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Avoid undefined behavior when subtracting tagged pointers #715

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merged 2 commits into from
Sep 12, 2024
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Avoid undefined behavior when subtracting tagged pointers #715

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38960e5
Avoid undefined behavior when subtracting tagged pointers
thurstond Sep 12, 2024
30b8826
Use uintptr_t, as suggested by phawkins
thurstond Sep 12, 2024
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5 changes: 4 additions & 1 deletion src/nb_type.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -124,7 +124,10 @@ PyObject *inst_new_ext(PyTypeObject *tp, void *value) {
}

// Compute offset to instance value
int32_t offset = (int32_t) ((intptr_t) value - (intptr_t) self);
// Use uint64_t because subtracting tagged pointers (e.g., with
// HardwareAddressSanitizer) may overflow, which is undefined behavior for
// signed integers.
int32_t offset = (int32_t) ((uintptr_t) value - (uintptr_t) self);

bool direct = (intptr_t) self + offset == (intptr_t) value;
if (NB_UNLIKELY(!direct)) {
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