Merge bitcoin/bitcoin#28832: fuzz: rule-out too deep derivation paths in descriptor parsing targets

a44808fb437864878c2d9696b8a96193091446ee fuzz: rule-out too deep derivation paths in descriptor parsing targets (Antoine Poinsot)

Pull request description:

  This fixes the `mocked_descriptor_parse` timeout reported in #28812 and direct the targets more toward what they are intended to fuzz: the descriptor syntax.

ACKs for top commit:
  sipa:
    utACK a44808fb437864878c2d9696b8a96193091446ee
  achow101:
    ACK a44808fb437864878c2d9696b8a96193091446ee
  dergoegge:
    ACK a44808fb437864878c2d9696b8a96193091446ee - Not running into timeouts anymore
  TheCharlatan:
    ACK a44808fb437864878c2d9696b8a96193091446ee

Tree-SHA512: a5dd1dbe9adf8f088bdc435addab88b56f435e6d7d2065bd6d5c6d80a32e3f1f97d3d2323131ab233618cd6dcc477c458abe3c4c865ab569449b8bc176231e93

1 files changed, 12 insertions, 0 deletions

diff --git a/src/wallet/test/fuzz/scriptpubkeyman.cpp b/src/wallet/test/fuzz/scriptpubkeyman.cpp
index b0c955f482..228e9629ed 100644
--- a/src/wallet/test/fuzz/scriptpubkeyman.cpp
+++ b/src/wallet/test/fuzz/scriptpubkeyman.cpp
@@ -49,9 +49,21 @@ void initialize_spkm()
     MOCKED_DESC_CONVERTER.Init();
 }
 
+/**
+ * Key derivation is expensive. Deriving deep derivation paths take a lot of compute and we'd rather spend time
+ * elsewhere in this target, like on actually fuzzing the DescriptorScriptPubKeyMan. So rule out strings which could
+ * correspond to a descriptor containing a too large derivation path.
+ */
+static bool TooDeepDerivPath(std::string_view desc)
+{
+    const FuzzBufferType desc_buf{reinterpret_cast<const unsigned char *>(desc.data()), desc.size()};
+    return HasDeepDerivPath(desc_buf);
+}
+
 static std::optional<std::pair<WalletDescriptor, FlatSigningProvider>> CreateWalletDescriptor(FuzzedDataProvider& fuzzed_data_provider)
 {
     const std::string mocked_descriptor{fuzzed_data_provider.ConsumeRandomLengthString()};
+    if (TooDeepDerivPath(mocked_descriptor)) return {};
     const auto desc_str{MOCKED_DESC_CONVERTER.GetDescriptor(mocked_descriptor)};
     if (!desc_str.has_value()) return std::nullopt;