fuzz: Abort when global PRNG is used before SeedRand::ZEROS

Author: MarcoFalke

src/test/fuzz/fuzz.cpp

@@ -79,7 +79,7 @@ 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)
+static void test_one_input(FuzzBufferType buffer)
 {
     CheckGlobals check{};
     (*Assert(g_test_one_input))(buffer);
@@ -108,12 +108,12 @@ void ResetCoverageCounters() {}
 #endif
 
 
-void initialize()
+static void initialize()
 {
     // By default, make the RNG deterministic with a fixed seed. This will affect all
     // randomness during the fuzz test, except:
     // - GetStrongRandBytes(), which is used for the creation of private key material.
-    // - Creating a BasicTestingSetup or derived class will switch to a random seed.
+    // - Randomness obtained before this call in g_rng_temp_path_init
     SeedRandomStateForTest(SeedRand::ZEROS);
 
     // Terminate immediately if a fuzzing harness ever tries to create a socket.

src/test/util/random.cpp

@@ -1,4 +1,4 @@
-// Copyright (c) 2023 The Bitcoin Core developers
+// Copyright (c) 2023-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.
 
@@ -24,7 +24,8 @@ void SeedRandomStateForTest(SeedRand seedtype)
     // MakeRandDeterministicDANGEROUS is called, the output of GetRandHash is
     // no longer truly random. It should be enough to get the seed once for the
     // process.
-    static const uint256 ctx_seed = []() {
+    static const auto g_ctx_seed = []() -> std::optional<uint256> {
+        if constexpr (G_FUZZING) return {};
         // If RANDOM_CTX_SEED is set, use that as seed.
         if (const char* num{std::getenv(RANDOM_CTX_SEED)}) {
             if (auto num_parsed{uint256::FromUserHex(num)}) {
@@ -41,8 +42,9 @@ void SeedRandomStateForTest(SeedRand seedtype)
     g_seeded_g_prng_zero = seedtype == SeedRand::ZEROS;
     if constexpr (G_FUZZING) {
         Assert(g_seeded_g_prng_zero); // Only SeedRandomStateForTest(SeedRand::ZEROS) is allowed in fuzz tests
+        Assert(!g_used_g_prng);       // The global PRNG must not have been used before SeedRandomStateForTest(SeedRand::ZEROS)
     }
-    const uint256& seed{seedtype == SeedRand::FIXED_SEED ? ctx_seed : uint256::ZERO};
+    const uint256& seed{seedtype == SeedRand::FIXED_SEED ? g_ctx_seed.value() : uint256::ZERO};
     LogInfo("Setting random seed for current tests to %s=%s\n", RANDOM_CTX_SEED, seed.GetHex());
     MakeRandDeterministicDANGEROUS(seed);
 }

src/test/util/setup_common.cpp

@@ -79,7 +79,9 @@ constexpr inline auto TEST_DIR_PATH_ELEMENT{"test_common bitcoin"}; // Includes
 static FastRandomContext g_rng_temp_path;
 static const bool g_rng_temp_path_init{[] {
     // Must be initialized before any SeedRandomForTest
+    Assert(!g_used_g_prng);
     (void)g_rng_temp_path.rand64();
+    g_used_g_prng = false;
     return true;
 }()};