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Amaury Chamayou (achamayou) wants to merge 2 commits into from
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Reject unrepresentable deserialised trees #82

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5778593
Make tree deserialisation exception-safe
achamayou Aug 24, 2026
2deaf52
Reject deserialised trees without retained leaves
achamayou Aug 24, 2026
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37 changes: 25 additions & 12 deletions merklecpp.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1557,6 +1557,11 @@ namespace merkle
const size_t deserialised_num_flushed =
deserialise_size_t(bytes, position);

if (num_leaf_nodes == 0 && deserialised_num_flushed != 0)
{
throw std::runtime_error("serialised tree has no retained leaves");
}

// A binary tree has 2 * leaves - 1 nodes, which must fit in Node::size.
constexpr size_t max_num_leaves =
std::numeric_limits<size_t>::max() / 2 + 1;
Expand All @@ -1579,17 +1584,19 @@ namespace merkle
throw std::runtime_error("not enough bytes");
}

num_flushed = deserialised_num_flushed;
leaf_nodes.reserve(num_leaf_nodes);
std::vector<Node*> deserialised_leaf_nodes;
deserialised_leaf_nodes.reserve(num_leaf_nodes);
std::vector<std::unique_ptr<Node>> level;
level.reserve(num_leaf_nodes);
for (size_t i = 0; i < num_leaf_nodes; i++)
{
Node* n = Node::make(Hash(bytes, position));
leaf_nodes.push_back(n);
auto n = std::unique_ptr<Node>(Node::make(Hash(bytes, position)));
deserialised_leaf_nodes.push_back(n.get());
level.push_back(std::move(n));
}

std::vector<Node*> level = leaf_nodes;
std::vector<Node*> next_level;
size_t it = num_flushed;
std::vector<std::unique_ptr<Node>> next_level;
size_t it = deserialised_num_flushed;
uint8_t level_no = 0;
while (it != 0 || level.size() > 1)
{
Expand All @@ -1598,11 +1605,11 @@ namespace merkle
{
Hash h(bytes, position);
MERKLECPP_TRACE(MERKLECPP_TOUT << "+";);
auto n = Node::make(h);
auto n = std::unique_ptr<Node>(Node::make(std::move(h)));
n->height = level_no + 1;
n->size = Node::full_size(n->height);
assert(n->invariant());
level.insert(level.begin(), n);
level.insert(level.begin(), std::move(n));
}

MERKLECPP_TRACE(
Expand All @@ -1615,11 +1622,15 @@ namespace merkle
{
if (i + 1 >= level.size())
{
next_level.push_back(level.at(i));
next_level.push_back(std::move(level.at(i)));
}
else
{
next_level.push_back(Node::make(level.at(i), level.at(i + 1)));
auto parent = std::unique_ptr<Node>(
Node::make(level.at(i).get(), level.at(i + 1).get()));
level.at(i).release();
level.at(i + 1).release();
next_level.push_back(std::move(parent));
}
}

Expand All @@ -1634,9 +1645,11 @@ namespace merkle

if (level.size() == 1)
{
_root = level.at(0);
_root = level.at(0).release();
assert(_root->invariant());
}
leaf_nodes = std::move(deserialised_leaf_nodes);
num_flushed = deserialised_num_flushed;
}

/// @brief Operator to serialise the tree
Expand Down
20 changes: 20 additions & 0 deletions test/unit_tests.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -296,6 +296,16 @@ TEST_CASE("TreeT rejects invalid serialised leaf data")
"not enough bytes",
std::runtime_error);

std::vector<uint8_t> no_retained_leaves;
merkle::serialise_uint64_t(0, no_retained_leaves);
merkle::serialise_uint64_t(1, no_retained_leaves);
no_retained_leaves.resize(
no_retained_leaves.size() + merkle::Hash::size_bytes);
REQUIRE_THROWS_WITH_AS(
(void)merkle::Tree(no_retained_leaves),
"serialised tree has no retained leaves",
std::runtime_error);

std::vector<uint8_t> overflowing_leaf_count;
merkle::serialise_uint64_t(1, overflowing_leaf_count);
merkle::serialise_uint64_t(
Expand Down Expand Up @@ -325,6 +335,16 @@ TEST_CASE("TreeT rejects invalid serialised leaf data")
(void)merkle::Tree(truncated_extra_hash),
"not enough bytes",
std::runtime_error);

std::vector<uint8_t> truncated_flushed_hashes;
merkle::serialise_uint64_t(1, truncated_flushed_hashes);
merkle::serialise_uint64_t(3, truncated_flushed_hashes);
truncated_flushed_hashes.resize(
truncated_flushed_hashes.size() + 2 * merkle::Hash::size_bytes);
REQUIRE_THROWS_WITH_AS(
(void)merkle::Tree(truncated_flushed_hashes),
"not enough bytes",
std::runtime_error);
}

TEST_CASE("TreeT deserialises flushed counts beyond signed shift width")
Expand Down
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