Static Analysis Internals in Java

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4 Static Analysis Internals
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Throughout this discussion, we have quickly moved from an equation such as (x < y) (x = v) (x < y) to the conclusion that an assertion will succeed or fail For a static analysis tool to make this same conclusion, it needs to use a constraint solver Some static analysis tools have their own specialized constraint solvers, while others use independently developed solvers Writing a good solver is a hard problem all by itself, so if you create your own, be sure to create a well-de ned interface between it and your constraint-generation code Different solvers are good for different problems, so be sure your solver is well matched to the problems that need to be solved Popular approaches to constraint solving include the Nelson-Oppen architecture for cooperating decision procedures [Nelson, 1981] as implemented by Simplify [Detlefs et al, 1996] Simplify is used by the static analysis tools Esc/Java [Flanagan et al, 2002] and Eau Claire [Chess, 2002] In recent years, Boolean satis ability solvers (SAT solvers) such as zChaff [Moskewicz et al, 2001] have become ef cient enough to make them effective for static analysis purposes The static analysis tool SATURN [Xie and Aiken, 2005] uses zChaff Packages for manipulating binary decision diagrams (BDDs), such as BuDDy (http://sourceforgenet/projects/buddy/), are also seeing use in tools such as Microsoft SLAM [Ball et al, 2001] Examples of static analysis tools that use custom solvers include the buffer over ow detectors ARCHER [Xie et al, 2003] and BOON [Wagner et al, 2000]
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The rules that de ne what a security tool should report are just as important, if not more important, than the analysis algorithms and heuristics that the tool implements The analysis algorithms do the heavy lifting, but the rules call the shots Analysis algorithms sometimes get lucky and reach the right conclusions for the wrong reasons, but a tool can never report a problem outside its rule set Early security tools were sometimes compared simply by counting the number of rules that each tool came packaged with by default More recent static analysis tools are harder to compare Rules might work together to
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detect an issue, and an individual rule might refer to abstract interfaces or match method names against a regular expression Just as more code does not always make a better program, more rules do not always make a better static analysis tool Code quality tools sometimes infer rules from the code they are analyzing If a program calls the same method in 100 different locations, and in 99 of those locations it pays attention to the method s return value, there is a decent chance that there is a bug at the single location that does not check the return value This statistical approach to inferring rules does not work so well for identifying security problems If a programmer did not understand that a particular construct represents a security risk, the code might uniformly apply the construct incorrectly throughout the program, which would result in a 100% false negative rate given only a statistical approach Rules are not just for de ning security properties They re also used to de ne any program behavior not explicitly included in the program text, such as the behavior of any system or third-party libraries that the program uses For example, if a Java program uses the javautilHashtable class, the static analysis tool needs rules that de ne the behavior of a Hashtable object and all its methods It s a big job to create and maintain a good set of modeling rules for system libraries and popular third-party libraries Rule Formats Good static analysis tools externalize the rules they check so that rules can be added, subtracted, or altered without having to modify the tool itself The best static analysis tools externalize all the rules they check In addition to adjusting the out-of-the-box behavior of a tool, an external rules interface enables the end user to add checks for new kinds of defects or to extend existing checks in ways that are speci c to the semantics of the program being analyzed Specialized Rule Files Maintaining external les that use a specialized format for describing rules allows the rule format to be tailored to the capabilities of the analysis engine Example 410 shows the RATS rule describing a command injection problem related to the system call system() RATS will report a violation of the rule whenever it sees a call to system() where the rst argument is not constant It gives the function name, the argument number for the untrusted buffer (so that it can avoid reporting cases in which the argument is a constant), and the severity associated with a violation of the rule
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