Deciphering Code Structures in .NET

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Deciphering Code Structures
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Assembly Language Code
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High-Level Code
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eax, [SomeVariable]
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if (SomeVariable == 0) CallAFunction(); ...
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call CallAFunction AfterCondition: ...
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Figure A.1 High-level/low-level view of a single branch conditional sequence.
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Two-Way Conditionals
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Another fundamental functionality of high-level languages is to allow the use of two-way conditionals, typically implemented in high-level languages using the if-else keyword pair. A two-way conditional is different from a singlebranch conditional in the sense that if the condition is not satisfied, the program executes an alternative code block and only then proceeds to the code that follows the if-else statement. These constructs are called two-way conditionals because the flow of the program is split into one of two different possible paths: the one in the if block, or the one in the else block. Let s take a quick look at how compilers implement two-way conditionals. First of all, in two-way conditionals the conditional branch points to the else block and not to the code that follows the conditional statement. Second, the condition itself is almost always reversed (so that the jump to the else block only takes place when the condition is not satisfied), and the primary conditional block is placed right after the conditional jump (so that the conditional code gets executed if the condition is satisfied). The conditional block always ends with an unconditional jump that essentially skips the else block this is a good indicator for identifying two-way conditionals. The else block is placed at the end of the conditional block, right after that unconditional jump. Figure A.2 shows what an average if-else statement looks like in assembly language.
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Appendix A
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Assembly Language Code
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High-Level Code
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cmp jne call jmp
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[Variable1], 7 ElseBlock SomeFunction AfterConditionalBlock
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Reversed
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if (SomeVariable == 7) SomeFunction(); else SomeOtherFunction();
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ElseBlock: call SomeOtherFunction
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AfterConditionalBlock:
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Figure A.2 High-level/low-level view of a two-way conditional.
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Notice the unconditional JMP right after the function call. That is where the first condition skips the else block and jumps to the code that follows. The basic pattern to look for when trying to detect a simple if-else statement in a disassembled program is a condition where the code that follows it ends with an unconditional jump. Most high-level languages also support a slightly more complex version of a two-way conditional where a separate conditional statement is used for each of the two code blocks. This is usually implemented by combining the if and else-if keywords where each statement is used with a separate conditional statement. This way, if the first condition is not satisfied, the program jumps to the second condition, evaluates that one, and simply skips the entire conditional block if neither condition is satisfied. If one of the conditions is satisfied, the corresponding conditional block is executed, and execution just flows into the next program statement. Figure A.3 provides a high-level/lowlevel view of this type of control flow construct.
Multiple-Alternative Conditionals
Sometimes programmers create long statements with multiple conditions, where each condition leads to the execution of a different code block. One way to implement this in high-level languages is by using a switch block (discussed later), but it is also possible to do this using conventional if statements. The reason that programmers sometimes must use if statements is that they allow for more flexible conditional statements. The problem is that switch blocks don t support complex conditions, only the use of hardcoded constants. In contrast, a sequence of else-if statements allows for any kind of complex condition on each of the blocks it is just more flexible.
Deciphering Code Structures
Assembly Language Code
High-Level Code
cmp jae call jmp
[Variable1], 10 AlternateBlock SomeFunction AfterIfBlock
Reversed
if (SomeVariable < 10) SomeFunction(); else if (SomeVariable == 345) SomeOtherFunction();
AlternateBlock: cmp jne call [Variable1], 345 AfterIfBlock
Reversed
SomeOtherFunction
AfterIfBlock: ...
Figure A.3 High-level/low-level view of a two-way conditional with two conditional statements.
The guidelines for identifying such blocks are very similar to the ones used for plain two-way conditionals in the previous section. The difference here is that the compiler adds additional alternate blocks that consist of one or more logical checks, the actual conditional code block, and the final JMP that skips to the end of the entire block. Of course, the JMP only gets executed if the condition is satisfied. Unlike switch blocks where several conditions can lead to the same code block, with these kinds of else-if blocks each condition is linked to just one code block. Figure A.4 demonstrates a four-way conditional sequence with one if and three alternate else-if paths that follow.