compare_exchange(std::__addressof(_M_i),
std::__addressof(__e),
std::__addressof(__i),
false, int(__s), int(__f));
return __atomic_impl::__compare_exchange(_M_i, __e, __i, false,
__s, __f);
}
bool
compare_exchange_strong(_Tp& __e, _Tp __i, memory_order __s,
memory_order __f) volatile noexcept
{
__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
return __atomic_compare_exchange(std::__addressof(_M_i),
std::__addressof(__e),
std::__addressof(__i),
false, int(__s), int(__f));
return __atomic_impl::__compare_exchange(_M_i, __e, __i, false,
__s, __f);
}
bool
@ -390,7 +386,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ return compare_exchange_strong(__e, __i, __m,
__cmpexch_failure_order(__m)); }
#if __cpp_lib_atomic_wait
#if __cpp_lib_atomic_wait
void
wait(_Tp __old, memory_order __m = memory_order_seq_cst) const noexcept
{
@ -407,7 +403,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
void
notify_all() noexcept
{ std::__atomic_notify_address(&_M_i, true); }
#endif // __cpp_lib_atomic_wait
#endif // __cpp_lib_atomic_wait
};
#undef _GLIBCXX20_INIT

42
libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc Normal file
View file

@ -0,0 +1,42 @@
// { dg-options "-std=gnu++20" }
// { dg-do run { target c++20 } }
// { dg-add-options libatomic }
#include <atomic>
#include <testsuite_hooks.h>
struct S { char c; short s; };
void __attribute__((noinline,noipa))
fill_struct(S& s)
{ __builtin_memset(&s, 0xff, sizeof(S)); }
bool
compare_struct(const S& a, const S& b)
{ return __builtin_memcmp(&a, &b, sizeof(S)) == 0; }
int
main ()
{
S s;
fill_struct(s);
s.c = 'a';
s.s = 42;
std::atomic<S> as{ s };
auto ts = as.load();
VERIFY( !compare_struct(s, ts) ); // padding cleared on construction
as.exchange(s);
auto es = as.load();
VERIFY( compare_struct(ts, es) ); // padding cleared on exchange
S n;
fill_struct(n);
n.c = 'b';
n.s = 71;
// padding cleared on compexchg
VERIFY( as.compare_exchange_weak(s, n) );
VERIFY( as.compare_exchange_strong(n, s) );
return 0;
}

43
libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc Normal file
View file

@ -0,0 +1,43 @@
// { dg-options "-std=gnu++20" }
// { dg-do run { target c++20 } }
// { dg-add-options libatomic }
#include <atomic>
#include <testsuite_hooks.h>
struct S { char c; short s; };
void __attribute__((noinline,noipa))
fill_struct(S& s)
{ __builtin_memset(&s, 0xff, sizeof(S)); }
bool
compare_struct(const S& a, const S& b)
{ return __builtin_memcmp(&a, &b, sizeof(S)) == 0; }
int
main ()
{
S s;
fill_struct(s);
s.c = 'a';
s.s = 42;
S ss{ s };
std::atomic_ref<S> as{ s };
auto ts = as.load();
VERIFY( !compare_struct(ss, ts) ); // padding cleared on construction
as.exchange(ss);
auto es = as.load();
VERIFY( compare_struct(ts, es) ); // padding cleared on exchange
S n;
fill_struct(n);
n.c = 'b';
n.s = 71;
// padding cleared on compexchg
VERIFY( as.compare_exchange_weak(s, n) );
VERIFY( as.compare_exchange_strong(n, s) );
return 0;
}