Fix applications that use JIT on Apple silicon but don't know about pthread_jit_write_protect_np

fixjit.c

// The usual: compile with clang libfixjit.c -arch arm64 -arch arm64e -shared -o libfixjit.dylib, add to DYLD_INSERT_LIBRARIES.

#include <errno.h>
#include <pthread.h>
#include <stdatomic.h>

__attribute__((constructor)) static void fix_jit() {
	unsigned long long mask;
	__asm__ volatile("mrs %0, s3_4_c15_c2_7" : "=r"(mask): :);
	__asm__ volatile("msr s3_4_c15_c2_7, %0" : : "r"(mask & 0xfffffffff0ffffff) :);
}

typedef struct {
	void *(*function)();
	void *argument;
} pthread_context;

static pthread_context context_arena[4096 /* ought to be enough for anybody */];
static atomic_size_t arena_top = sizeof(context_arena) / sizeof(*context_arena);

static void *fix_jit_trampoline(void *argument) {
	fix_jit();
	pthread_context *context = (pthread_context *)argument;
	return context->function(context->argument);
}

int overriden_pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void *), void *arg) {
	size_t context;
	if ((context = --arena_top)) {
		context_arena[context] = (pthread_context) { start_routine, arg };
		return pthread_create(thread, attr, fix_jit_trampoline, context_arena + context);
	} else {
		return EAGAIN;
	}
}


__attribute__((used, section("__DATA,__interpose"))) static struct {
	int (*overriden_pthread_create)();
	int (*pthread_create)();
} pthread_create_interpose = {overriden_pthread_create, pthread_create};