Merge pull request #4179 from janisozaur/network-checksum-sprites

Send a checksum of all the sprites occasionally in multiplayer for better desync detection.

src/network/network.cpp

@@ -586,7 +586,10 @@ bool Network::CheckSRAND(uint32 tick, uint32 srand0)
 
 	if (tick == server_srand0_tick) {
 		server_srand0_tick = 0;
-		if (srand0 != server_srand0) {
+		// Check that the server and client sprite hashes match
+		const bool sprites_mismatch = server_sprite_hash[0] != '\0' && strcmp(sprite_checksum(), server_sprite_hash);
+		// Check PRNG values and sprite hashes, if exist
+		if ((srand0 != server_srand0) || sprites_mismatch) {
 			return false;
 		}
 	}
@@ -1081,6 +1084,22 @@ void Network::Server_Send_TICK()
 	last_tick_sent_time = SDL_GetTicks();
 	std::unique_ptr<NetworkPacket> packet = std::move(NetworkPacket::Allocate());
 	*packet << (uint32)NETWORK_COMMAND_TICK << (uint32)gCurrentTicks << (uint32)gScenarioSrand0;
+	uint32 flags = 0;
+	// Simple counter which limits how often a sprite checksum gets sent.
+	// This can get somewhat expensive, so we don't want to push it every tick in release,
+	// but debug version can check more often.
+	static int checksum_counter = 0;
+	checksum_counter++;
+	if (checksum_counter >= 100) {
+		checksum_counter = 0;
+		flags |= NETWORK_TICK_FLAG_CHECKSUMS;
+	}
+	// Send flags always, so we can understand packet structure on the other end,
+	// and allow for some expansion.
+	*packet << flags;
+	if (flags & NETWORK_TICK_FLAG_CHECKSUMS) {
+		packet->WriteString(sprite_checksum());
+	}
 	SendPacketToClients(*packet);
 }
 
@@ -1739,10 +1758,23 @@ void Network::Server_Handle_GAMECMD(NetworkConnection& connection, NetworkPacket
 void Network::Client_Handle_TICK(NetworkConnection& connection, NetworkPacket& packet)
 {
 	uint32 srand0;
-	packet >> server_tick >> srand0;
+	uint32 flags;
+	// Note: older server version may not advertise flags at all.
+	// NetworkPacket will return 0, if trying to read past end of buffer,
+	// so flags == 0 is expected in such cases.
+	packet >> server_tick >> srand0 >> flags;
 	if (server_srand0_tick == 0) {
 		server_srand0 = srand0;
 		server_srand0_tick = server_tick;
+		server_sprite_hash[0] = '\0';
+		if (flags & NETWORK_TICK_FLAG_CHECKSUMS)
+		{
+			const char* text = packet.ReadString();
+			if (text != nullptr)
+			{
+				safe_strcpy(server_sprite_hash, text, sizeof(server_sprite_hash));
+			}
+		}
 	}
 }
 

src/network/network.h

@@ -43,6 +43,7 @@ extern "C" {
 #include "../game.h"
 #include "../platform/platform.h"
 #include "../localisation/string_ids.h"
+#include <openssl/evp.h>
 #ifdef __cplusplus
 }
 #endif // __cplusplus
@@ -77,6 +78,10 @@ extern "C" {
 #include "NetworkUser.h"
 #include "TcpSocket.h"
 
+enum {
+	NETWORK_TICK_FLAG_CHECKSUMS = 1 << 0,
+};
+
 class Network
 {
 public:
@@ -191,6 +196,7 @@ private:
 	uint32 server_tick = 0;
 	uint32 server_srand0 = 0;
 	uint32 server_srand0_tick = 0;
+	char server_sprite_hash[EVP_MAX_MD_SIZE + 1];
 	uint8 player_id = 0;
 	std::list<std::unique_ptr<NetworkConnection>> client_connection_list;
 	std::multiset<GameCommand> game_command_queue;

src/openrct2.c

@@ -63,6 +63,11 @@ bool gOpenRCT2Headless = false;
 bool gOpenRCT2ShowChangelog;
 bool gOpenRCT2SilentBreakpad;
 
+#ifndef DISABLE_NETWORK
+// OpenSSL's message digest context used for calculating sprite checksums
+EVP_MD_CTX *gHashCTX = NULL;
+#endif // DISABLE_NETWORK
+
 /** If set, will end the OpenRCT2 game loop. Intentially private to this module so that the flag can not be set back to 0. */
 int _finished;
 
@@ -176,6 +181,11 @@ bool openrct2_initialise()
 {
 	utf8 userPath[MAX_PATH];
 
+#ifndef DISABLE_NETWORK
+	gHashCTX = EVP_MD_CTX_create();
+	assert(gHashCTX != NULL);
+#endif // DISABLE_NETWORK
+
 	platform_resolve_openrct_data_path();
 	platform_resolve_user_data_path();
 	platform_get_user_directory(userPath, NULL);
@@ -336,6 +346,9 @@ void openrct2_dispose()
 	language_close_all();
 	rct2_dispose();
 	config_release();
+#ifndef DISABLE_NETWORK
+	EVP_MD_CTX_destroy(gHashCTX);
+#endif // DISABLE_NETWORK
 	platform_free();
 }
 

src/openrct2.h

@@ -20,6 +20,10 @@
 #include "common.h"
 #include "platform/platform.h"
 
+#ifndef DISABLE_NETWORK
+#include <openssl/evp.h>
+#endif // DISABLE_NETWORK
+
 enum {
 	STARTUP_ACTION_INTRO,
 	STARTUP_ACTION_TITLE,
@@ -39,6 +43,10 @@ extern utf8 gCustomPassword[MAX_PATH];
 extern bool gOpenRCT2Headless;
 extern bool gOpenRCT2ShowChangelog;
 
+#ifndef DISABLE_NETWORK
+extern EVP_MD_CTX *gHashCTX;
+#endif // DISABLE_NETWORK
+
 #ifndef DISABLE_NETWORK
 extern int gNetworkStart;
 extern char gNetworkStartHost[128];

src/world/sprite.c

@@ -168,6 +168,50 @@ void game_command_reset_sprites(int* eax, int* ebx, int* ecx, int* edx, int* esi
 	*ebx = 0;
 }
 
+#ifndef DISABLE_NETWORK
+
+static unsigned char _spriteChecksum[EVP_MAX_MD_SIZE + 1];
+
+const char * sprite_checksum()
+{
+	if (EVP_DigestInit_ex(gHashCTX, EVP_sha1(), NULL) <= 0)
+	{
+		openrct2_assert(false, "Failed to initialise SHA1 engine");
+	}
+	for (size_t i = 0; i < MAX_SPRITES; i++)
+	{
+		rct_sprite *sprite = get_sprite(i);
+		if (sprite->unknown.sprite_identifier != SPRITE_IDENTIFIER_NULL)
+		{
+			if (EVP_DigestUpdate(gHashCTX, sprite, sizeof(rct_sprite)) <= 0)
+			{
+				openrct2_assert(false, "Failed to update digest");
+			}
+		}
+	}
+	unsigned char localhash[EVP_MAX_MD_SIZE + 1];
+	unsigned int size = sizeof(localhash);
+	EVP_DigestFinal(gHashCTX, localhash, &size);
+	assert(size <= sizeof(localhash));
+	localhash[sizeof(localhash) - 1] = '\0';
+	char *x = (char *)_spriteChecksum;
+	for (unsigned int i = 0; i < size; i++)
+	{
+		sprintf(x, "%02x", localhash[i]);
+		x += 2;
+	}
+	*x = '\0';
+	return (char *)_spriteChecksum;
+}
+#else
+
+const char * sprite_checksum()
+{
+	return NULL;
+}
+
+#endif // DISABLE_NETWORK
+
 /**
  * Clears all the unused sprite memory to zero. Probably so that it can be compressed better when saving.
  *  rct2: 0x0069EBA4

src/world/sprite.h

@@ -441,4 +441,6 @@ void crashed_vehicle_particle_update(rct_crashed_vehicle_particle *particle);
 void crash_splash_create(int x, int y, int z);
 void crash_splash_update(rct_crash_splash *splash);
 
+const char *sprite_checksum();
+
 #endif