rweichler / shitstrate

demo of why asm is necessary in ObjC hooking

Shitstrate

This is mostly for my reference, but this demonstrates why assembly is necessary in MobileSubstrate, even with hooking Objective C.

<rpetrich> rweichler: the problems are around calling through to the "original" method implementation

<rpetrich> rweichler: imagine you have classes Foo and Bar, where Bar is a subclass of Foo

<rpetrich> rweichler: Foo has a -baz method, which isn't overridden on Bar

<rpetrich> rweichler: your tweak hooks the -baz method on Bar to add its behaviours, and like a good citizen calls through to the original implementation

<rpetrich> rweichler: later on, my tweak decides it also wants to hook -baz, this time on Foo

<rpetrich> rweichler: will your call to the original method implementation call my -baz hook?

<rpetrich> rweichler: if you aren't careful about how you apply the hook on your end, the answer might be no!

The first thing I thought of was, if you don't have an original method, then you just have a dummy original method that just calls the superclass's.

NSString *generic_original(id self, SEL _cmd)
{
    Class super = class_getSuperclass(self);
    Method m;
    find_method(super, _cmd, &m);
    IMP i = method_getImplementation(m);

    return i(self, _cmd);
}

But the problem with that is the arguments can be anything! What do you do if it's something like this?

-(NSString *)takesArguments(int arg1)
{
    //...
}

Then you'd need to make something like:

NSString *generic_original2(id self, SEL _cmd, int arg1)
{
    ///...
}

And that's obviously not generic at all. You'd somehow need to figure out the # of arguments on the fly. Hence the assembly.

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A followup is I wonder if you could use method_getTypeEncoding to somehow create this generic function.