Decompilation in .NET

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Decompilation
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Additionally, the decompiled output might be structured somewhat differently from the original source code because of compiler optimizations. In this chapter, I will demonstrate several limitations imposed on native code decompilers by modern compilers and show how precious information is often eliminated from executable binaries.
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Typical Decompiler Architecture
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In terms of its basic architecture, a decompiler is somewhat similar to a compiler, except that, well . . . it works in the reverse order. The front end, which is the component that parses the source code in a conventional compiler, decodes low-level language instructions and translates them into some kind of intermediate representation. This intermediate representation is gradually improved by eliminating as much useless detail as possible, while emphasizing valuable details as they are gathered in order to improve the quality of the decompiled output. Finally, the back end takes this improved intermediate representation of the program and uses it to produce a high-level language representation. The following sections describe each of these stages in detail and attempt to demonstrate this gradual transition from low-level assembly language code to a high-level language representation.
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The first step in decompilation is to translate each individual low-level instruction into an intermediate representation that provides a higher-level view of the program. Intermediate representation is usually just a generic instruction set that can represent everything about the code. Intermediate representations are different from typical low-level instruction sets. For example, intermediate representations typically have an infinite number of registers available (this is also true in most compilers). Additionally, even though the instructions have support for basic operations such as addition or subtraction, there usually aren t individual instructions that perform these operations. Instead, instructions use expression trees (see the next section) as operands. This makes such intermediate representations extremely flexible because they can describe anything from assembly-language-like single-operation-per-instruction type code to a higher-level representation where a single instruction includes complex arithmetic or logical expressions. Some decompilers such as dcc [Cifuentes2] have more than one intermediate representation, one for providing a low-level representation of the program in the early stages of the process and another for representing a higher-level view of the program later on. Others use a single representation for the entire process and just gradually eliminate low-level detail from the code while adding high-level detail as the process progresses.
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Generally speaking, intermediate representations consist of tiny instruction sets, as opposed to the huge instruction sets of some processor architecture such as IA-32. Tiny instruction sets are possible because of complex expressions used in almost every instruction. The following is a generic description of the instruction set typically used by decompilers. Notice that this example describes a generic instruction set that can be used throughout the decompilation process, so that it can directly represent both a low-level representation that is very similar to the original assembly language code and a high-level representation that can be translated into a high-level language representation. Assignment This is a very generic instruction that represents an assignment operation into a register, variable, or other memory location (such as a global variable). An assignment instruction can typically contain complex expressions on either side. Push Push a value into the stack. Again, the value being pushed can be any kind of complex expression. These instructions are generally eliminated during data-flow analysis since they have no direct equivalent in high-level representations. Pop Pop a value from the stack. These instructions are generally eliminated during data-flow analysis since they have no direct equivalent in high-level representations. Call Call a subroutine and pass the listed parameters. Each parameter can be represented using a complex expression. Keep in mind that to obtain such a list of parameters, a decompiler would have to perform significant analysis of the low-level code. Ret Return from a subroutine. Typically supports a complex expression to represent the procedure s return value. Branch A branch instruction evaluates two operands using a specified conditional code and jumps to the specified address if the expression evaluates to True. The comparison is performed on two expression trees, where each tree can represent anything from a trivial expression (such as a constant), to a complex expression. Notice how this is a higher-level representation of what would require several instructions in native assembly language; that s a good example of how the intermediate representation has the flexibility of showing both an assembly-languagelike low-level representation of the code and a higher-level representation that s closer to a high-level language. Unconditional Jump An unconditional jump is a direct translation of the unconditional jump instruction in the original program. It is used during the construction of the control flow graph. The meanings of unconditional jumps are analyzed during the control flow analysis stage.
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