Custom hooks causing traced app to crash

[KiranPanesar]

I've created a custom hook to trace NSString's +stringWithFormat method. Seeing as there is no README or wiki post on how to do this, I followed the UIPasteboard hook. Here are the steps I have taken:

1. Create an NSStringHooks.xm in the /hooks directory. That file looks like this.
2. Added a method to PlistObjectConverter to convert NSString data. Those files look like this. Line 527 is the beginning of the implementation of +convertNSString:.
3. Added the NSStringHooks to Tweak.xmi. That file looks like this. Line 159 is where I am initialising the NSStringHooks group.

When I run make package in the /src directory, a deb file is successfully created. I can SFTP that to my device and successfully run dpkg -i introspy.deb to install the custom build onto my device. But when I open an app for which I have enabled tracing, it crashes on launch.

Troubleshooting

• I have removed the NSStringHooks tweak altogether, built it using make package and then installed it. That works fine. No crashes when launching a traced app.
• I have added NSStringHooks back in (using the above steps), except I removed all tracing logic from NSStringHooks.xm. So all it was doing was intercepting the call and then passing it straight to the original method. This still causes a crash on the device.

I feel like I am missing a step, because I can't get my custom trace's toggle to show up under the Introspy Settings. The Introspy2.plist files seem to be generated by the make routine, so any changes I make there to add a toggle are overwritten.

Can any contributors see what I am missing in my setup of a custom hook?

Once I've got all this figured out, I'll create a concise list of steps and add it to the README/Wiki.

[radj]

I'm not 100% sure but I'm guessing your hook to +stringWithFormat is doing an infinite recursion. Somewhere in your hook is making a call to +stringWithFormat so it hooks again and again. But it's just a theory.

Another theory is it may not be hooking the right class name. I experienced a crash when I was hooking NSURLSession. See #32

Can you share any crash/exception logs?

Additionally, you don't need convertNSString: as the PlistObjectConverter's methods are simply to convert non-stringable objects into helpful strings/dictionaries. Simply call -[addArgFromPlistObject:@"thestring" withKey:@"paramName"]

[radj]

Also, +stringWithFormat is a complex method that accepts variadic parameters. Try hooking a simpler method with definite number of parameters like - lengthOfBytesUsingEncoding: or - getCharacters:range:.

I am not sure how to hook variadic functions. This may not be an Introspy problem but a theos limitation like #32.

[KiranPanesar]

@radj Thanks for the tip! A couple of things:

1. You were right! I tried hooking into the -length method on __NSCFString and it worked. When It try and hook into -length on NSString it crashes. When I use the runtime to print out the class methods of NSString, -length is not there (but it is for __NSCFString).
2. However, __NSCFString doesn't have the +stringWithFormat method. NSString is the one who defines and implements this (confirmed by inspecting that class at runtime).
   3) I tried hooking +stringWithString: on NSString, which is also defined and implemented in NSString and it worked perfectly! I can see it printed out to the console.

Now I just need to figure out how I can do variadic parameters.

[KiranPanesar]

I created a demo iOS app. Pretty much the only thing it does is call [NSString stringWithFormat:@"asd"];. Ran it through Xcode, put down Exception Breakpoints and grabbed the backtrace using LLDB. Here's the trace of the crashing thread.

[KiranPanesar]

Closing this and opening a more relevant issue (#35)

[mfarhand-sa]

I want to find an important key in an app , so the only way , that I know is make a hook on nsstringwithformat method , but it's not working!

so do you have any idea to find that key ?
the application make a MD5 from mixed key (A-B-C) so I need to find the value of A-B -C
thank you

[mfarhand-sa]

@KiranPanesar how can I make a hook on nsstringwithformat ?
any idea ?