minecraft-pi-reborn/docs/MODDING.md

Modding

Modding Minecraft: Pi Edition is possible by patching the binary at runtime. To make this easier minecraft-pi-reborn includes a library called libreborn.so which provides several functions to help you patch the game.

Hex Addresses

Minecraft: Pi Edition has no symbols so you must patch the hex address of an instruction instead of using a function name. Hex addresses can be found using tools like Ghidra or RetDec. To find out what a function does, you can find its equivalent in Minecraft: Pocket Edition 0.6.1 and use its name for reference because Minecraft: Pocket Edition 0.6.1 includes symbols.

Loading Directories

minecraft-pi-reborn loads mods from two locations, /app/minecraft-pi/mods, and ~/.minecraft-pi/mods. The first location only exists in the Docker container and is immutable, so you should place your mods in ~/.minecraft-pi/mods which is mounted on the host.

C++ Strings

Minecraft: Pi Edition was compiled with an old version of GCC, so when interacting with C++ strings, make sure you set -D_GLIBCXX_USE_CXX11_ABI=0.

libreborn.so API

Header files and the shared library can be download from Jenkins.

void overwrite(void *start, void *target)

This method replaces a function with another function.

Parameters

• start: The function you are replacing.
• target: The function you are replacing it with.

Return Value

None

Warning

This should never be used on functions that are only 1 byte long because it overwrites 2 bytes.

Example

static int func_injection(int a, int b) {
    return a + 4;
}

__attribute__((constructor)) static void init() {
    overwrite((void *) 0xabcde, func_injection);
}

void overwrite_call(void *start, void *original)

This allows you to overwrite a specific call of a function rather than the function itself. This allows you to call the original function. However, this does not effect VTables.

Parameters

• start: The address of the function call to overwrite.
• target: The function call you are replacing it with.

Return Value

None

Warning

This method can only be safely used 512 times in total.

Example

typedef int (*func_t)(int a, int b);
static func_t func = (func_t) 0xabcde;
static void *func_original = NULL;

static int func_injection(int a, int b) {
    (*func)(a, b);

    return a + 4;
}

__attribute__((constructor)) static void init() {
    overwrite_call((void *) 0xabcd, func_injection);
}

void overwrite_calls(void *start, void *original)

This allows you to overwrite all calls of a function rather than the function itself. This allows you to call the original function. However, this does not effect VTables.

Parameters

• start: The function call to overwrite;
• target: The function call you are replacing it with.

Return Value

None

Warning

This method can only be safely used 512 times in total.

Example

typedef int (*func_t)(int a, int b);
static func_t func = (func_t) 0xabcde;
static void *func_original = NULL;

static int func_injection(int a, int b) {
    (*func)(a, b);

    return a + 4;
}

__attribute__((constructor)) static void init() {
    overwrite_calls((void *) func, func_injection);
}

void patch(void *start, unsigned char patch[])

This allows you to replace a specific instruction.

Parameters

• start: The target instruction.
• patch: The new instruction (array length must be 4).

Return Value

None

Example

__attribute__((constructor)) static void init() {
    unsigned char patch_data[4] = {0x00, 0x00, 0x00, 0x00};
    patch((void *) 0xabcde, patch_data);
}