Make Music.app run in iOS Simulator

sim_dylib_loader.m

//
//  sim_dylib_loader.m
//  objsee
//
//  Created by Ethan Arbuckle on 1/31/25.
//

#include <Foundation/Foundation.h>
#include <objc/runtime.h>
#include <dlfcn.h>

#define WHILE_IGNORING_SIGNALS(code) ({ \
int __result = 0; \
struct sigaction __old_sa_segv, __old_sa_bus; \
if (!g_sig_ignoring) { \
g_sig_ignoring = 1; \
install_signal_handlers(&__old_sa_segv, &__old_sa_bus); \
if ((__result = sigsetjmp(g_sig_ignore_jmpbuf, 1)) == 0) { \
code; \
} \
restore_signal_handlers(&__old_sa_segv, &__old_sa_bus); \
g_sig_ignoring = 0; \
} \
__result; \
})

static __thread sigjmp_buf g_sig_ignore_jmpbuf;
static __thread volatile sig_atomic_t g_sig_ignoring = 0;

static void _sig_ignoring_handler(int signo) {
    if (g_sig_ignoring) {
        siglongjmp(g_sig_ignore_jmpbuf, signo);
    }
}

static inline void install_signal_handlers(struct sigaction * _Nonnull old_sa_segv, struct sigaction * _Nonnull old_sa_bus) {
    struct sigaction sa;
    memset(&sa, 0, sizeof(sa));
    sa.sa_handler = _sig_ignoring_handler;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);
    
    sigaction(SIGSEGV, &sa, old_sa_segv);
    sigaction(SIGBUS, &sa, old_sa_bus);
}

static inline void restore_signal_handlers(const struct sigaction * _Nonnull old_sa_segv, const struct sigaction * _Nonnull old_sa_bus) {
    sigaction(SIGSEGV, old_sa_segv, NULL);
    sigaction(SIGBUS, old_sa_bus, NULL);
}


__attribute__((constructor)) static void launch_apple_music(void) {
    NSString *processName = [[NSProcessInfo processInfo] processName];
    if (![processName isEqualToString:@"Music"]) {
        return;
    }
    
    // Codesigning limitations make it such that Music possesses no entitlements when installed under Simulator.
    // Missing entitlements cause several issues that are worked around here with method swizzling
    dlopen("/System/Library/Frameworks/CloudKit.framework/CloudKit", RTLD_NOW);

    // CloudKit raises an exception about missing container ID. Fix by hardcoding a container ID in `-[CKContainerID initWithContainerIdentifier:environment:]`
    SEL initWithContainerIdentifier = sel_registerName("initWithContainerIdentifier:environment:");
    Method initWithContainerIdentifier_method = class_getInstanceMethod(objc_getClass("CKContainerID"), initWithContainerIdentifier);
    IMP orig_initWithContainerIdentifier_imp = method_getImplementation(initWithContainerIdentifier_method);
    IMP newImp = imp_implementationWithBlock(^(__unsafe_unretained id self, NSString *containerID, id environment) {
        return ((id (*)(id, SEL, id, id))orig_initWithContainerIdentifier_imp)(self, initWithContainerIdentifier, @"fake", environment);
    });
    method_setImplementation(initWithContainerIdentifier_method, newImp);
    
    // Mock in-process entitlement checks
    SEL hasBoolEntitlement = sel_registerName("hasBoolEntitlement:shouldLogForMissingEntitlement:");
    Method hasBoolEntitlementMethod = class_getClassMethod(objc_getClass("NSObject"), hasBoolEntitlement);
    IMP newHasBoolEntitlement = imp_implementationWithBlock(^BOOL(Class self, NSString *entitlement, BOOL shouldLog) {
        return YES;
    });
    method_setImplementation(hasBoolEntitlementMethod, newHasBoolEntitlement);
    
    // Crashes are expected, so kick off UIApplicationMain in a signal-ignoring context.
    // On success, run the runloop to keep the bootstrapped app alive.
    // On failure, the launch will be attempted again by the app's usual entry point
    WHILE_IGNORING_SIGNALS({
        void *_UIApplicationMain = dlsym(dlopen(NULL, 0), "UIApplicationMain");
        if (_UIApplicationMain) {
            ((void (*)(int, char **, id, id))_UIApplicationMain)(0, NULL, nil, @"_TtC5Music19ApplicationDelegate");
            CFRunLoopRun();
        }
    });
}