[libunwind] Undefined behaviour pointer arithmetic with null pointer

[DanielKristofKiss]

Fixes #91144

[llvmbot]

@llvm/pr-subscribers-libunwind

Author: Daniel Kiss (DanielKristofKiss)

Changes

Fixes #91144

---

Full diff: https://github.com/llvm/llvm-project/pull/98648.diff

1 Files Affected:

• (modified) libunwind/src/UnwindCursor.hpp (+2-2)

diff --git a/libunwind/src/UnwindCursor.hpp b/libunwind/src/UnwindCursor.hpp
index 2ec60e4c123d5..1fa95b4ebee19 100644
--- a/libunwind/src/UnwindCursor.hpp
+++ b/libunwind/src/UnwindCursor.hpp
@@ -230,8 +230,8 @@ void DwarfFDECache<A>::iterateCacheEntries(void (*func)(
 }
 #endif // defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
 
-
-#define arrayoffsetof(type, index, field) ((size_t)(&((type *)0)[index].field))
+#define arrayoffsetof(type, index, field)                                      \
+  (sizeof(type) * (index) + offsetof(type, field))
 
 #if defined(_LIBUNWIND_SUPPORT_COMPACT_UNWIND)
 template <typename A> class UnwindSectionHeader {

[arichardson]

This looks correct to me.