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Impossible Java

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Original source (barahilia.github.io)
Tags: java jvm bytecode
Clipped on: 2017-04-22

Impossible Java

Mar 26, 2017

Mysterious snippet

Some time ago I came by a very strange piece of Java code. It was extracted from a working application. In a simplified form it looks like this:

class C {
    void a(String s) {}
    int a(String s) { return 0; }
}

Well, this is a clear syntactic error. And sure enough, javac tells:

C.java:3: error: method a(String) is already defined in class C

Overloading functions

It cannot be otherwise. Any compiler of sound mind and memory will issue an error. Not only for Java. Same thing happens in any statically-typed object-oriented language allowing functions overloading in class: functions of the same name and number of arguments must have different types of arguments.

This rule is explained in basic textbooks and programming courses. And the reason is this: function invocation in such languages is usually an expression and the compiler must choose appropriate implementation at compile-time. Meaning the following code a.b(c, d) is regarded as a complete expression and the compiler will decide which function to be called at this point.

To make the decision, our compiler will check the types of a, c and d. We are talking about staticly-typed languages, Java included. All the type information is declared and known at compile time. Then the matching b(,) function will be chosen. Again, we are looking at expression and are aware of parameters’ types.

But the expected return type isn’t known. On the contrary it is be inferred from the chosen matching function. And if there is more than one option, the compiler is at loss. This is the case in the example above. Both the first and the second function accept the same one argument of type String. Which is the right one?

A bit about Android app

How such a snippet could have possibly get into working application? It is impossible. There is no way to compile it. So I thought. Then wiped my eyes. And then - eureka! I actually wasn’t looking at compilable code. But rather it was decompiled from a virtual machine bytecode.

The application was an Android application. They are usually written in Java, compiled and packed into APK files. When we install an app our smart phone downloads the APK file, which is actually a ZIP archive with a specified structure. There are a number of files inside like images, fonts, certificate. One of files is called classes.dex. It is a DEX file containing the binary code for Dalvik - the Android Java Virtual Machine.

There are a number of tools for converting DEX into human readable mnemonics, just like Assembler, but more high-level. And there are also tools capable of decompiling Dalvik opcodes back to Java. Frankly, the result may differ a lot from the original source code, but semantically they should be equal. In the past I worked with IlSpy - a decompiler for .NET. It had achieved spectacular results. Sometimes the decompiled version was almost indistinguishable from the origin.

In mnemonics our example looks this way:

# L and ; decorate the reference type name
.class LC;

.method a(Ljava/lang/String;)V  # V stands for "return type void"
    # statements...
.end method

.method a(Ljava/lang/String;)I  # I stands for "return type int"
    # statements...
.end method

Method invocation

In order to call C.a() the following opcode will be used:

    invoke-direct {v0, v1}, LC;->a(Ljava/lang/String;)I
    #                       ^.........................^

And here is the clue: the entire signature is being used including the function name, the argument types and the return type! For the reference I would recommend to browse http://pallergabor.uw.hu/androidblog/dalvik_opcodes.html and to search for invoke-direct opcode. You see the point? To Dalvik, there is simply no such thing as a function name by its own. In order to declare or to call a function entire signature, including the return type, must be specified.

Now the answer to “how is this possible,” is clear. Decompiler simply built Java code from Dalvik opcodes. And what was sane and executable to the Virtual Machine suddenly presented itself as high level code that compiler rejects. The next question is: how and why this has happened? We may assume that originally there was valid Java code. How our example has come into being?

Concealing the code

Like with assembler, one can write Dalvik mnemonics directly and produce machine code without any aid from high level language compiler. This indeed happens for various reasons. In Android world, much like .NET, Java, JavaScript and more, there is one more player - obfuscator. Existence of decompilers is a problem: once an app was developed and published, nothing prevents adversaries taking the bytecode, putting it back to readable form and seeing what and how the app does.

One may want to guard its code and conceal how exactly it ticks. Obfuscators try to help to some extent. The easiest thing they can do is replacing names of all packages, classes and functions to some meaningless strings in a consistent way. To Dalvik it does not matter. But whoever tries to read the code will need to dig much deeper. E.g. byte[] downloadFileFromUrl(String url) can become byte[] a(String b) or double computeLoanInterest(int days) can be renamed to double c(int d).

Apparently some clever obfuscator used the fact that Dalvik requires entire function signature together with its name at method invocation. And so it took

class C {
    void first(String s) {}
    int second(String s) { return 0; }
}

And instead of renaming first to a and second to b it renamed them both to a, which is still valid in VM because of the different return types! That’s it. End of story. Dalvik happily executed the code. And Java decompiler diligently translated methods to the high level language unaware of the catch.

Insane Java

Well, until now the snippet was simply impossible. Now, to raise the level, a really insane thing is going to happen. Try to imagine a class with two different fields with the same name:

class C {
    int a;
    String a;
}

It was discovered in the same application. And again the clue comes from the Dalvik opcodes reference. Let’s say v1 is of type C as above and we need to read value of the second member String a into v0 like: v0 = v1.a. The following opcode will be used:

    iget v0, v1, LC;->a:Ljava/lang/String;
    #            ^.......................^

Again, the Dalvik syntax involves the fully resolved class name, the field name and the field’s type. No problem to have a dozen of fields with the same name assuming all of them are of different types.

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