Feeds

Reverse engineering Apple's OS X

A thunking good time

New hybrid storage solutions

The Wrinkle

There's one wrinkle that you need to be aware of, especially when looking at framework code. In order to implement position-independent code, the GCC compiler implements a "thunking" technique that provides register-based access to global variables, selector strings, and so forth. Thunk calls always occur at the beginning of a method. A typical method prologue might look like this:

+(void)[SomeClass SomeMethod]
0000fa60  55              pushl       %ebp
0000fa61  89e5            movl        %esp,%ebp
0000fa63  57              pushl       %edi
0000fa64  56              pushl       %esi
0000fa65  53              pushl       %ebx
0000fa66  e800000000      calll       0x0000fa6b
0000fa6b  5b              popl        %ebx

If you look carefully at the above, you'll see that the function call at address 0000FA66 simply calls the very next address. In other words, execution continues at address 0000FA6B, but with the same address on the stack.

Popping this return address into EBX gives a position-independent offset that can be used to access global variables. For every EBX-relative data reference in this method (sometimes ECX or even EAX is used), we need to add the "thunk offset" (0000FA6B in this case) to determine the absolute address of the data that's being accessed.

This makes it more tedious to figure out which global variables, string constants or selectors are being accessed. Fortunately, though, otx gets it right much of the time. For those times when otx gets it wrong, I've written a little utility for myself - a kind of otool "after-after-burner" - that processes the output of otx and massages every thunked address reference, so that the absolute address is always shown in the disassembly. It also recognizes and correctly handles old-style thunk calls such as "___i686.get_pc_thunk".

OTX for Cocoa reverse engineering

Tick the three Objective-C check boxes when reverse engineering with otx

There's one last weapon in my reverse-engineering armory that definitely deserves a mention, and that's IDA Pro. Currently available here, IDA Pro is a sophisticated multi-processor disassembler that's equally at home munching away at x86 code (my preferred MO), PowerPC or even iPhone ARM executables. Using the output of IDA Pro, it's very easy - for example - to see how a specific function call is actually implemented inside some external dylib or framework.

With all these tools in your arsenal, reverse engineering Cocoa executables is actually very simple. In fact, it's a good deal more straightforward than most Windows executables, with the exception of Delphi and .NET where - like Cocoa - a good deal of runtime type information is contained within the executable. ®

Secure remote control for conventional and virtual desktops

More from The Register

next story
Not appy with your Chromebook? Well now it can run Android apps
Google offers beta of tricky OS-inside-OS tech
Keep that consumer browser tat away from our software says Oracle
Big Red decides it will only support Firefox's Extended Support Releases
Greater dev access to iOS 8 will put us AT RISK from HACKERS
Knocking holes in Apple's walled garden could backfire, says securo-chap
WordPress 4.0 is here, complete with one-click upgrade process
Don't relax yet, sysadmins, there's still a chance for some big messes here
NHS grows a NoSQL backbone and rips out its Oracle Spine
Open source? In the government? Ha ha! What, wait ...?
Google extends app refund window to two hours
You now have 120 minutes to finish that game instead of 15
Intel: Hey, enterprises, drop everything and DO HADOOP
Big Data analytics projected to run on more servers than any other app
prev story

Whitepapers

Secure remote control for conventional and virtual desktops
Balancing user privacy and privileged access, in accordance with compliance frameworks and legislation. Evaluating any potential remote control choice.
Intelligent flash storage arrays
Tegile Intelligent Storage Arrays with IntelliFlash helps IT boost storage utilization and effciency while delivering unmatched storage savings and performance.
Reg Reader Research: SaaS based Email and Office Productivity Tools
Read this Reg reader report which provides advice and guidance for SMBs towards the use of SaaS based email and Office productivity tools.
Security for virtualized datacentres
Legacy security solutions are inefficient due to the architectural differences between physical and virtual environments.
Providing a secure and efficient Helpdesk
A single remote control platform for user support is be key to providing an efficient helpdesk. Retain full control over the way in which screen and keystroke data is transmitted.