ARM __moddi3

When I was debugging an app on iPhone 4 (armv7), I found a function call:

Since ARM does not have Division/Modulo operators, compilers use other ways making your code work on devices like iPhone.

I followed the __moddi3 on iPhone 4, the moddi3() is provided by llvm’s compiler-rt (lib/builtins/moddi3.c).

Parameters a and b are 64-bit long and returned value is also 64-bit.

But in ARMv7, registers are 32-bit long. I’m trying to find out which registers are used.

I wrote a simple C file and compile to armv7 and x86_64

Compile to x86_64

Disassembe x86_64

x86_64 uses idivq, in this program:

Compile to armv7

Disassemble armv7

In armv7:

If both dividend and divisor are less than 0x100000000.

$r2 may be negative, like 0xfffffff3. ($r3 == 0).

means $r0 – $r2 == divisor

If signed integer $r2 is smaller than 0x80000000,
((long long) $r1 << 32 | $r0) - ((long long ) $r3 << 32 | $r2) == divisor.

ARM __moddi3 by @sskaje:

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error: failed to launch process debugserver: Security

I jailbroken my iPhone 4s with iOS 8.2 and tried to ran debugserver, then I met this error:

Google doesn’t help.

Just because, the App I’m trying to crack is an In-House app, not from AppStore, I have to ran it after installation and choose to trust the developer.

error: failed to launch process debugserver: Security by @sskaje:

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Set up DebugServer on iOS 7

I had my iPhone 4 jailbroken, so I can debug/crack apps on iPhone.

Server: iPhone 4 + debugserver
Client: Mac OS X + lldb



Debugserver can be found on iOS: /Developer/usr/bin/debugserver
Just follow instructions: debugserver on iPhone Wiki

Save following content as a plist like dbg.plist

Apply the entitlement

If entitlement above is not applied, debugserver won’t be able to listen to a TCP port.

Remove FairPlay

FairPlay is Apple’s DRM applied to apps on AppStore.
If you see cryptid 1 like below, try Clutch!

Disable ASLR

Try otool -hv to your App, if you see PIE flags, you have to disable ASLR.

See: Mach-O Disable ASLR/PIE

Make sure you have python installed on your iPhone.

Find your target app.


Copy a decrypted and de-aslr-ed app binary to your OS X and:

Set up DebugServer on iOS 7 by @sskaje:

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OS X 下采集程序调用:sample


sample — Profile a process during a time interval

sample is a command-line tool for gathering data about the running behavior of a process. It suspends
the process at specified intervals (by default, every 1 millisecond), records the call stacks of all
threads in the process at that time, then resumes the process. The analysis done by sample is called
sampling” because it only checks the state of the program at the sampling points. The analysis may
miss execution of some functions that are not executing during one of the samples, but sample still
provides useful data about commonly executing functions.


OS X 下采集程序调用:sample by @sskaje:

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FernFlower inside IDEA

IntelliJ IDEA 14 has FernFlower integrated.

To use:

FernFlower inside IDEA by @sskaje: