Compute the attribution pc with integer arithmetic

ddprof-lib/src/main/cpp/stackWalker.inline.h

@@ -60,8 +60,14 @@ inline void fillFrame(ASGCT_CallFrame& frame, FrameTypeId type, int bci, jmethod
 // zero-size-symbol cases, not zero-gap adjacency (see
 // BinarySearchPicksNextSymbolAtZeroGapBoundary), which is why the adjustment
 // is needed.
+//
+// The adjustment is done on uintptr_t rather than on a char pointer: pc can be
+// garbage read from a return-address slot during an optimistic unwind (UBSan
+// has caught both nullptr and 0x1 here), and offsetting a pointer to or from
+// null is undefined behavior, while unsigned integer wrap-around is not. Such
+// an address still matches no library and no FDE either way.
 inline const void* attributionPC(const void* pc, bool pc_is_return_address) {
-    return pc_is_return_address ? (const void*)((const char*)pc - 1) : pc;
+    return pc_is_return_address ? (const void*)((uintptr_t)pc - 1) : pc;
 }
 
 // The walking pc and "was it loaded from a return-address slot?" are one

ddprof-lib/src/test/cpp/returnAddressAttribution_ut.cpp

@@ -90,6 +90,24 @@ TEST(ReturnAddressAttributionCharacterizationTest, AttributionPcArithmetic) {
     EXPECT_EQ((const void*)((const char*)p - 1), attributionPC(p, true));
 }
 
+// An optimistic unwind can read garbage such as 0 or 1 from a return-address
+// slot and hand it to attributionPC(). Offsetting a pointer to or from null is
+// undefined behavior, which the asan config's -fsanitize=pointer-overflow with
+// -fno-sanitize-recover=all turns into an abort; outside sanitizer builds this
+// pins the wrapped values. The inputs go through volatile so the arithmetic
+// runs at run time where UBSan instruments it.
+TEST(ReturnAddressAttributionCharacterizationTest, AttributionPcOfGarbageNearNullIsDefined) {
+    volatile uintptr_t zero = 0;
+    volatile uintptr_t one = 1;
+    const void* null_pc = (const void*)zero;
+    const void* one_pc = (const void*)one;
+
+    EXPECT_EQ(null_pc, attributionPC(null_pc, false));
+    EXPECT_EQ(one_pc, attributionPC(one_pc, false));
+    EXPECT_EQ((const void*)UINTPTR_MAX, attributionPC(null_pc, true));
+    EXPECT_EQ(nullptr, attributionPC(one_pc, true));
+}
+
 // Supporting characterization (not gating): pins findFrameDesc's and
 // binarySearch's row-selection semantics directly, independent of the live
 // walker. These do NOT change with the fix -- they document why Test 3's