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AK+LibJS: Handle NaN-boxing pointers on AArch64
JS::Value stores 48 bit pointers to separately allocated objects in its payload. On x86-64, canonical addresses have their top 16 bits set to the same value as bit 47, effectively meaning that the value has to be sign-extended to get the pointer. AArch64, however, expects the topmost bits to be all zeros. This commit gates sign extension behind `#if ARCH(X86_64)`, and adds an `#error` for unsupported architectures, so that we do not forget to think about pointer handling when porting to a new architecture. Fixes #15290 Fixes SerenityOS/ladybird#56
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4 changed files with 42 additions and 24 deletions
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@ -398,6 +398,24 @@ public:
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template<typename... Args>
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[[nodiscard]] ALWAYS_INLINE ThrowCompletionOr<Value> invoke(VM&, PropertyKey const& property_key, Args... args);
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static constexpr FlatPtr extract_pointer_bits(u64 encoded)
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{
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#ifdef AK_ARCH_32_BIT
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// For 32-bit system the pointer fully fits so we can just return it directly.
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static_assert(sizeof(void*) == sizeof(u32));
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return static_cast<FlatPtr>(encoded & 0xffff'ffff);
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#elif ARCH(X86_64)
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// For x86_64 the top 16 bits should be sign extending the "real" top bit (47th).
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// So first shift the top 16 bits away then using the right shift it sign extends the top 16 bits.
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return static_cast<FlatPtr>((static_cast<i64>(encoded << 16)) >> 16);
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#elif ARCH(AARCH64)
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// For AArch64 the top 16 bits of the pointer should be zero.
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return static_cast<FlatPtr>(encoded & 0xffff'ffff'ffffULL);
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#else
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# error "Unknown architecture. Don't know whether pointers need to be sign-extended."
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#endif
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}
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private:
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Value(u64 tag, u64 val)
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{
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@ -444,15 +462,7 @@ private:
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PointerType* extract_pointer() const
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{
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VERIFY(is_cell());
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// For 32-bit system the pointer fully fits so we can just return it directly.
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if constexpr (sizeof(PointerType*) < sizeof(u64))
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return reinterpret_cast<PointerType*>(static_cast<u32>(m_value.encoded & 0xffffffff));
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// For x86_64 the top 16 bits should be sign extending the "real" top bit (47th).
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// So first shift the top 16 bits away then using the right shift it sign extends the top 16 bits.
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u64 ptr_val = (u64)(((i64)(m_value.encoded << 16)) >> 16);
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return reinterpret_cast<PointerType*>(ptr_val);
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return reinterpret_cast<PointerType*>(extract_pointer_bits(m_value.encoded));
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}
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[[nodiscard]] ThrowCompletionOr<Value> invoke_internal(VM&, PropertyKey const&, Optional<MarkedVector<Value>> arguments);
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