CStringSyntaxChecker.cpp revision 360784 
1//== CStringSyntaxChecker.cpp - CoreFoundation containers API *- C++ -*-==//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// An AST checker that looks for common pitfalls when using C string APIs.
10//  - Identifies erroneous patterns in the last argument to strncat - the number
11//    of bytes to copy.
12//
13//===----------------------------------------------------------------------===//
14#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
15#include "clang/AST/Expr.h"
16#include "clang/AST/OperationKinds.h"
17#include "clang/AST/StmtVisitor.h"
18#include "clang/Analysis/AnalysisDeclContext.h"
19#include "clang/Basic/TargetInfo.h"
20#include "clang/Basic/TypeTraits.h"
21#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
22#include "clang/StaticAnalyzer/Core/Checker.h"
23#include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
24#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
25#include "llvm/ADT/SmallString.h"
26#include "llvm/Support/raw_ostream.h"
27
28using namespace clang;
29using namespace ento;
30
31namespace {
32class WalkAST: public StmtVisitor<WalkAST> {
33  const CheckerBase *Checker;
34  BugReporter &BR;
35  AnalysisDeclContext* AC;
36
37  /// Check if two expressions refer to the same declaration.
38  bool sameDecl(const Expr *A1, const Expr *A2) {
39    if (const auto *D1 = dyn_cast<DeclRefExpr>(A1->IgnoreParenCasts()))
40      if (const auto *D2 = dyn_cast<DeclRefExpr>(A2->IgnoreParenCasts()))
41        return D1->getDecl() == D2->getDecl();
42    return false;
43  }
44
45  /// Check if the expression E is a sizeof(WithArg).
46  bool isSizeof(const Expr *E, const Expr *WithArg) {
47    if (const auto *UE = dyn_cast<UnaryExprOrTypeTraitExpr>(E))
48      if (UE->getKind() == UETT_SizeOf && !UE->isArgumentType())
49        return sameDecl(UE->getArgumentExpr(), WithArg);
50    return false;
51  }
52
53  /// Check if the expression E is a strlen(WithArg).
54  bool isStrlen(const Expr *E, const Expr *WithArg) {
55    if (const auto *CE = dyn_cast<CallExpr>(E)) {
56      const FunctionDecl *FD = CE->getDirectCallee();
57      if (!FD)
58        return false;
59      return (CheckerContext::isCLibraryFunction(FD, "strlen") &&
60              sameDecl(CE->getArg(0), WithArg));
61    }
62    return false;
63  }
64
65  /// Check if the expression is an integer literal with value 1.
66  bool isOne(const Expr *E) {
67    if (const auto *IL = dyn_cast<IntegerLiteral>(E))
68      return (IL->getValue().isIntN(1));
69    return false;
70  }
71
72  StringRef getPrintableName(const Expr *E) {
73    if (const auto *D = dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
74      return D->getDecl()->getName();
75    return StringRef();
76  }
77
78  /// Identify erroneous patterns in the last argument to strncat - the number
79  /// of bytes to copy.
80  bool containsBadStrncatPattern(const CallExpr *CE);
81
82  /// Identify erroneous patterns in the last argument to strlcpy - the number
83  /// of bytes to copy.
84  /// The bad pattern checked is when the size is known
85  /// to be larger than the destination can handle.
86  ///   char dst[2];
87  ///   size_t cpy = 4;
88  ///   strlcpy(dst, "abcd", sizeof("abcd") - 1);
89  ///   strlcpy(dst, "abcd", 4);
90  ///   strlcpy(dst + 3, "abcd", 2);
91  ///   strlcpy(dst, "abcd", cpy);
92  /// Identify erroneous patterns in the last argument to strlcat - the number
93  /// of bytes to copy.
94  /// The bad pattern checked is when the last argument is basically
95  /// pointing to the destination buffer size or argument larger or
96  /// equal to.
97  ///   char dst[2];
98  ///   strlcat(dst, src2, sizeof(dst));
99  ///   strlcat(dst, src2, 2);
100  ///   strlcat(dst, src2, 10);
101  bool containsBadStrlcpyStrlcatPattern(const CallExpr *CE);
102
103public:
104  WalkAST(const CheckerBase *Checker, BugReporter &BR, AnalysisDeclContext *AC)
105      : Checker(Checker), BR(BR), AC(AC) {}
106
107  // Statement visitor methods.
108  void VisitChildren(Stmt *S);
109  void VisitStmt(Stmt *S) {
110    VisitChildren(S);
111  }
112  void VisitCallExpr(CallExpr *CE);
113};
114} // end anonymous namespace
115
116// The correct size argument should look like following:
117//   strncat(dst, src, sizeof(dst) - strlen(dest) - 1);
118// We look for the following anti-patterns:
119//   - strncat(dst, src, sizeof(dst) - strlen(dst));
120//   - strncat(dst, src, sizeof(dst) - 1);
121//   - strncat(dst, src, sizeof(dst));
122bool WalkAST::containsBadStrncatPattern(const CallExpr *CE) {
123  if (CE->getNumArgs() != 3)
124    return false;
125  const Expr *DstArg = CE->getArg(0);
126  const Expr *SrcArg = CE->getArg(1);
127  const Expr *LenArg = CE->getArg(2);
128
129  // Identify wrong size expressions, which are commonly used instead.
130  if (const auto *BE = dyn_cast<BinaryOperator>(LenArg->IgnoreParenCasts())) {
131    // - sizeof(dst) - strlen(dst)
132    if (BE->getOpcode() == BO_Sub) {
133      const Expr *L = BE->getLHS();
134      const Expr *R = BE->getRHS();
135      if (isSizeof(L, DstArg) && isStrlen(R, DstArg))
136        return true;
137
138      // - sizeof(dst) - 1
139      if (isSizeof(L, DstArg) && isOne(R->IgnoreParenCasts()))
140        return true;
141    }
142  }
143  // - sizeof(dst)
144  if (isSizeof(LenArg, DstArg))
145    return true;
146
147  // - sizeof(src)
148  if (isSizeof(LenArg, SrcArg))
149    return true;
150  return false;
151}
152
153bool WalkAST::containsBadStrlcpyStrlcatPattern(const CallExpr *CE) {
154  if (CE->getNumArgs() != 3)
155    return false;
156  const Expr *DstArg = CE->getArg(0);
157  const Expr *LenArg = CE->getArg(2);
158
159  const auto *DstArgDRE = dyn_cast<DeclRefExpr>(DstArg->IgnoreParenImpCasts());
160  const auto *LenArgDRE =
161      dyn_cast<DeclRefExpr>(LenArg->IgnoreParenLValueCasts());
162  uint64_t DstOff = 0;
163  if (isSizeof(LenArg, DstArg))
164    return false;
165
166  // - size_t dstlen = sizeof(dst)
167  if (LenArgDRE) {
168    const auto *LenArgVal = dyn_cast<VarDecl>(LenArgDRE->getDecl());
169    // If it's an EnumConstantDecl instead, then we're missing out on something.
170    if (!LenArgVal) {
171      assert(isa<EnumConstantDecl>(LenArgDRE->getDecl()));
172      return false;
173    }
174    if (LenArgVal->getInit())
175      LenArg = LenArgVal->getInit();
176  }
177
178  // - integral value
179  // We try to figure out if the last argument is possibly longer
180  // than the destination can possibly handle if its size can be defined.
181  if (const auto *IL = dyn_cast<IntegerLiteral>(LenArg->IgnoreParenImpCasts())) {
182    uint64_t ILRawVal = IL->getValue().getZExtValue();
183
184    // Case when there is pointer arithmetic on the destination buffer
185    // especially when we offset from the base decreasing the
186    // buffer length accordingly.
187    if (!DstArgDRE) {
188      if (const auto *BE =
189              dyn_cast<BinaryOperator>(DstArg->IgnoreParenImpCasts())) {
190        DstArgDRE = dyn_cast<DeclRefExpr>(BE->getLHS()->IgnoreParenImpCasts());
191        if (BE->getOpcode() == BO_Add) {
192          if ((IL = dyn_cast<IntegerLiteral>(BE->getRHS()->IgnoreParenImpCasts()))) {
193            DstOff = IL->getValue().getZExtValue();
194          }
195        }
196      }
197    }
198    if (DstArgDRE) {
199      if (const auto *Buffer =
200              dyn_cast<ConstantArrayType>(DstArgDRE->getType())) {
201        ASTContext &C = BR.getContext();
202        uint64_t BufferLen = C.getTypeSize(Buffer) / 8;
203        auto RemainingBufferLen = BufferLen - DstOff;
204        if (RemainingBufferLen < ILRawVal)
205          return true;
206      }
207    }
208  }
209
210  return false;
211}
212
213void WalkAST::VisitCallExpr(CallExpr *CE) {
214  const FunctionDecl *FD = CE->getDirectCallee();
215  if (!FD)
216    return;
217
218  if (CheckerContext::isCLibraryFunction(FD, "strncat")) {
219    if (containsBadStrncatPattern(CE)) {
220      const Expr *DstArg = CE->getArg(0);
221      const Expr *LenArg = CE->getArg(2);
222      PathDiagnosticLocation Loc =
223        PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
224
225      StringRef DstName = getPrintableName(DstArg);
226
227      SmallString<256> S;
228      llvm::raw_svector_ostream os(S);
229      os << "Potential buffer overflow. ";
230      if (!DstName.empty()) {
231        os << "Replace with 'sizeof(" << DstName << ") "
232              "- strlen(" << DstName <<") - 1'";
233        os << " or u";
234      } else
235        os << "U";
236      os << "se a safer 'strlcat' API";
237
238      BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
239                         "C String API", os.str(), Loc,
240                         LenArg->getSourceRange());
241    }
242  } else if (CheckerContext::isCLibraryFunction(FD, "strlcpy") ||
243             CheckerContext::isCLibraryFunction(FD, "strlcat")) {
244    if (containsBadStrlcpyStrlcatPattern(CE)) {
245      const Expr *DstArg = CE->getArg(0);
246      const Expr *LenArg = CE->getArg(2);
247      PathDiagnosticLocation Loc =
248        PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
249
250      StringRef DstName = getPrintableName(DstArg);
251
252      SmallString<256> S;
253      llvm::raw_svector_ostream os(S);
254      os << "The third argument allows to potentially copy more bytes than it should. ";
255      os << "Replace with the value ";
256      if (!DstName.empty())
257          os << "sizeof(" << DstName << ")";
258      else
259          os << "sizeof(<destination buffer>)";
260      os << " or lower";
261
262      BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
263              "C String API", os.str(), Loc,
264              LenArg->getSourceRange());
265    }
266  }
267
268  // Recurse and check children.
269  VisitChildren(CE);
270}
271
272void WalkAST::VisitChildren(Stmt *S) {
273  for (Stmt *Child : S->children())
274    if (Child)
275      Visit(Child);
276}
277
278namespace {
279class CStringSyntaxChecker: public Checker<check::ASTCodeBody> {
280public:
281
282  void checkASTCodeBody(const Decl *D, AnalysisManager& Mgr,
283      BugReporter &BR) const {
284    WalkAST walker(this, BR, Mgr.getAnalysisDeclContext(D));
285    walker.Visit(D->getBody());
286  }
287};
288}
289
290void ento::registerCStringSyntaxChecker(CheckerManager &mgr) {
291  mgr.registerChecker<CStringSyntaxChecker>();
292}
293
294bool ento::shouldRegisterCStringSyntaxChecker(const LangOptions &LO) {
295  return true;
296}
297