the root node has no sibling

Author: arvidn

src/merkle_tree.cpp

@@ -384,7 +384,9 @@ namespace {
 		// already-known hash is backed by downloaded data (set_block()),
 		// and only that may be reported as passed below.
 		bool const insert_root_already_known = has_node(insert_root_idx);
+		// insert_root_idx > 0 since the root node has no sibling.
 		bool const sibling_already_known = !uncle_hashes.empty() && insert_root_idx >= first_leaf
+			&& insert_root_idx > 0
 			&& insert_root_idx - first_leaf < m_num_blocks
 			&& has_node(merkle_get_sibling(insert_root_idx));
 
@@ -398,13 +400,10 @@ namespace {
 		// successful return means anything touched is proven correct. With
 		// no uncle hashes there was no walk, so the sibling wasn't touched
 		// and nothing was learned about it.
-		if (!uncle_hashes.empty()
+		if (!uncle_hashes.empty() && insert_root_idx > 0
 			&& insert_root_idx >= first_leaf && insert_root_idx - first_leaf < m_num_blocks)
 		{
-			// insert_root_idx == 0 only happens for a single-block tree, where
-			// the root is the leaf itself and has no sibling. callers (e.g.
-			// hash_picker) are expected to reject such requests before they
-			// reach here.
+			// the root node has no sibling.
 			TORRENT_ASSERT(insert_root_idx > 0);
 			int const sibling_idx = merkle_get_sibling(insert_root_idx);
 			int const sibling_block = sibling_idx - first_leaf;

test/test_merkle_tree.cpp

@@ -1176,3 +1176,17 @@ TORRENT_TEST(single_block_tree_allocate_full_preserves_verified_bit)
 
 	TEST_CHECK(t.blocks_verified(0, 1));
 }
+
+// insert_root_idx == 0 only happens for a single-block tree, where the root
+// is the leaf itself and has no sibling. merkle_get_sibling() asserts
+// tree_node > 0, so add_hashes() must not evaluate it for this tree's lone
+// leaf, even when a peer sends a (bogus, since none can exist) non-empty
+// uncle_hashes list for it.
+TORRENT_TEST(add_hashes_single_block_tree_rejects_uncle_hashes)
+{
+	aux::merkle_tree t(1, 1, f[0].data());
+
+	std::vector<sha256_hash> const bogus_proof{rand_sha256()};
+	auto const result = t.add_hashes(0, pdiff(0), range(f, 0, 1), bogus_proof);
+	TEST_CHECK(!result);
+}