297 lines · 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-exception6//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 number11// 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 number79 /// of bytes to copy.80 bool containsBadStrncatPattern(const CallExpr *CE);81 82 /// Identify erroneous patterns in the last argument to strlcpy - the number83 /// of bytes to copy.84 /// The bad pattern checked is when the size is known85 /// 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 number93 /// of bytes to copy.94 /// The bad pattern checked is when the last argument is basically95 /// pointing to the destination buffer size or argument larger or96 /// 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 namespace115 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) - 1139 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 value179 // We try to figure out if the last argument is possibly longer180 // 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 buffer185 // especially when we offset from the base decreasing the186 // 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 } else235 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 else259 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 CheckerManager &mgr) {295 return true;296}297