//== CStringSyntaxChecker.cpp - CoreFoundation containers API *- C++ -*-==//

//

// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.

// See https://llvm.org/LICENSE.txt for license information.

// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

//

//===----------------------------------------------------------------------===//

//

// An AST checker that looks for common pitfalls when using C string APIs.

//  - Identifies erroneous patterns in the last argument to strncat - the number

//    of bytes to copy.

//

//===----------------------------------------------------------------------===//

#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"

#include "clang/AST/Expr.h"

#include "clang/AST/OperationKinds.h"

#include "clang/AST/StmtVisitor.h"

#include "clang/Analysis/AnalysisDeclContext.h"

#include "clang/Basic/TargetInfo.h"

#include "clang/Basic/TypeTraits.h"

#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"

#include "clang/StaticAnalyzer/Core/Checker.h"

#include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"

#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"

#include "llvm/ADT/SmallString.h"

#include "llvm/Support/raw_ostream.h"

using namespace clang;

using namespace ento;

namespace {

class WalkAST: public StmtVisitor<WalkAST> {

  const CheckerBase *Checker;

  BugReporter &BR;

  AnalysisDeclContext* AC;

  /// Check if two expressions refer to the same declaration.

  bool sameDecl(const Expr *A1, const Expr *A2) {

    if (const auto *D1 = dyn_cast<DeclRefExpr>(A1->IgnoreParenCasts()))

      if (const auto *D2 = dyn_cast<DeclRefExpr>(A2->IgnoreParenCasts()))

        return D1->getDecl() == D2->getDecl();

    return false;

  }

  /// Check if the expression E is a sizeof(WithArg).

  bool isSizeof(const Expr *E, const Expr *WithArg) {

    if (const auto *UE = dyn_cast<UnaryExprOrTypeTraitExpr>(E))

      if (UE->getKind() == UETT_SizeOf && !UE->isArgumentType())

        return sameDecl(UE->getArgumentExpr(), WithArg);

    return false;

  }

  /// Check if the expression E is a strlen(WithArg).

  bool isStrlen(const Expr *E, const Expr *WithArg) {

    if (const auto *CE = dyn_cast<CallExpr>(E)) {

      const FunctionDecl *FD = CE->getDirectCallee();

      if (!FD)

        return false;

      return (CheckerContext::isCLibraryFunction(FD, "strlen") &&

              sameDecl(CE->getArg(0), WithArg));

    }

    return false;

  }

  /// Check if the expression is an integer literal with value 1.

  bool isOne(const Expr *E) {

    if (const auto *IL = dyn_cast<IntegerLiteral>(E))

      return (IL->getValue().isIntN(1));

    return false;

  }

  StringRef getPrintableName(const Expr *E) {

    if (const auto *D = dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))

      return D->getDecl()->getName();

    return StringRef();

  }

  /// Identify erroneous patterns in the last argument to strncat - the number

  /// of bytes to copy.

  bool containsBadStrncatPattern(const CallExpr *CE);

  /// Identify erroneous patterns in the last argument to strlcpy - the number

  /// of bytes to copy.

  /// The bad pattern checked is when the size is known

  /// to be larger than the destination can handle.

  ///   char dst[2];

  ///   size_t cpy = 4;

  ///   strlcpy(dst, "abcd", sizeof("abcd") - 1);

  ///   strlcpy(dst, "abcd", 4);

  ///   strlcpy(dst + 3, "abcd", 2);

  ///   strlcpy(dst, "abcd", cpy);

  /// Identify erroneous patterns in the last argument to strlcat - the number

  /// of bytes to copy.

  /// The bad pattern checked is when the last argument is basically

  /// pointing to the destination buffer size or argument larger or

  /// equal to.

  ///   char dst[2];

  ///   strlcat(dst, src2, sizeof(dst));

  ///   strlcat(dst, src2, 2);

  ///   strlcat(dst, src2, 10);

  bool containsBadStrlcpyStrlcatPattern(const CallExpr *CE);

public:

  WalkAST(const CheckerBase *Checker, BugReporter &BR, AnalysisDeclContext *AC)

      : Checker(Checker), BR(BR), AC(AC) {}

  // Statement visitor methods.

  void VisitChildren(Stmt *S);

  void VisitStmt(Stmt *S) {

    VisitChildren(S);

  }

  void VisitCallExpr(CallExpr *CE);

};

} // end anonymous namespace

// The correct size argument should look like following:

//   strncat(dst, src, sizeof(dst) - strlen(dest) - 1);

// We look for the following anti-patterns:

//   - strncat(dst, src, sizeof(dst) - strlen(dst));

//   - strncat(dst, src, sizeof(dst) - 1);

//   - strncat(dst, src, sizeof(dst));

bool WalkAST::containsBadStrncatPattern(const CallExpr *CE) {

  if (CE->getNumArgs() != 3)

    return false;

  const Expr *DstArg = CE->getArg(0);

  const Expr *SrcArg = CE->getArg(1);

  const Expr *LenArg = CE->getArg(2);

  // Identify wrong size expressions, which are commonly used instead.

  if (const auto *BE = dyn_cast<BinaryOperator>(LenArg->IgnoreParenCasts())) {

    // - sizeof(dst) - strlen(dst)

    if (BE->getOpcode() == BO_Sub) {

      const Expr *L = BE->getLHS();

      const Expr *R = BE->getRHS();

      if (isSizeof(L, DstArg) && isStrlen(R, DstArg))

        return true;

      // - sizeof(dst) - 1

      if (isSizeof(L, DstArg) && isOne(R->IgnoreParenCasts()))

        return true;

    }

  }

  // - sizeof(dst)

  if (isSizeof(LenArg, DstArg))

    return true;

  // - sizeof(src)

  if (isSizeof(LenArg, SrcArg))

    return true;

  return false;

}

bool WalkAST::containsBadStrlcpyStrlcatPattern(const CallExpr *CE) {

  if (CE->getNumArgs() != 3)

    return false;

  const Expr *DstArg = CE->getArg(0);

  const Expr *LenArg = CE->getArg(2);

  const auto *DstArgDRE = dyn_cast<DeclRefExpr>(DstArg->IgnoreParenImpCasts());

  const auto *LenArgDRE =

      dyn_cast<DeclRefExpr>(LenArg->IgnoreParenLValueCasts());

  uint64_t DstOff = 0;

  if (isSizeof(LenArg, DstArg))

    return false;

  // - size_t dstlen = sizeof(dst)

  if (LenArgDRE) {

    const auto *LenArgVal = dyn_cast<VarDecl>(LenArgDRE->getDecl());

    // If it's an EnumConstantDecl instead, then we're missing out on something.

    if (!LenArgVal) {

      assert(isa<EnumConstantDecl>(LenArgDRE->getDecl()));

      return false;

    }

    if (LenArgVal->getInit())

      LenArg = LenArgVal->getInit();

  }

  // - integral value

  // We try to figure out if the last argument is possibly longer

  // than the destination can possibly handle if its size can be defined.

  if (const auto *IL = dyn_cast<IntegerLiteral>(LenArg->IgnoreParenImpCasts())) {

    uint64_t ILRawVal = IL->getValue().getZExtValue();

    // Case when there is pointer arithmetic on the destination buffer

    // especially when we offset from the base decreasing the

    // buffer length accordingly.

    if (!DstArgDRE) {

      if (const auto *BE =

              dyn_cast<BinaryOperator>(DstArg->IgnoreParenImpCasts())) {

        DstArgDRE = dyn_cast<DeclRefExpr>(BE->getLHS()->IgnoreParenImpCasts());

        if (BE->getOpcode() == BO_Add) {

          if ((IL = dyn_cast<IntegerLiteral>(BE->getRHS()->IgnoreParenImpCasts()))) {

            DstOff = IL->getValue().getZExtValue();

          }

        }

      }

    }

    if (DstArgDRE) {

      if (const auto *Buffer =

              dyn_cast<ConstantArrayType>(DstArgDRE->getType())) {

        ASTContext &C = BR.getContext();

        uint64_t BufferLen = C.getTypeSize(Buffer) / 8;

        auto RemainingBufferLen = BufferLen - DstOff;

        if (RemainingBufferLen < ILRawVal)

          return true;

      }

    }

  }

  return false;

}

void WalkAST::VisitCallExpr(CallExpr *CE) {

  const FunctionDecl *FD = CE->getDirectCallee();

  if (!FD)

    return;

  if (CheckerContext::isCLibraryFunction(FD, "strncat")) {

    if (containsBadStrncatPattern(CE)) {

      const Expr *DstArg = CE->getArg(0);

      const Expr *LenArg = CE->getArg(2);

      PathDiagnosticLocation Loc =

        PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);

      StringRef DstName = getPrintableName(DstArg);

      SmallString<256> S;

      llvm::raw_svector_ostream os(S);

      os << "Potential buffer overflow. ";

      if (!DstName.empty()) {

        os << "Replace with 'sizeof(" << DstName << ") "

              "- strlen(" << DstName <<") - 1'";

        os << " or u";

      } else

        os << "U";

      os << "se a safer 'strlcat' API";

      BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",

                         "C String API", os.str(), Loc,

                         LenArg->getSourceRange());

    }

  } else if (CheckerContext::isCLibraryFunction(FD, "strlcpy") ||

             
CheckerContext::isCLibraryFunction(FD, "strlcat")5.12k
) {

    if (containsBadStrlcpyStrlcatPattern(CE)) {

      const Expr *DstArg = CE->getArg(0);

      const Expr *LenArg = CE->getArg(2);

      PathDiagnosticLocation Loc =

        PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);

      StringRef DstName = getPrintableName(DstArg);

      SmallString<256> S;

      llvm::raw_svector_ostream os(S);

      os << "The third argument allows to potentially copy more bytes than it should. ";

      os << "Replace with the value ";

      if (!DstName.empty())

          os << "sizeof(" << DstName << ")";

      else

          os << "sizeof(<destination buffer>)";

      os << " or lower";

      BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",

              "C String API", os.str(), Loc,

              LenArg->getSourceRange());

    }

  }

  // Recurse and check children.

  VisitChildren(CE);

}

void WalkAST::VisitChildren(Stmt *S) {

  for (Stmt *Child : S->children())

    if (Child)

      Visit(Child);

}

namespace {

class CStringSyntaxChecker: public Checker<check::ASTCodeBody> {

public:

  void checkASTCodeBody(const Decl *D, AnalysisManager& Mgr,

      BugReporter &BR) const {

    WalkAST walker(this, BR, Mgr.getAnalysisDeclContext(D));

    walker.Visit(D->getBody());

  }

};

}

void ento::registerCStringSyntaxChecker(CheckerManager &mgr) {

  mgr.registerChecker<CStringSyntaxChecker>();

}

bool ento::shouldRegisterCStringSyntaxChecker(const CheckerManager &mgr) {

  return true;

}