Detecting Domain-Specific Coding Errors with Static Analysis

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INTRODUCTION

Static analysis is a term that describes techniques that compute run-time properties of programs, without actually executing them. Static analysis tools are typically used to find program defects. The first generation of static-analysis tools, exemplified by the lint family of tools, had limited capability and were only capable of finding superficial defects. The latest generation, such as GrammaTech’s CodeSonar® use highly sophisticated whole-program analyses to find deep semantic problems [1]. This paper is about this class of tools and refers to them as advanced static-analysis tools.

Out of the box, advanced static analysis tools are good at finding programming defects that arise because a fundamental rule of the languages is being violated (e.g., a buffer overrun), or because an API is being used incorrectly (such as closing the same file descriptor twice). Many tools are also capable of finding violations of stylistic rules (e.g., don’t use goto). These tools have proven to be effective at finding serious problems, and they have been widely adopted. For example, see [2, 3].

An often under-appreciated aspect of these tools is that they are extensible. They can be configured or programmed to find violations of domain-specific rules too. So if you have rules for using an internal API, or require programmers to use a particular idiom, then it is often possible to write a checker that signals violations of those rules. Programmers can, often with a little programming effort, dramatically increase the value they get from the tools.

This paper describes some of the ways in which static-analysis tools can be extended. Section 2 describes how these tools work. Section 3 uses examples to describe several ways in which the tools can be customized. Section 4 concludes with some recommendations.

HOW ADVANCED STATIC ANALYSIS WORKS

In order to understand the different ways in which extensions can be written, it first helps to know what static analysis is, and how these tools work.

Static analysis tools work very much like compilers. They take source code as input, which they then parse and convert to an intermediate representation (IR). The IR typically comprises the program’s abstract syntax tree (AST), the symbol table, the control-flow graph (CFG), and the call graph. A block diagram of the architecture of an advanced static analysis tool is shown below in Figure 1. Whereas a compiler would use the IR to generate object code, static-analysis tools retain the IR, and checkers are usually implemented by traversing or querying the IR looking for particular properties or patterns that indicate defects. A simple checker, such as one that looks for simple syntactic properties (e.g., goto statements), would search the abstract syntax tree for constructs that match that pattern. A more complex checker might examine the call graph or CFG.

The advanced tools get their power from sophisticated symbolic execution techniques that explore paths through the control-flow graph. These algorithms keep track of the abstract state of the program and know how to use that state to exclude consideration of infeasible paths.

This level of complexity is required in order to find serious bugs while keeping the level of false positives low.

Although all advanced static analysis tools have an architecture much like the one shown, the details differ somewhat. In GrammaTech CodeSonar, all of the intermediate representations are retained, and an interface allows end users to author code that inspects these representations in various ways. The techniques with which new checkers can be written are described in the next section.

SPECIAL CHALLENGES FACING AUTOMOTIVE SOFTWARE DEVELOPERS

No two tools provide exactly the same interface or techniques for implementing custom checkers, but three mechanisms are common, and are described in more detail in the subsequent subsections:

» Existing checkers can be extended by adding directives to a configuration file.

» The user can add annotations to their code that instruct the analysis to look for certain properties. In CodeSonar, these annotations can be done on the side in an aspect-oriented fashion if users do not wish to perturb the source code.

» An API allows users access to all of the intermediate representations. Typically, a visitor pattern is used that allows extensions to piggyback on traversals the analysis is already doing.

Although most tools have similar mechanisms, the examples are shown for CodeSonar.

CONFIGURATION FILES

These advanced static analysis tools implement dozens of checkers. Often, a user needs a checker that is only slightly different than a built-in one, and many of them have been designed to be extensible. One class of checkers finds functions whose use is forbidden. For example, the C library function gets is notoriously insecure (and is now officially deprecated). The checker is implemented by a phase that finds references to function names and then matches them against a set of regular expressions. It is a simple matter to add additional regular expressions to this set by adding lines to a configuration file.

A similar process applies to extend the set of functions whose return value should always be checked or to specify which functions take format string parameters.

CODE ANNOTATIONS

The second way in to write checkers is to add annotations to the code. Suppose for example that there is an internal function named foo that takes a single integer parameter and that it is dangerous for that parameter to have the value -1. A check for this case could be implemented by adding some code to the body of foo as follows:

The #ifdef construct ensures that this new code is not seen by the regular compiler. However, when this code is analyzed by CodeSonar, the call to csonar_trigger is seen. Thus this call is never actually executed but is instead interpreted by the tool as a directive to the underlying analysis engine. If the analysis considers that the trigger condition may be satisfied, then it will issue a warning with the given message.

Of course, in most cases, it is not appropriate to require that programmers interleave these annotations with the code, so there is an alternative way to implement this kind of check that allows it to be written in a separate file, which avoids the need for the code to be edited. This approach is also appropriate when the source code for foo is not available, such as when it is in a third-party library. To do this, the author of the checker would write a replacement function as follows:

When the analysis sees the definition of csonar_replace_foo, it treats all calls in the code to foo (except the one inside csonar_replace_foo) as if they were calls to csonar_re-
place_foo instead.

This trigger idiom is good for checking temporal properties, particularly sequences of function calls. Suppose there is a rule that says that bar should never be called while foo is executing. A check might be implemented as follows:

Note that a global state variable is used to record whether or not foo is active. Before entry to foo, it is set to one and then reset to zero after for returns. This variable is then checked in the trigger in the bar, and if set to one, a warning will be issued.

The example above shows how a global property might be checked. The same mechanism can be used to write checks for the properties of objects. For example, it is illegal to read from a file object after it has been closed. In such a case the state being checked must be associated with the file object. The extension API allows users to specify attributes that can be attached to objects. These can be thought of as state variables that can be associated with objects. In the example just given, an attribute can be used to encode whether a file object is open or closed, and the checker for a reader function can test the value of that attribute and issue a warning if the file is in fact closed.

This approach allows users to author static checks almost as if they were writing dynamic checks. The API for this kind of check is small, and the language is regular C, so there is a shallow learning curve. This simplicity is deceptive — the technique can be used to implement fairly sophisticated checks. In CodeSonar, many of the out-of-the-box checks are implemented this way.

API FOR INTERMEDIATE REPRESENTATIONS

The final way to implement a custom check is to use the API that gives access to the underlying IR. This technique has been used by CodeSonar users to implement a variety of checkers.

For example, one company makes a highly sophisticated electronics manufacturing system, controlled by in excess of a million lines of code. They employ a custom idiom for handling hardware errors. If this is not followed consistently, then it can lead to excessive expensive downtime for their customers. They created a custom checker in CodeSonar that finds locations in the code where the idiom is incorrect.

This API can be used for other program analysis tasks too. A medical device company uses CodeSonar to identify potential tasking problems in their code and to emit information that allows them to interactively explore properties of stack configurations.

Many checks can be written using a visitor pattern. A visitor is a function that is invoked on every IR element of the appropriate type; there are different visitor types for different IR constructs. When they are present, visitors piggyback on the various traversals carried out by the analysis. Visitors can be defined for files, identifiers, subprograms, and nodes in the control-flow graph (which correspond roughly to program statements), and the syntax tree. The interface to a visitor allows for pattern matching against the construct. This way a checker author can easily search for constructs without having to know exactly how they are represented.

For example, suppose there is a rule that no variable is allowed to contain upper-case characters. The checker for this would be best implemented by writing a function that takes an identifier as input, checks that it represents a variable, scans it for upper-case letters, and issues a warning if one is found. This function would be registered as a visitor for the table of identifiers.

CodeSonar has an additional kind of visitor, which is invoked during the path exploration of the control-flow graph. At each step along the path, the check can query the abstract state of the program, so the implementation can ask questions such as “What is the value of this variable at this point”. This allows the checker author to write sophisticated checks that leverage the built-in program analysis capabilities of the tool. A key aspect of this kind of checker is that it uses the part of the analysis that eliminates the exploration of infeasible paths, which automatically reduces the probability of false positive results.

RECOMMENDATIONS

Advanced static analysis tools have become essential tools for software developers because they have proven effective at finding serious flaws. Users of such tools should consider writing custom checkers in order to dramatically increase their return on investment.

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