clang/lib/StaticAnalyzer/Checkers/CStringSyntaxChecker.cpp

   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
  28 using namespace clang;
  29 using namespace ento;
  30
  31 namespace {
  32 class 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
 103 public:
 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));
 122 bool 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
 153 bool 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
 213 void 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
 272 void WalkAST::VisitChildren(Stmt *S) {
 273   for (Stmt *Child : S->children())
 274     if (Child)
 275       Visit(Child);
 276 }
 277
 278 namespace {
 279 class CStringSyntaxChecker: public Checker<check::ASTCodeBody> {
 280 public:
 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
 290 void ento::registerCStringSyntaxChecker(CheckerManager &mgr) {
 291   mgr.registerChecker<CStringSyntaxChecker>();
 292 }
 293
 294 bool ento::shouldRegisterCStringSyntaxChecker(const CheckerManager &mgr) {
 295   return true;
 296 }