Simple hack to disable joystick scanning and remove stutters from Papers Please.

papers_please_fix.cpp

#include <string>
#include <dlfcn.h>
#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>

using namespace std::literals;

template<class T>
constexpr T* page_round_down(T* ptr, size_t page_size) {
	size_t page = reinterpret_cast<size_t>(ptr) & ~(page_size - 1);
	return reinterpret_cast<T*>(page);
}

void patch_function(void* ptr, size_t offset) {
	char* func = reinterpret_cast<char*>(ptr);
	func += offset;
	const long page_size = sysconf(_SC_PAGESIZE);
	int result = mprotect(page_round_down(func, page_size), 1, PROT_READ | PROT_WRITE | PROT_EXEC);
	if(result) {
		fprintf(stderr, "result %d %d\n", result, errno);
		return;
	}
	func[0] = 0xc3;
	mprotect(page_round_down(func, page_size), 1, PROT_READ | PROT_EXEC);
}

using dlopen_t = void* (*)(const char*, int);

extern "C"
void* dlopen(const char* filename, int flags) {
	static dlopen_t real_dlopen = nullptr;
    if(!real_dlopen)
        real_dlopen = reinterpret_cast<dlopen_t>(dlsym(RTLD_NEXT, "dlopen"));
	void* handle = real_dlopen(filename, flags);
	if(!filename || filename != "././lime.ndll"s) {
		return handle;
	}
	char* symbol = getenv("PP_SYMBOL");
	if(!symbol) {
		fprintf(stderr, "PP_SYMBOL not set\n");
		return handle;
	}
	char* offset = getenv("PP_OFFSET");
	if(!offset) {
		fprintf(stderr, "PP_OFFSET not set\n");
		return handle;
	}
	void* func = dlsym(handle, symbol);
	if(!func) {
		fprintf(stderr, "could not find symbol: %s\n", symbol);
		return handle;
	}
	patch_function(func, atoi(offset));
	return handle;
}⏎

Very impressive analysis and fix! What arguments are you passing to gcc to compile this? Is there a Makefile?

Hi. Great and educational article. Although I am not sure if it wouldn't be easier to just create a stub for the global symbol SDL_JoystickInit.

Very impressive article, it was an amazing read.

Kudos and great write-up!

Great article! It came really handy to me as yesterday I had the same problem

@tomioe here's the commands to compile into a shared library:
g++ -g -ggdb -fPIC -rdynamic -I../static -c papers_please_fix.cpp -o papers_please_fix.o
g++ -g -ggdb -fPIC -rdynamic -shared -o papers_please_fix.so papers_please_fix.o

This was a bug that was fixed in lime. Now you can just download the latest version and replace lime.ndll. See https://www.gog.com/forum/papers_please/terrible_lags_on_linux_with_nvidia_gpu