Introduction to Static Analysis in Java

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2 Introduction to Static Analysis
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Figure 21 Fujaba enables programmers to move back and forth between a UML view and source code Fujaba has a reverse-engineering capability that allows it to read source code and identify a limited set of design patterns
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Program Veri cation and Property Checking A program veri cation tool accepts a speci cation and a body of code and then attempts to prove that the code is a faithful implementation of the speci cation If the speci cation is a complete description of everything the program should do, the program veri cation tool can perform equivalence checking to make sure that the code and the speci cation exactly match1
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1 Equivalence checking is not used much for software, but in the world of hardware design, where a circuit might go through a long series of complex transformations on its way to becoming a piece of silicon, equivalence checking is widely used to make sure that a transformed design remains true to the original design
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Rarely do programmers have a speci cation that is detailed enough that it can be used for equivalence checking, and the job of creating such a speci cation can end up being more work than writing the code, so this style of formal veri cation does not happen very often Even more limiting is the fact that, historically, equivalence checking tools have not been able to process programs of any signi cant size See the sidebar Formal Veri cation and the Orange Book for a 1980s attempt at pulling formal veri cation toward the mainstream More commonly, verification tools check software against a partial specification that details only part of the behavior of a program This endeavor sometimes goes by the name property checking The majority of property checking tools work either by applying logical inference or by performing model checking (We discuss analysis algorithms in more detail in 4) Many property checking tools focus on temporal safety properties A temporal safety property specifies an ordered sequence of events that a program must not carry out An example of a temporal safety property is a memory location should not be read after it is freed Most propertychecking tools enable programmers to write their own specifications to check program-specific properties When a property checking tool discovers that the code might not match the specification, it traditionally explains its finding to the user by reporting a counterexample: a hypothetical event or sequence of events that takes place within the program that will lead to the property being violated Example 26 gives a few C statements that contain a memory leak, and Example 27 shows how a property checking tool might go about reporting a violation of the property using a counterexample to tell the story of the leaking memory
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Example 26 A memory leak If the rst call to malloc() succeeds and the second fails, the memory allocated in the rst call is leaked
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1 2 3 4 5 6 inBuf = (char*) malloc(bufSz); if (inBuf == NULL) return -1; outBuf = (char*) malloc(bufSz); if (outBuf == NULL) return -1;
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2 Introduction to Static Analysis
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Example 27 A counterexample from a property checking tool running against the code in Example 26 The sequence of events describes a way in which the program can violate the property allocated memory should always be freed
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Violation line 2: line 5: line 6: of property "allocated memory should always be freed": inBuf != NULL outBuf == NULL function returns (-1) without freeing inBuf
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A property checking tool is said to be sound with respect to the speci cation if it will always report a problem if one exists In other words, the tool will never suffer a false negative Most tools that claim to be sound require that the program being evaluated meet certain conditions Some disallow function pointers, while others disallow recursion or assume that two pointers never alias (point to the same memory location) Soundness is an important characteristic in an academic context, where anything less might garner the label unprincipled But for large real-world bodies of code, it is almost impossible to meet the conditions stipulated by the tool, so the soundness guarantee is not meaningful For this reason, soundness is rarely a requirement from a practitioner s point of view In striving for soundness or because of other complications, a property checking tool might produce false positives In the case of a false positive, the counterexample will contain one or more events that could not actually take place Example 28 gives a second counterexample for a memory leak This time, the property checker has gone wrong; it does not understand that, by returning NULL, malloc() is indicating that no memory has been allocated This could indicate a problem with the way the property is speci ed, or it could be a problem with the way the property checker works
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Example 28 An errant counterexample from a property checking tool running against the code in Example 26 The tool does not understand that when malloc() returns NULL, no memory has been allocated, and therefore no memory needs to be freed
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Violation of property "allocated memory should always be freed": line 2: inBuf == NULL line 3: function returns (-1) without freeing inBuf
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Praxis High Integrity Systems (http://wwwpraxis-hiscom) offers a commercial program veri cation tool for a subset of the Ada programming
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