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Remove undefined behavior from JSTypedArray.GetJSValueForMemory #295

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May 15, 2024
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Remove undefined behavior from JSTypedArray.GetJSValueForMemory #295

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19 changes: 3 additions & 16 deletions src/NodeApi/JSTypedArray.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -106,26 +106,13 @@ public unsafe JSTypedArray(ReadOnlyMemory<T> data)
/// returns that JS value.
/// </summary>
/// <returns>The JS value, or null if the memory is external to JS.</returns>
private static unsafe JSValue? GetJSValueForMemory(ReadOnlyMemory<T> data)
private static JSValue? GetJSValueForMemory(ReadOnlyMemory<T> data)
{
// This assumes the owner object of a Memory struct is stored as a reference in the
// first (private) field of the struct. If the Memory internal structure ever changes
// (in a future major version of the .NET Runtime), this unsafe code could crash.
// Unfortunately there's no public API to get the Memory owner object.
void* memoryPointer = Unsafe.AsPointer(ref data);
object? memoryOwner = Unsafe.Read<object?>(memoryPointer);
if (memoryOwner is MemoryManager manager)
if (MemoryMarshal.TryGetMemoryManager(data, out MemoryManager? manager, out int index, out int length))
{
// The Memory was created from a JS TypedArray.

// Strip the high bit of the index - it has a special meaning.
void* memoryIndexPointer = (byte*)memoryPointer + Unsafe.SizeOf<object?>();
int index = Unsafe.Read<int>(memoryIndexPointer) & ~int.MinValue;

void* memoryLengthPointer = (byte*)memoryIndexPointer + Unsafe.SizeOf<int>();
int length = Unsafe.Read<int>(memoryLengthPointer);

JSValue value = manager.JSValue;
JSValue value = manager!.JSValue;
int valueLength = value.GetTypedArrayLength(out _);

if (index != 0 || length != valueLength)
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