Hooking Variadic Functions With Substrate

If you’re used to writing iOS tweaks using theos and the Logos preprocessor directives, you may run into problems when trying to hook methods or functions that accept an arbitrary number of arguments. One example of a variadic method is NSString’s+ (instancetype)stringWithFormat:(NSString *)format, ....

To hook variadic methods or C functions, the Substrate C API has to be used directly. I wrote a quick proof of concept for the function open():

// Hook open() to catch the path of all files being accessed
static int (*original_open)(const char *path, int oflag, ...);

static int replaced_open(const char *path, int oflag, ...) {
    int result = 0;

    // Handle the optional third argument
    if (oflag & O_CREAT) {
        mode_t mode;
        va_list args;

        va_start(args, oflag);
        mode = (mode_t)va_arg(args, int);
        va_end(args);
        result = original_open(path, oflag, mode);
    }
    else {
        result = original_open(path, oflag);
    }
    NSLog(@"OPEN: %s", path);
    return result;
}

void hookLibC(void) {
    MSHookFunction(open, replaced_open, (void **) &original_open);
}

The full code is available as a gist on GitHub.