clang  3.7.0
CStringSyntaxChecker.cpp
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1 //== CStringSyntaxChecker.cpp - CoreFoundation containers API *- C++ -*-==//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // An AST checker that looks for common pitfalls when using C string APIs.
11 // - Identifies erroneous patterns in the last argument to strncat - the number
12 // of bytes to copy.
13 //
14 //===----------------------------------------------------------------------===//
15 #include "ClangSACheckers.h"
16 #include "clang/AST/Expr.h"
18 #include "clang/AST/StmtVisitor.h"
20 #include "clang/Basic/TargetInfo.h"
21 #include "clang/Basic/TypeTraits.h"
26 #include "llvm/ADT/SmallString.h"
27 #include "llvm/Support/raw_ostream.h"
28 
29 using namespace clang;
30 using namespace ento;
31 
32 namespace {
33 class WalkAST: public StmtVisitor<WalkAST> {
34  const CheckerBase *Checker;
35  BugReporter &BR;
37 
38  /// Check if two expressions refer to the same declaration.
39  inline bool sameDecl(const Expr *A1, const Expr *A2) {
40  if (const DeclRefExpr *D1 = dyn_cast<DeclRefExpr>(A1->IgnoreParenCasts()))
41  if (const DeclRefExpr *D2 = dyn_cast<DeclRefExpr>(A2->IgnoreParenCasts()))
42  return D1->getDecl() == D2->getDecl();
43  return false;
44  }
45 
46  /// Check if the expression E is a sizeof(WithArg).
47  inline bool isSizeof(const Expr *E, const Expr *WithArg) {
48  if (const UnaryExprOrTypeTraitExpr *UE =
49  dyn_cast<UnaryExprOrTypeTraitExpr>(E))
50  if (UE->getKind() == UETT_SizeOf)
51  return sameDecl(UE->getArgumentExpr(), WithArg);
52  return false;
53  }
54 
55  /// Check if the expression E is a strlen(WithArg).
56  inline bool isStrlen(const Expr *E, const Expr *WithArg) {
57  if (const CallExpr *CE = dyn_cast<CallExpr>(E)) {
58  const FunctionDecl *FD = CE->getDirectCallee();
59  if (!FD)
60  return false;
61  return (CheckerContext::isCLibraryFunction(FD, "strlen") &&
62  sameDecl(CE->getArg(0), WithArg));
63  }
64  return false;
65  }
66 
67  /// Check if the expression is an integer literal with value 1.
68  inline bool isOne(const Expr *E) {
69  if (const IntegerLiteral *IL = dyn_cast<IntegerLiteral>(E))
70  return (IL->getValue().isIntN(1));
71  return false;
72  }
73 
74  inline StringRef getPrintableName(const Expr *E) {
75  if (const DeclRefExpr *D = dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
76  return D->getDecl()->getName();
77  return StringRef();
78  }
79 
80  /// Identify erroneous patterns in the last argument to strncat - the number
81  /// of bytes to copy.
82  bool containsBadStrncatPattern(const CallExpr *CE);
83 
84 public:
85  WalkAST(const CheckerBase *checker, BugReporter &br, AnalysisDeclContext *ac)
86  : Checker(checker), BR(br), AC(ac) {}
87 
88  // Statement visitor methods.
89  void VisitChildren(Stmt *S);
90  void VisitStmt(Stmt *S) {
91  VisitChildren(S);
92  }
93  void VisitCallExpr(CallExpr *CE);
94 };
95 } // end anonymous namespace
96 
97 // The correct size argument should look like following:
98 // strncat(dst, src, sizeof(dst) - strlen(dest) - 1);
99 // We look for the following anti-patterns:
100 // - strncat(dst, src, sizeof(dst) - strlen(dst));
101 // - strncat(dst, src, sizeof(dst) - 1);
102 // - strncat(dst, src, sizeof(dst));
103 bool WalkAST::containsBadStrncatPattern(const CallExpr *CE) {
104  if (CE->getNumArgs() != 3)
105  return false;
106  const Expr *DstArg = CE->getArg(0);
107  const Expr *SrcArg = CE->getArg(1);
108  const Expr *LenArg = CE->getArg(2);
109 
110  // Identify wrong size expressions, which are commonly used instead.
111  if (const BinaryOperator *BE =
112  dyn_cast<BinaryOperator>(LenArg->IgnoreParenCasts())) {
113  // - sizeof(dst) - strlen(dst)
114  if (BE->getOpcode() == BO_Sub) {
115  const Expr *L = BE->getLHS();
116  const Expr *R = BE->getRHS();
117  if (isSizeof(L, DstArg) && isStrlen(R, DstArg))
118  return true;
119 
120  // - sizeof(dst) - 1
121  if (isSizeof(L, DstArg) && isOne(R->IgnoreParenCasts()))
122  return true;
123  }
124  }
125  // - sizeof(dst)
126  if (isSizeof(LenArg, DstArg))
127  return true;
128 
129  // - sizeof(src)
130  if (isSizeof(LenArg, SrcArg))
131  return true;
132  return false;
133 }
134 
135 void WalkAST::VisitCallExpr(CallExpr *CE) {
136  const FunctionDecl *FD = CE->getDirectCallee();
137  if (!FD)
138  return;
139 
140  if (CheckerContext::isCLibraryFunction(FD, "strncat")) {
141  if (containsBadStrncatPattern(CE)) {
142  const Expr *DstArg = CE->getArg(0);
143  const Expr *LenArg = CE->getArg(2);
145  PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
146 
147  StringRef DstName = getPrintableName(DstArg);
148 
150  llvm::raw_svector_ostream os(S);
151  os << "Potential buffer overflow. ";
152  if (!DstName.empty()) {
153  os << "Replace with 'sizeof(" << DstName << ") "
154  "- strlen(" << DstName <<") - 1'";
155  os << " or u";
156  } else
157  os << "U";
158  os << "se a safer 'strlcat' API";
159 
160  BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
161  "C String API", os.str(), Loc,
162  LenArg->getSourceRange());
163  }
164  }
165 
166  // Recurse and check children.
167  VisitChildren(CE);
168 }
169 
170 void WalkAST::VisitChildren(Stmt *S) {
171  for (Stmt *Child : S->children())
172  if (Child)
173  Visit(Child);
174 }
175 
176 namespace {
177 class CStringSyntaxChecker: public Checker<check::ASTCodeBody> {
178 public:
179 
180  void checkASTCodeBody(const Decl *D, AnalysisManager& Mgr,
181  BugReporter &BR) const {
182  WalkAST walker(this, BR, Mgr.getAnalysisDeclContext(D));
183  walker.Visit(D->getBody());
184  }
185 };
186 }
187 
188 void ento::registerCStringSyntaxChecker(CheckerManager &mgr) {
189  mgr.registerChecker<CStringSyntaxChecker>();
190 }
191 
StringRef getName() const
Definition: Decl.h:168
Expr * getArg(unsigned Arg)
getArg - Return the specified argument.
Definition: Expr.h:2216
Defines enumerations for the type traits support.
A builtin binary operation expression such as "x + y" or "x <= y".
Definition: Expr.h:2918
Expr * IgnoreParenCasts() LLVM_READONLY
Definition: Expr.cpp:2439
const Decl * getDecl() const
AnalysisDeclContext * getAnalysisDeclContext(const Decl *D)
static bool isCLibraryFunction(const FunctionDecl *FD, StringRef Name=StringRef())
Returns true if the callee is an externally-visible function in the top-level namespace, such as malloc.
static PathDiagnosticLocation createBegin(const Decl *D, const SourceManager &SM)
Create a location for the beginning of the declaration.
CHECKER * registerChecker()
Used to register checkers.
virtual Stmt * getBody() const
Definition: DeclBase.h:840
FunctionDecl * getDirectCallee()
If the callee is a FunctionDecl, return it. Otherwise return 0.
Definition: Expr.cpp:1184
unsigned getNumArgs() const
Definition: Expr.h:2205
Defines the clang::TargetInfo interface.
A reference to a declared variable, function, enum, etc. [C99 6.5.1p2].
Definition: Expr.h:899