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QAtomicScopedValueRollback: fix UB (passing rel/acq_rel to std::atomic::load())

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It's explicitly undefined behavior to pass release/acq_rel
memory_order to load(), so don't.

This is private API, so no ChangeLog needed.

Reported-by: Fabian Kosmale <fabian.kosmale@qt.io>
Task-number: QTBUG-115107
Pick-to: 6.5
Change-Id: Iee119303d790c31937238ef92d900a25020e9713
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 5797f29e8c249e13da6285ae9b2d50fff4ffb505)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
This commit is contained in:
Marc Mutz 2023-09-29 15:13:00 +02:00 committed by Qt Cherry-pick Bot
parent f0286b60ba
commit 401950a76f
1 changed files with 20 additions and 2 deletions
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22
src/corelib/tools/qatomicscopedvaluerollback_p.h
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@ -45,6 +45,24 @@ class QAtomicScopedValueRollback
#if !defined(Q_CC_GNU_ONLY) || (Q_CC_GNU >= 900)
// NOLINTNEXTLINE(qt-use-unreachable-return): Triggers on Clang, breaking GCC 8
Q_UNREACHABLE();
#endif
return std::memory_order_seq_cst;
}
static constexpr std::memory_order load_part(std::memory_order mo) noexcept
{
switch (mo) {
case std::memory_order_relaxed:
case std::memory_order_release: return std::memory_order_relaxed;
case std::memory_order_consume: return std::memory_order_consume;
case std::memory_order_acquire:
case std::memory_order_acq_rel: return std::memory_order_acquire;
case std::memory_order_seq_cst: return std::memory_order_seq_cst;
}
// GCC 8.x does not treat __builtin_unreachable() as constexpr
#if !defined(Q_CC_GNU_ONLY) || (Q_CC_GNU >= 900)
// NOLINTNEXTLINE(qt-use-unreachable-return): Triggers on Clang, breaking GCC 8
Q_UNREACHABLE();
#endif
return std::memory_order_seq_cst;
}
@ -56,7 +74,7 @@ public:
explicit constexpr
QAtomicScopedValueRollback(std::atomic<T> &var,
std::memory_order mo = std::memory_order_seq_cst)
: m_atomic(var), m_value(var.load(mo)), m_mo(mo) {}
: m_atomic(var), m_value(var.load(load_part(mo))), m_mo(mo) {}
Q_NODISCARD_CTOR
explicit constexpr
@ -104,7 +122,7 @@ public:
constexpr void commit()
{
m_value = m_atomic.load(m_mo);
m_value = m_atomic.load(load_part(m_mo));
}
};