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Reversed Compilation Techniques

Teddy Rogers

About This File

Techniques for writing reverse compilers or decompilers are presented in this thesis. These techniques are based on compiler and optimization theory, and are applied to decompilation in a unique way; these techniques have never before been published.

A decompiler is composed of several phases which are grouped into modules dependent on language or machine features. The front-end is a machine dependent module that parses the binary program, analyzes the semantics of the instructions in the program, and generates an intermediate low-level representation of the program, as well as a control flow graph of each subroutine. The universal decompiling machine is a language and machine independent module that analyzes the low-level intermediate code and transforms it into a high-level representation available in any high-level language, and analyzes the structure of the control ow graph(s) and transform them into graphs that make use of high-level control structures. Finally, the back-end is a target language dependent module that generates code for the target language.

Decompilation is a process that involves the use of tools to load the binary program into memory, parse or disassemble such a program, and decompile or analyze the program to generate a high-level language program. This process bene ts from compiler and library signatures to recognize particular compilers and library subroutines. Whenever a compiler signature is recognized in the binary program, all compiler start-up and library subroutines are not decompiled; in the former case, the routines are eliminated from the nal target program and the entry point to the main program is used for the decompiler analysis, in the latter case the subroutines are replaced by their library name.

The presented techniques were implemented in a prototype decompiler for the Intel i80286 architecture running under the DOS operating system, dcc, which produces target C programs for source .exeor .com les. Sample decompiled programs, comparisons against the initial high-level language program, and an analysis of results is presented in Chapter 9. Chapter 1 gives an introduction to decompilation from a compiler point of view, Chapter 2 gives an overview of the history of decompilation since its appearance in the early 1960s, Chapter 3 presents the relations between the static binary code of the source binary program and the actions performed at run-time to implement the program, Chapter 4 describes the phases of the front-end module, Chapter 5 describes data optimization techniques to analyze the intermediate code and transform it into a higher-representation, Chapter 6 de nes control structure transformation techniques to analyze the structure of the control ow graph and transform it into a graph of high-level control structures, Chapter 7 describes the back-end module, Chapter 8 presents the decompilation tool programs, Chapter 9 gives an overview of the implementation of dcc and the results obtained, and Chapter 10 gives the conclusions and future work of this research.

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