What Is Decompiler

[Mica Withington]

A decompiler is a computer program that translates an executable file to high-level source code. It does therefore the opposite of a typical compiler, which translates a high-level language to a low-level language. While disassemblers translate an executable into assembly language, decompilers go a step further and translate the code into a higher level language such as C or Java, requiring more sophisticated techniques. Decompilers are usually unable to perfectly reconstruct the original source code, thus will frequently produce obfuscated code. Nonetheless, they remain an important tool in the reverse engineering of computer software.

The term decompiler is most commonly applied to a program which translates executable programs (the output from a compiler) into source code in a (relatively) high level language which, when compiled, will produce an executable whose behavior is the same as the original executable program. By comparison, a disassembler translates an executable program into assembly language (and an assembler could be used for assembling it back into an executable program).

Decompilation is the act of using a decompiler, although the term can also refer to the output of a decompiler. It can be used for the recovery of lost source code, and is also useful in some cases for computer security, interoperability and error correction.[1] The success of decompilation depends on the amount of information present in the code being decompiled and the sophistication of the analysis performed on it. The bytecode formats used by many virtual machines (such as the Java Virtual Machine or the .NET Framework Common Language Runtime) often include extensive metadata and high-level features that make decompilation quite feasible. The application of debug data, i.e. debug-symbols, may enable to reproduce the original names of variables and structures and even the line numbers. Machine language without such metadata or debug data is much harder to decompile.[2]

Some compilers and post-compilation tools produce obfuscated code (that is, they attempt to produce output that is very difficult to decompile, or that decompiles to confusing output). This is done to make it more difficult to reverse engineer the executable.

The success level achieved by decompilers can be impacted by various factors. These include the abstraction level of the source language, if the object code contains explicit class structure information, it aids the decompilation process. Descriptive information, especially with naming details, also accelerates the compiler's work. Moreover, less optimized code is quicker to decompile since optimization can cause greater deviation from the original code.[5]

The first decompilation phase loads and parses the input machine code or intermediate language program's binary file format. It should be able to discover basic facts about the input program, such as the architecture (Pentium, PowerPC, etc.) and the entry point. In many cases, it should be able to find the equivalent of the main function of a C program, which is the start of the user written code. This excludes the runtime initialization code, which should not be decompiled if possible. If available the symbol tables and debug data are also loaded. The front end may be able to identify the libraries used even if they are linked with the code, this will provide library interfaces. If it can determine the compiler or compilers used it may provide useful information in identifying code idioms.[6]

Idiomatic machine code sequences are sequences of code whose combined semantics are not immediately apparent from the instructions' individual semantics. Either as part of the disassembly phase, or as part of later analyses, these idiomatic sequences need to be translated into known equivalent IR. For example, the x86 assembly code:

Some idiomatic sequences are machine independent; some involve only one instruction. For example, xor eax, eax clears the eax register (sets it to zero). This can be implemented with a machine independent simplification rule, such as a = 0.

In general, it is best to delay detection of idiomatic sequences if possible, to later stages that are less affected by instruction ordering. For example, the instruction scheduling phase of a compiler may insert other instructions into an idiomatic sequence, or change the ordering of instructions in the sequence. A pattern matching process in the disassembly phase would probably not recognize the altered pattern. Later phases group instruction expressions into more complex expressions, and modify them into a canonical (standardized) form, making it more likely that even the altered idiom will match a higher level pattern later in the decompilation.

The places where register contents are defined and used must be traced using data flow analysis. The same analysis can be applied to locations that are used for temporaries and local data. A different name can then be formed for each such connected set of value definitions and uses. It is possible that the same local variable location was used for more than one variable in different parts of the original program. Even worse it is possible for the data flow analysis to identify a path whereby a value may flow between two such uses even though it would never actually happen or matter in reality. This may in bad cases lead to needing to define a location as a union of types. The decompiler may allow the user to explicitly break such unnatural dependencies which will lead to clearer code. This of course means a variable is potentially used without being initialized and so indicates a problem in the original program.[citation needed]

A good machine code decompiler will perform type analysis. Here, the way registers or memory locations are used result in constraints on the possible type of the location. For example, an and instruction implies that the operand is an integer; programs do not use such an operation on floating point values (except in special library code) or on pointers. An add instruction results in three constraints, since the operands may be both integer, or one integer and one pointer (with integer and pointer results respectively; the third constraint comes from the ordering of the two operands when the types are different).[7]

Various high level expressions can be recognized which trigger recognition of structures or arrays. However, it is difficult to distinguish many of the possibilities, because of the freedom that machine code or even some high level languages such as C allow with casts and pointer arithmetic.

The final phase is the generation of the high level code in the back end of the decompiler. Just as a compiler may have several back ends for generating machine code for different architectures, a decompiler may have several back ends for generating high level code in different high level languages.

Just before code generation, it may be desirable to allow an interactive editing of the IR, perhaps using some form of graphical user interface. This would allow the user to enter comments, and non-generic variable and function names. However, these are almost as easily entered in a post decompilation edit. The user may want to change structural aspects, such as converting a while loop to a for loop. These are less readily modified with a simple text editor, although source code refactoring tools may assist with this process. The user may need to enter information that failed to be identified during the type analysis phase, e.g. modifying a memory expression to an array or structure expression. Finally, incorrect IR may need to be corrected, or changes made to cause the output code to be more readable.

In Europe, the 1991 Software Directive explicitly provides for a right to decompile in order to achieve interoperability. The result of a heated debate between, on the one side, software protectionists, and, on the other, academics as well as independent software developers, Article 6 permits decompilation only if a number of conditions are met:

Overall, the decompilation right provided by Article 6 codifies what is claimed to be common practice in the software industry. Few European lawsuits are known to have emerged from the decompilation right. This could be interpreted as meaning one of three things:

To decompile means to convert executable or ready-to-run program code -- sometimes called object code -- into some form of higher-level programming language that humans can easily understand. Decompilation is a type of reverse-engineering that performs the opposite operations of a compiler. Decompilation was first used in the 1960s to facilitate the migration of a program from one platform to another.

To convert a programming code into a working program, it must first be compiled -- i.e., converted into a series of binary bits or digits (that is, 1s and 0s) that can be understood by the computer. This operation can be reversed by decompiling the final program (which is why decompiling is described as a type of reverse-engineering).

A decompiler is a computer program that receives input in the form of an executable file. If the file's source code is lost or corrupted for some reason, the decompilation process attempts to recover the code, or at least most of it. Unlike a compiler, whose recipient (or final user) is the computer, a decompiler's recipient is a human user.

A disassembler is a similar tool that translates object code into assembler language. It provides a "dump" of the machine code that makes a program work. Some reverse compilers are in fact disassemblers.

Decompilation can rarely reconstruct the original source code perfectly or completely. One reason is successful decompilation depends on available code information. It also depends on the reverse engineer's skills, expertise, and the tools and techniques they use.

The meaningful names given to variables and Functions by programmers to make them easily identifiable are not usually stored in executable files, and are therefore not always recovered in decompiling. In short, because the source code rarely includes such "identifiers," perfect decompilation almost never happens.

c80f0f1006