xref: /llvm-project/clang/lib/StaticAnalyzer/Checkers/CStringSyntaxChecker.cpp (revision 2ea81639bd61c4480ba57535cb142017b07e2633) 
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"
17 #include "clang/AST/OperationKinds.h"
18 #include "clang/AST/StmtVisitor.h"
19 #include "clang/Analysis/AnalysisDeclContext.h"
20 #include "clang/Basic/TargetInfo.h"
21 #include "clang/Basic/TypeTraits.h"
22 #include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
23 #include "clang/StaticAnalyzer/Core/Checker.h"
24 #include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
25 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.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;
36   AnalysisDeclContext* AC;
37 
38   /// Check if two expressions refer to the same declaration.
39   bool sameDecl(const Expr *A1, const Expr *A2) {
40     if (const auto *D1 = dyn_cast<DeclRefExpr>(A1->IgnoreParenCasts()))
41       if (const auto *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   bool isSizeof(const Expr *E, const Expr *WithArg) {
48     if (const auto *UE = dyn_cast<UnaryExprOrTypeTraitExpr>(E))
49       if (UE->getKind() == UETT_SizeOf && !UE->isArgumentType())
50         return sameDecl(UE->getArgumentExpr(), WithArg);
51     return false;
52   }
53 
54   /// Check if the expression E is a strlen(WithArg).
55   bool isStrlen(const Expr *E, const Expr *WithArg) {
56     if (const auto *CE = dyn_cast<CallExpr>(E)) {
57       const FunctionDecl *FD = CE->getDirectCallee();
58       if (!FD)
59         return false;
60       return (CheckerContext::isCLibraryFunction(FD, "strlen") &&
61               sameDecl(CE->getArg(0), WithArg));
62     }
63     return false;
64   }
65 
66   /// Check if the expression is an integer literal with value 1.
67   bool isOne(const Expr *E) {
68     if (const auto *IL = dyn_cast<IntegerLiteral>(E))
69       return (IL->getValue().isIntN(1));
70     return false;
71   }
72 
73   StringRef getPrintableName(const Expr *E) {
74     if (const auto *D = dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
75       return D->getDecl()->getName();
76     return StringRef();
77   }
78 
79   /// Identify erroneous patterns in the last argument to strncat - the number
80   /// of bytes to copy.
81   bool containsBadStrncatPattern(const CallExpr *CE);
82 
83   /// Identify erroneous patterns in the last argument to strlcpy - the number
84   /// of bytes to copy.
85   /// The bad pattern checked is when the size is known
86   /// to be larger than the destination can handle.
87   ///   char dst[2];
88   ///   size_t cpy = 4;
89   ///   strlcpy(dst, "abcd", sizeof("abcd") - 1);
90   ///   strlcpy(dst, "abcd", 4);
91   ///   strlcpy(dst + 3, "abcd", 2);
92   ///   strlcpy(dst, "abcd", cpy);
93   bool containsBadStrlcpyPattern(const CallExpr *CE);
94 
95 public:
96   WalkAST(const CheckerBase *Checker, BugReporter &BR, AnalysisDeclContext *AC)
97       : Checker(Checker), BR(BR), AC(AC) {}
98 
99   // Statement visitor methods.
100   void VisitChildren(Stmt *S);
101   void VisitStmt(Stmt *S) {
102     VisitChildren(S);
103   }
104   void VisitCallExpr(CallExpr *CE);
105 };
106 } // end anonymous namespace
107 
108 // The correct size argument should look like following:
109 //   strncat(dst, src, sizeof(dst) - strlen(dest) - 1);
110 // We look for the following anti-patterns:
111 //   - strncat(dst, src, sizeof(dst) - strlen(dst));
112 //   - strncat(dst, src, sizeof(dst) - 1);
113 //   - strncat(dst, src, sizeof(dst));
114 bool WalkAST::containsBadStrncatPattern(const CallExpr *CE) {
115   if (CE->getNumArgs() != 3)
116     return false;
117   const Expr *DstArg = CE->getArg(0);
118   const Expr *SrcArg = CE->getArg(1);
119   const Expr *LenArg = CE->getArg(2);
120 
121   // Identify wrong size expressions, which are commonly used instead.
122   if (const auto *BE = dyn_cast<BinaryOperator>(LenArg->IgnoreParenCasts())) {
123     // - sizeof(dst) - strlen(dst)
124     if (BE->getOpcode() == BO_Sub) {
125       const Expr *L = BE->getLHS();
126       const Expr *R = BE->getRHS();
127       if (isSizeof(L, DstArg) && isStrlen(R, DstArg))
128         return true;
129 
130       // - sizeof(dst) - 1
131       if (isSizeof(L, DstArg) && isOne(R->IgnoreParenCasts()))
132         return true;
133     }
134   }
135   // - sizeof(dst)
136   if (isSizeof(LenArg, DstArg))
137     return true;
138 
139   // - sizeof(src)
140   if (isSizeof(LenArg, SrcArg))
141     return true;
142   return false;
143 }
144 
145 bool WalkAST::containsBadStrlcpyPattern(const CallExpr *CE) {
146   if (CE->getNumArgs() != 3)
147     return false;
148   const Expr *DstArg = CE->getArg(0);
149   const Expr *LenArg = CE->getArg(2);
150 
151   const auto *DstArgDecl = dyn_cast<DeclRefExpr>(DstArg->IgnoreParenImpCasts());
152   const auto *LenArgDecl = dyn_cast<DeclRefExpr>(LenArg->IgnoreParenLValueCasts());
153   uint64_t DstOff = 0;
154   // - size_t dstlen = sizeof(dst)
155   if (LenArgDecl) {
156     const auto *LenArgVal = dyn_cast<VarDecl>(LenArgDecl->getDecl());
157     if (LenArgVal->getInit())
158       LenArg = LenArgVal->getInit();
159   }
160 
161   // - integral value
162   // We try to figure out if the last argument is possibly longer
163   // than the destination can possibly handle if its size can be defined.
164   if (const auto *IL = dyn_cast<IntegerLiteral>(LenArg->IgnoreParenImpCasts())) {
165     uint64_t ILRawVal = IL->getValue().getZExtValue();
166 
167     // Case when there is pointer arithmetic on the destination buffer
168     // especially when we offset from the base decreasing the
169     // buffer length accordingly.
170     if (!DstArgDecl) {
171       if (const auto *BE = dyn_cast<BinaryOperator>(DstArg->IgnoreParenImpCasts())) {
172         DstArgDecl = dyn_cast<DeclRefExpr>(BE->getLHS()->IgnoreParenImpCasts());
173         if (BE->getOpcode() == BO_Add) {
174           if ((IL = dyn_cast<IntegerLiteral>(BE->getRHS()->IgnoreParenImpCasts()))) {
175             DstOff = IL->getValue().getZExtValue();
176           }
177         }
178       }
179     }
180     if (DstArgDecl) {
181       if (const auto *Buffer = dyn_cast<ConstantArrayType>(DstArgDecl->getType())) {
182         ASTContext &C = BR.getContext();
183         uint64_t BufferLen = C.getTypeSize(Buffer) / 8;
184         if ((BufferLen - DstOff) < ILRawVal)
185           return true;
186       }
187     }
188   }
189 
190   return false;
191 }
192 
193 void WalkAST::VisitCallExpr(CallExpr *CE) {
194   const FunctionDecl *FD = CE->getDirectCallee();
195   if (!FD)
196     return;
197 
198   if (CheckerContext::isCLibraryFunction(FD, "strncat")) {
199     if (containsBadStrncatPattern(CE)) {
200       const Expr *DstArg = CE->getArg(0);
201       const Expr *LenArg = CE->getArg(2);
202       PathDiagnosticLocation Loc =
203         PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
204 
205       StringRef DstName = getPrintableName(DstArg);
206 
207       SmallString<256> S;
208       llvm::raw_svector_ostream os(S);
209       os << "Potential buffer overflow. ";
210       if (!DstName.empty()) {
211         os << "Replace with 'sizeof(" << DstName << ") "
212               "- strlen(" << DstName <<") - 1'";
213         os << " or u";
214       } else
215         os << "U";
216       os << "se a safer 'strlcat' API";
217 
218       BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
219                          "C String API", os.str(), Loc,
220                          LenArg->getSourceRange());
221     }
222   } else if (CheckerContext::isCLibraryFunction(FD, "strlcpy")) {
223     if (containsBadStrlcpyPattern(CE)) {
224       const Expr *DstArg = CE->getArg(0);
225       const Expr *LenArg = CE->getArg(2);
226       PathDiagnosticLocation Loc =
227         PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
228 
229       StringRef DstName = getPrintableName(DstArg);
230 
231       SmallString<256> S;
232       llvm::raw_svector_ostream os(S);
233       os << "The third argument is larger than the size of the input buffer. ";
234       if (!DstName.empty())
235         os << "Replace with the value 'sizeof(" << DstName << ")` or lower";
236 
237       BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
238                          "C String API", os.str(), Loc,
239                          LenArg->getSourceRange());
240     }
241   }
242 
243   // Recurse and check children.
244   VisitChildren(CE);
245 }
246 
247 void WalkAST::VisitChildren(Stmt *S) {
248   for (Stmt *Child : S->children())
249     if (Child)
250       Visit(Child);
251 }
252 
253 namespace {
254 class CStringSyntaxChecker: public Checker<check::ASTCodeBody> {
255 public:
256 
257   void checkASTCodeBody(const Decl *D, AnalysisManager& Mgr,
258       BugReporter &BR) const {
259     WalkAST walker(this, BR, Mgr.getAnalysisDeclContext(D));
260     walker.Visit(D->getBody());
261   }
262 };
263 }
264 
265 void ento::registerCStringSyntaxChecker(CheckerManager &mgr) {
266   mgr.registerChecker<CStringSyntaxChecker>();
267 }
268 
269