fuzz: Abort when using global PRNG without re-seed

Author: MarcoFalke

src/random.cpp

@@ -671,9 +671,11 @@ void MakeRandDeterministicDANGEROUS(const uint256& seed) noexcept
 {
     GetRNGState().MakeDeterministic(seed);
 }
+std::atomic<bool> g_used_g_prng{false}; // Only accessed from tests
 
 void GetRandBytes(Span<unsigned char> bytes) noexcept
 {
+    g_used_g_prng = true;
     ProcRand(bytes.data(), bytes.size(), RNGLevel::FAST, /*always_use_real_rng=*/false);
 }
 

src/test/fuzz/fuzz.cpp

@@ -6,6 +6,7 @@
 
 #include <netaddress.h>
 #include <netbase.h>
+#include <test/fuzz/util/check_globals.h>
 #include <test/util/random.h>
 #include <test/util/setup_common.h>
 #include <util/check.h>
@@ -78,6 +79,12 @@ void FuzzFrameworkRegisterTarget(std::string_view name, TypeTestOneInput target,
 static std::string_view g_fuzz_target;
 static const TypeTestOneInput* g_test_one_input{nullptr};
 
+inline void test_one_input(FuzzBufferType buffer)
+{
+    CheckGlobals check{};
+    (*Assert(g_test_one_input))(buffer);
+}
+
 const std::function<std::string()> G_TEST_GET_FULL_NAME{[]{
     return std::string{g_fuzz_target};
 }};
@@ -210,7 +217,6 @@ void signal_handler(int signal)
 // This function is used by libFuzzer
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
-    static const auto& test_one_input = *Assert(g_test_one_input);
     test_one_input({data, size});
     return 0;
 }
@@ -227,7 +233,6 @@ extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv)
 int main(int argc, char** argv)
 {
     initialize();
-    static const auto& test_one_input = *Assert(g_test_one_input);
 #ifdef __AFL_LOOP
     // Enable AFL persistent mode. Requires compilation using afl-clang-fast++.
     // See fuzzing.md for details.

src/test/fuzz/util/CMakeLists.txt

@@ -3,6 +3,7 @@
 # file COPYING or https://opensource.org/license/mit/.
 
 add_library(test_fuzz STATIC EXCLUDE_FROM_ALL
+  check_globals.cpp
   descriptor.cpp
   mempool.cpp
   net.cpp

src/test/fuzz/util/check_globals.cpp

@@ -0,0 +1,41 @@
+// Copyright (c) 2024-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/util/check_globals.h>
+
+#include <test/util/random.h>
+
+#include <iostream>
+#include <memory>
+#include <optional>
+#include <string>
+
+struct CheckGlobalsImpl {
+    CheckGlobalsImpl()
+    {
+        g_used_g_prng = false;
+        g_seeded_g_prng_zero = false;
+    }
+    ~CheckGlobalsImpl()
+    {
+        if (g_used_g_prng && !g_seeded_g_prng_zero) {
+            std::cerr << "\n\n"
+                         "The current fuzz target used the global random state.\n\n"
+
+                         "This is acceptable, but requires the fuzz target to call \n"
+                         "SeedRandomStateForTest(SeedRand::ZEROS) at the beginning \n"
+                         "of processing the fuzz input.\n\n"
+
+                         "An alternative solution would be to avoid any use of globals.\n\n"
+
+                         "Without a solution, fuzz stability and determinism can lead \n"
+                         "to non-reproducible bugs or inefficient fuzzing.\n\n"
+                      << std::endl;
+            std::abort(); // Abort, because AFL may try to recover from a std::exit
+        }
+    }
+};
+
+CheckGlobals::CheckGlobals() : m_impl(std::make_unique<CheckGlobalsImpl>()) {}
+CheckGlobals::~CheckGlobals() = default;

src/test/fuzz/util/check_globals.h

@@ -0,0 +1,19 @@
+// Copyright (c) 2024-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.
+
+#ifndef BITCOIN_TEST_FUZZ_UTIL_CHECK_GLOBALS_H
+#define BITCOIN_TEST_FUZZ_UTIL_CHECK_GLOBALS_H
+
+#include <memory>
+#include <optional>
+#include <string>
+
+struct CheckGlobalsImpl;
+struct CheckGlobals {
+    CheckGlobals();
+    ~CheckGlobals();
+    std::unique_ptr<CheckGlobalsImpl> m_impl;
+};
+
+#endif // BITCOIN_TEST_FUZZ_UTIL_CHECK_GLOBALS_H

src/test/util/random.cpp

@@ -12,6 +12,8 @@
 #include <cstdlib>
 #include <iostream>
 
+std::atomic<bool> g_seeded_g_prng_zero{false};
+
 extern void MakeRandDeterministicDANGEROUS(const uint256& seed) noexcept;
 
 void SeedRandomStateForTest(SeedRand seedtype)
@@ -36,6 +38,7 @@ void SeedRandomStateForTest(SeedRand seedtype)
         return GetRandHash();
     }();
 
+    g_seeded_g_prng_zero = seedtype == SeedRand::ZEROS;
     const uint256& seed{seedtype == SeedRand::FIXED_SEED ? ctx_seed : uint256::ZERO};
     LogInfo("Setting random seed for current tests to %s=%s\n", RANDOM_CTX_SEED, seed.GetHex());
     MakeRandDeterministicDANGEROUS(seed);

src/test/util/random.h

@@ -9,6 +9,7 @@
 #include <random.h>
 #include <uint256.h>
 
+#include <atomic>
 #include <cstdint>
 
 enum class SeedRand {
@@ -27,6 +28,9 @@ enum class SeedRand {
 /** Seed the global RNG state for testing and log the seed value. This affects all randomness, except GetStrongRandBytes(). */
 void SeedRandomStateForTest(SeedRand seed);
 
+extern std::atomic<bool> g_seeded_g_prng_zero;
+extern std::atomic<bool> g_used_g_prng;
+
 template <RandomNumberGenerator Rng>
 inline CAmount RandMoney(Rng&& rng)
 {