macho/research/custom_loader/a.mm

#import <Foundation/Foundation.h>
#include <objc/message.h>
#include <stdio.h>

void* sel_lookUpByName(const char* name);


@interface Foo : NSObject
@end

@implementation Foo
- (void)bar {
  NSLog(@"Invoke instance method %@", self);
}

- (void)tobehijacked:(NSString*)input {
  NSLog(@"Invoke tobehijacked method %@", input);
}
@end

@interface Bar : NSObject
@end

@implementation Bar
static int x;

+ (void)load {
  x = 1;
  printf("Invoke +load method\n");
}

- (void)dummy {
  NSLog(@"Static value check after +load should be 1: x=%d", x);
}
@end



@interface FakeNSDateFormatter : NSDateFormatter {
}
@end

@implementation FakeNSDateFormatter
- (NSDate*)dateFromString:(NSString*)dateString {
  NSLog(@"Hijacked the NSDateFormatter");
  return [super dateFromString:dateString];
}
@end


__attribute__((constructor)) static void
hmmge(int argc, char** argv) {
  printf("Invoke C constructor\n");
  printf("Checking for arguments to be passed correctly\n");
  printf("  argc=%d\n", argc);
  for (int i = 0; i < argc; i++) {
    printf("    argv[%d]=%s\n", i, argv[i]);
  }
  NSLog(@"Using Objective-C in C constructor");
  NSLog(@"Test static Objective-C class is initialized and +load completed");
  Bar *bar = [[Bar alloc] init];
  [bar dummy];
}

int main(int argc, const char * argv[], char* envp[]) {
  @autoreleasepool {
    NSLog(@"Invoke main()");

    // Foo bar using Objective-C syntax
    Foo *foo = [[Foo alloc] init];
    [foo bar];

    // Foo bar with selector and msgSend
    NSLog(@"Directly call \"bar\" %p through objc_msgSend %p with object foo %p", @selector(bar), objc_msgSend, foo);
    typedef void (*barfunc)(id, SEL);
    barfunc bar_ = (barfunc)&objc_msgSend;
    bar_(foo, @selector(bar));

    NSString *dummyinput = @"dummy input";
    [foo tobehijacked:dummyinput];
    NSLog(@"The above invocation should be hijacked with input at %p", dummyinput);

    NSString *dateString =  @"2024-01-01T00:00:00.000Z";

    // NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    // [dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSSZ"];
    // NSDate *date = [dateFormatter dateFromString:dateString];

    // this is to test the idea for hooking,
    // basically, we create a middle-class inherits the class to be used
    //
    // example using NSDateFormatter:
    // - Create a FakeNSDateFormatter inherits NSDateFormatter
    // - Have an overloaded function that calls [super inherited]
    // - The internal struct class_t has superclass points to NSDateFormatter
    // FakeNSDateFormatter *dateFormatter = [[FakeNSDateFormatter alloc] init];
    // [dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSSZ"];
    // NSDate *date = [dateFormatter dateFromString:dateString];
    // NSLog(@"Test hijacked/hooked Objective-C result %@", date);

    NSLog(@"Selector \"dateFromString:\" using @selector %p", @selector(dateFromString:));
    NSLog(@"Selector \"bar:\" using @selector %p", @selector(bar:));
    NSLog(@"Selector \"dummy\" using @selector %p", @selector(dummy));

    NSLog(@"[Bar dummy] implementation is at %p\n", [foo methodForSelector:@selector(bar:)]);
  }

  printf("Selector lookup 'dateFromString:' addr: %p\n", sel_lookUpByName("dateFromString:"));
  printf("Selector lookup 'bar:' addr: %p\n", sel_lookUpByName("bar:"));
  printf("Selector lookup 'dummy' addr: %p\n", sel_lookUpByName("dummy"));

  printf("Test if arguments are passed correctly to main(argc, argv, env)\n");
  printf("  argc=%d\n", argc);
  for (int i = 0; i < argc; i++) {
    printf("    argv[%d]=%s\n", i, argv[i]);
  }

  while (*envp) {
    printf("    env[]=%s\n", *envp);
    envp++;
  }
  return 0;
}