fuzz: Add fuzz test for checked and saturating add and left shift

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/test/fuzz/CMakeLists.txt

@@ -71,6 +71,7 @@ add_executable(fuzz
   netaddress.cpp
   netbase_dns_lookup.cpp
   node_eviction.cpp
+  overflow.cpp
   p2p_handshake.cpp
   p2p_headers_presync.cpp
   p2p_transport_serialization.cpp

src/test/fuzz/overflow.cpp

@@ -0,0 +1,50 @@
+// Copyright (c) 2025-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <test/fuzz/FuzzedDataProvider.h>
+#include <test/fuzz/fuzz.h>
+#include <util/check.h>
+#include <util/overflow.h>
+
+#include <algorithm>
+#include <limits>
+#include <optional>
+
+namespace {
+//! Test overflow operations for type T using a wider type, W, to verify results.
+template <typename T, typename W>
+void TestOverflow(FuzzedDataProvider& fuzzed_data_provider)
+{
+    constexpr auto min{std::numeric_limits<T>::min()};
+    constexpr auto max{std::numeric_limits<T>::max()};
+    // Range needs to be at least twice as big to allow two numbers to be added without overflowing.
+    static_assert(min >= std::numeric_limits<W>::min() / 2);
+    static_assert(max <= std::numeric_limits<W>::max() / 2);
+
+    auto widen = [](T value) -> W { return value; };
+    auto clamp = [](W value) -> W { return std::clamp<W>(value, min, max); };
+    auto check = [](W value) -> std::optional<W> { if (value >= min && value <= max) return value; else return std::nullopt; };
+
+    const T i = fuzzed_data_provider.ConsumeIntegral<T>();
+    const T j = fuzzed_data_provider.ConsumeIntegral<T>();
+    const unsigned shift = fuzzed_data_provider.ConsumeIntegralInRange<unsigned>(0, std::numeric_limits<W>::digits - std::numeric_limits<T>::digits);
+
+    Assert(clamp(widen(i) + widen(j)) == SaturatingAdd(i, j));
+    Assert(check(widen(i) + widen(j)) == CheckedAdd(i, j));
+
+    Assert(clamp(widen(i) << shift) == SaturatingLeftShift(i, shift));
+    Assert(check(widen(i) << shift) == CheckedLeftShift(i, shift));
+}
+} // namespace
+
+FUZZ_TARGET(overflow)
+{
+    FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
+    TestOverflow<int8_t, int64_t>(fuzzed_data_provider);
+    TestOverflow<int16_t, int64_t>(fuzzed_data_provider);
+    TestOverflow<int32_t, int64_t>(fuzzed_data_provider);
+    TestOverflow<uint8_t, uint64_t>(fuzzed_data_provider);
+    TestOverflow<uint16_t, uint64_t>(fuzzed_data_provider);
+    TestOverflow<uint32_t, uint64_t>(fuzzed_data_provider);
+}