117 lines
3.8 KiB
Markdown
117 lines
3.8 KiB
Markdown
# Modding
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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.
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## Hex Addresses
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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](https://ghidra-sre.org) or [RetDec](https://retdec.com). 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.
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## Loading Directories
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``minecraft-pi-reborn`` loads mods from two locations, ``/app/minecraft-pi/mods``, and ``~/.minecraft/mods``. The first location only exists in the Docker container and is immutable, so you should place your mods in ``~/.minecraft/mods`` which is mounted on the host as ``~/.minecraft-pi/mods``.
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## C++ Strings
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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``.
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## ``libreborn.so`` API
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Header files and the shared library can be download from [Jenkins](https://jenkins.thebrokenrail.com/job/minecraft-pi-reborn/job/master/lastSuccessfulBuild/artifact/out/lib).
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### ``void overwrite(void *start, void *target)``
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This method replaces a function with another function.
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#### Parameters
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- **start:** The function you are replacing.
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- **target:** The function you are replacing it with.
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#### Return Value
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None
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#### Warning
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This should never be used on functions that are only 1 byte long because it overwrites 2 bytes.
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#### Example
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```c
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static int func_injection(int a, int b) {
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return a + 4;
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}
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__attribute__((constructor)) static void init() {
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overwrite((void *) 0xabcde, func_injection);
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}
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```
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### ``void overwrite_call(void *start, void *original)``
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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.
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#### Parameters
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- **start:** The address of the function call to overwrite.
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- **target:** The function call you are replacing it with.
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#### Return Value
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None
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#### Warning
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This method can only be safely used 512 times in total.
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#### Example
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```c
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typedef int (*func_t)(int a, int b);
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static func_t func = (func_t) 0xabcde;
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static void *func_original = NULL;
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static int func_injection(int a, int b) {
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(*func)(a, b);
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return a + 4;
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}
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__attribute__((constructor)) static void init() {
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overwrite_call((void *) 0xabcd, func_injection);
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}
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```
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### ``void overwrite_calls(void *start, void *original)``
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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.
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#### Parameters
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- **start:** The function call to overwrite;
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- **target:** The function call you are replacing it with.
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#### Return Value
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None
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#### Warning
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This method can only be safely used 512 times in total.
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#### Example
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```c
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typedef int (*func_t)(int a, int b);
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static func_t func = (func_t) 0xabcde;
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static void *func_original = NULL;
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static int func_injection(int a, int b) {
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(*func)(a, b);
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return a + 4;
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}
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__attribute__((constructor)) static void init() {
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overwrite_calls((void *) func, func_injection);
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}
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```
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### ``void patch(void *start, unsigned char patch[])``
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This allows you to replace a specific instruction.
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#### Parameters
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- **start:** The target instruction.
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- **patch:** The new instruction (array length must be 4).
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#### Return Value
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None
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#### Example
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```c
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__attribute__((constructor)) static void init() {
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unsigned char patch_data[4] = {0x00, 0x00, 0x00, 0x00};
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patch((void *) 0xabcde, patch_data);
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}
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```
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