1 //===----------------------------------------------------------------------===// 2 // Clang Static Analyzer 3 //===----------------------------------------------------------------------===// 4 5 = Library Structure = 6 7 The analyzer library has two layers: a (low-level) static analysis 8 engine (GRExprEngine.cpp and friends), and some static checkers 9 (*Checker.cpp). The latter are built on top of the former via the 10 Checker and CheckerVisitor interfaces (Checker.h and 11 CheckerVisitor.h). The Checker interface is designed to be minimal 12 and simple for checker writers, and attempts to isolate them from much 13 of the gore of the internal analysis engine. 14 15 = How It Works = 16 17 The analyzer is inspired by several foundational research papers ([1], 18 [2]). (FIXME: kremenek to add more links) 19 20 In a nutshell, the analyzer is basically a source code simulator that 21 traces out possible paths of execution. The state of the program 22 (values of variables and expressions) is encapsulated by the state 23 (ProgramState). A location in the program is called a program point 24 (ProgramPoint), and the combination of state and program point is a 25 node in an exploded graph (ExplodedGraph). The term "exploded" comes 26 from exploding the control-flow edges in the control-flow graph (CFG). 27 28 Conceptually the analyzer does a reachability analysis through the 29 ExplodedGraph. We start at a root node, which has the entry program 30 point and initial state, and then simulate transitions by analyzing 31 individual expressions. The analysis of an expression can cause the 32 state to change, resulting in a new node in the ExplodedGraph with an 33 updated program point and an updated state. A bug is found by hitting 34 a node that satisfies some "bug condition" (basically a violation of a 35 checking invariant). 36 37 The analyzer traces out multiple paths by reasoning about branches and 38 then bifurcating the state: on the true branch the conditions of the 39 branch are assumed to be true and on the false branch the conditions 40 of the branch are assumed to be false. Such "assumptions" create 41 constraints on the values of the program, and those constraints are 42 recorded in the ProgramState object (and are manipulated by the 43 ConstraintManager). If assuming the conditions of a branch would 44 cause the constraints to be unsatisfiable, the branch is considered 45 infeasible and that path is not taken. This is how we get 46 path-sensitivity. We reduce exponential blow-up by caching nodes. If 47 a new node with the same state and program point as an existing node 48 would get generated, the path "caches out" and we simply reuse the 49 existing node. Thus the ExplodedGraph is not a DAG; it can contain 50 cycles as paths loop back onto each other and cache out. 51 52 ProgramState and ExplodedNodes are basically immutable once created. Once 53 one creates a ProgramState, you need to create a new one to get a new 54 ProgramState. This immutability is key since the ExplodedGraph represents 55 the behavior of the analyzed program from the entry point. To 56 represent these efficiently, we use functional data structures (e.g., 57 ImmutableMaps) which share data between instances. 58 59 Finally, individual Checkers work by also manipulating the analysis 60 state. The analyzer engine talks to them via a visitor interface. 61 For example, the PreVisitCallExpr() method is called by GRExprEngine 62 to tell the Checker that we are about to analyze a CallExpr, and the 63 checker is asked to check for any preconditions that might not be 64 satisfied. The checker can do nothing, or it can generate a new 65 ProgramState and ExplodedNode which contains updated checker state. If it 66 finds a bug, it can tell the BugReporter object about the bug, 67 providing it an ExplodedNode which is the last node in the path that 68 triggered the problem. 69 70 = Notes about C++ = 71 72 Since now constructors are seen before the variable that is constructed 73 in the CFG, we create a temporary object as the destination region that 74 is constructed into. See ExprEngine::VisitCXXConstructExpr(). 75 76 In ExprEngine::processCallExit(), we always bind the object region to the 77 evaluated CXXConstructExpr. Then in VisitDeclStmt(), we compute the 78 corresponding lazy compound value if the variable is not a reference, and 79 bind the variable region to the lazy compound value. If the variable 80 is a reference, just use the object region as the initilizer value. 81 82 Before entering a C++ method (or ctor/dtor), the 'this' region is bound 83 to the object region. In ctors, we synthesize 'this' region with 84 CXXRecordDecl*, which means we do not use type qualifiers. In methods, we 85 synthesize 'this' region with CXXMethodDecl*, which has getThisType() 86 taking type qualifiers into account. It does not matter we use qualified 87 'this' region in one method and unqualified 'this' region in another 88 method, because we only need to ensure the 'this' region is consistent 89 when we synthesize it and create it directly from CXXThisExpr in a single 90 method call. 91 92 = Working on the Analyzer = 93 94 If you are interested in bringing up support for C++ expressions, the 95 best place to look is the visitation logic in GRExprEngine, which 96 handles the simulation of individual expressions. There are plenty of 97 examples there of how other expressions are handled. 98 99 If you are interested in writing checkers, look at the Checker and 100 CheckerVisitor interfaces (Checker.h and CheckerVisitor.h). Also look 101 at the files named *Checker.cpp for examples on how you can implement 102 these interfaces. 103 104 = Debugging the Analyzer = 105 106 There are some useful command-line options for debugging. For example: 107 108 $ clang -cc1 -help | grep analyze 109 -analyze-function <value> 110 -analyzer-display-progress 111 -analyzer-viz-egraph-graphviz 112 ... 113 114 The first allows you to specify only analyzing a specific function. 115 The second prints to the console what function is being analyzed. The 116 third generates a graphviz dot file of the ExplodedGraph. This is 117 extremely useful when debugging the analyzer and viewing the 118 simulation results. 119 120 Of course, viewing the CFG (Control-Flow Graph) is also useful: 121 122 $ clang -cc1 -help | grep cfg 123 -cfg-add-implicit-dtors Add C++ implicit destructors to CFGs for all analyses 124 -cfg-add-initializers Add C++ initializers to CFGs for all analyses 125 -cfg-dump Display Control-Flow Graphs 126 -cfg-view View Control-Flow Graphs using GraphViz 127 -unoptimized-cfg Generate unoptimized CFGs for all analyses 128 129 -cfg-dump dumps a textual representation of the CFG to the console, 130 and -cfg-view creates a GraphViz representation. 131 132 = References = 133 134 [1] Precise interprocedural dataflow analysis via graph reachability, 135 T Reps, S Horwitz, and M Sagiv, POPL '95, 136 http://portal.acm.org/citation.cfm?id=199462 137 138 [2] A memory model for static analysis of C programs, Z Xu, T 139 Kremenek, and J Zhang, http://lcs.ios.ac.cn/~xzx/memmodel.pdf 140
