140 lines
6.6 KiB
Text
140 lines
6.6 KiB
Text
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//===----------------------------------------------------------------------===//
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// Clang Static Analyzer
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//===----------------------------------------------------------------------===//
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= Library Structure =
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The analyzer library has two layers: a (low-level) static analysis
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engine (GRExprEngine.cpp and friends), and some static checkers
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(*Checker.cpp). The latter are built on top of the former via the
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Checker and CheckerVisitor interfaces (Checker.h and
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CheckerVisitor.h). The Checker interface is designed to be minimal
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and simple for checker writers, and attempts to isolate them from much
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of the gore of the internal analysis engine.
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= How It Works =
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The analyzer is inspired by several foundational research papers ([1],
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[2]). (FIXME: kremenek to add more links)
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In a nutshell, the analyzer is basically a source code simulator that
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traces out possible paths of execution. The state of the program
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(values of variables and expressions) is encapsulated by the state
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(ProgramState). A location in the program is called a program point
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(ProgramPoint), and the combination of state and program point is a
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node in an exploded graph (ExplodedGraph). The term "exploded" comes
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from exploding the control-flow edges in the control-flow graph (CFG).
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Conceptually the analyzer does a reachability analysis through the
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ExplodedGraph. We start at a root node, which has the entry program
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point and initial state, and then simulate transitions by analyzing
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individual expressions. The analysis of an expression can cause the
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state to change, resulting in a new node in the ExplodedGraph with an
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updated program point and an updated state. A bug is found by hitting
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a node that satisfies some "bug condition" (basically a violation of a
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checking invariant).
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The analyzer traces out multiple paths by reasoning about branches and
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then bifurcating the state: on the true branch the conditions of the
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branch are assumed to be true and on the false branch the conditions
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of the branch are assumed to be false. Such "assumptions" create
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constraints on the values of the program, and those constraints are
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recorded in the ProgramState object (and are manipulated by the
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ConstraintManager). If assuming the conditions of a branch would
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cause the constraints to be unsatisfiable, the branch is considered
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infeasible and that path is not taken. This is how we get
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path-sensitivity. We reduce exponential blow-up by caching nodes. If
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a new node with the same state and program point as an existing node
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would get generated, the path "caches out" and we simply reuse the
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existing node. Thus the ExplodedGraph is not a DAG; it can contain
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cycles as paths loop back onto each other and cache out.
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ProgramState and ExplodedNodes are basically immutable once created. Once
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one creates a ProgramState, you need to create a new one to get a new
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ProgramState. This immutability is key since the ExplodedGraph represents
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the behavior of the analyzed program from the entry point. To
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represent these efficiently, we use functional data structures (e.g.,
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ImmutableMaps) which share data between instances.
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Finally, individual Checkers work by also manipulating the analysis
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state. The analyzer engine talks to them via a visitor interface.
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For example, the PreVisitCallExpr() method is called by GRExprEngine
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to tell the Checker that we are about to analyze a CallExpr, and the
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checker is asked to check for any preconditions that might not be
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satisfied. The checker can do nothing, or it can generate a new
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ProgramState and ExplodedNode which contains updated checker state. If it
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finds a bug, it can tell the BugReporter object about the bug,
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providing it an ExplodedNode which is the last node in the path that
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triggered the problem.
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= Notes about C++ =
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Since now constructors are seen before the variable that is constructed
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in the CFG, we create a temporary object as the destination region that
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is constructed into. See ExprEngine::VisitCXXConstructExpr().
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In ExprEngine::processCallExit(), we always bind the object region to the
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evaluated CXXConstructExpr. Then in VisitDeclStmt(), we compute the
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corresponding lazy compound value if the variable is not a reference, and
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bind the variable region to the lazy compound value. If the variable
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is a reference, just use the object region as the initilizer value.
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Before entering a C++ method (or ctor/dtor), the 'this' region is bound
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to the object region. In ctors, we synthesize 'this' region with
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CXXRecordDecl*, which means we do not use type qualifiers. In methods, we
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synthesize 'this' region with CXXMethodDecl*, which has getThisType()
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taking type qualifiers into account. It does not matter we use qualified
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'this' region in one method and unqualified 'this' region in another
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method, because we only need to ensure the 'this' region is consistent
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when we synthesize it and create it directly from CXXThisExpr in a single
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method call.
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= Working on the Analyzer =
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If you are interested in bringing up support for C++ expressions, the
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best place to look is the visitation logic in GRExprEngine, which
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handles the simulation of individual expressions. There are plenty of
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examples there of how other expressions are handled.
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If you are interested in writing checkers, look at the Checker and
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CheckerVisitor interfaces (Checker.h and CheckerVisitor.h). Also look
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at the files named *Checker.cpp for examples on how you can implement
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these interfaces.
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= Debugging the Analyzer =
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There are some useful command-line options for debugging. For example:
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$ clang -cc1 -help | grep analyze
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-analyze-function <value>
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-analyzer-display-progress
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-analyzer-viz-egraph-graphviz
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...
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The first allows you to specify only analyzing a specific function.
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The second prints to the console what function is being analyzed. The
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third generates a graphviz dot file of the ExplodedGraph. This is
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extremely useful when debugging the analyzer and viewing the
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simulation results.
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Of course, viewing the CFG (Control-Flow Graph) is also useful:
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$ clang -cc1 -help | grep cfg
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-cfg-add-implicit-dtors Add C++ implicit destructors to CFGs for all analyses
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-cfg-add-initializers Add C++ initializers to CFGs for all analyses
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-cfg-dump Display Control-Flow Graphs
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-cfg-view View Control-Flow Graphs using GraphViz
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-unoptimized-cfg Generate unoptimized CFGs for all analyses
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-cfg-dump dumps a textual representation of the CFG to the console,
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and -cfg-view creates a GraphViz representation.
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= References =
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[1] Precise interprocedural dataflow analysis via graph reachability,
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T Reps, S Horwitz, and M Sagiv, POPL '95,
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http://portal.acm.org/citation.cfm?id=199462
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[2] A memory model for static analysis of C programs, Z Xu, T
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Kremenek, and J Zhang, http://lcs.ios.ac.cn/~xzx/memmodel.pdf
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