1e5dd7070Spatrick //== CStringSyntaxChecker.cpp - CoreFoundation containers API *- C++ -*-==//
2e5dd7070Spatrick //
3e5dd7070Spatrick // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4e5dd7070Spatrick // See https://llvm.org/LICENSE.txt for license information.
5e5dd7070Spatrick // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6e5dd7070Spatrick //
7e5dd7070Spatrick //===----------------------------------------------------------------------===//
8e5dd7070Spatrick //
9e5dd7070Spatrick // An AST checker that looks for common pitfalls when using C string APIs.
10e5dd7070Spatrick // - Identifies erroneous patterns in the last argument to strncat - the number
11e5dd7070Spatrick // of bytes to copy.
12e5dd7070Spatrick //
13e5dd7070Spatrick //===----------------------------------------------------------------------===//
14e5dd7070Spatrick #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
15e5dd7070Spatrick #include "clang/AST/Expr.h"
16e5dd7070Spatrick #include "clang/AST/OperationKinds.h"
17e5dd7070Spatrick #include "clang/AST/StmtVisitor.h"
18e5dd7070Spatrick #include "clang/Analysis/AnalysisDeclContext.h"
19e5dd7070Spatrick #include "clang/Basic/TargetInfo.h"
20e5dd7070Spatrick #include "clang/Basic/TypeTraits.h"
21e5dd7070Spatrick #include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
22e5dd7070Spatrick #include "clang/StaticAnalyzer/Core/Checker.h"
23e5dd7070Spatrick #include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
24e5dd7070Spatrick #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
25e5dd7070Spatrick #include "llvm/ADT/SmallString.h"
26e5dd7070Spatrick #include "llvm/Support/raw_ostream.h"
27e5dd7070Spatrick
28e5dd7070Spatrick using namespace clang;
29e5dd7070Spatrick using namespace ento;
30e5dd7070Spatrick
31e5dd7070Spatrick namespace {
32e5dd7070Spatrick class WalkAST: public StmtVisitor<WalkAST> {
33e5dd7070Spatrick const CheckerBase *Checker;
34e5dd7070Spatrick BugReporter &BR;
35e5dd7070Spatrick AnalysisDeclContext* AC;
36e5dd7070Spatrick
37e5dd7070Spatrick /// Check if two expressions refer to the same declaration.
sameDecl(const Expr * A1,const Expr * A2)38e5dd7070Spatrick bool sameDecl(const Expr *A1, const Expr *A2) {
39e5dd7070Spatrick if (const auto *D1 = dyn_cast<DeclRefExpr>(A1->IgnoreParenCasts()))
40e5dd7070Spatrick if (const auto *D2 = dyn_cast<DeclRefExpr>(A2->IgnoreParenCasts()))
41e5dd7070Spatrick return D1->getDecl() == D2->getDecl();
42e5dd7070Spatrick return false;
43e5dd7070Spatrick }
44e5dd7070Spatrick
45e5dd7070Spatrick /// Check if the expression E is a sizeof(WithArg).
isSizeof(const Expr * E,const Expr * WithArg)46e5dd7070Spatrick bool isSizeof(const Expr *E, const Expr *WithArg) {
47e5dd7070Spatrick if (const auto *UE = dyn_cast<UnaryExprOrTypeTraitExpr>(E))
48e5dd7070Spatrick if (UE->getKind() == UETT_SizeOf && !UE->isArgumentType())
49e5dd7070Spatrick return sameDecl(UE->getArgumentExpr(), WithArg);
50e5dd7070Spatrick return false;
51e5dd7070Spatrick }
52e5dd7070Spatrick
53e5dd7070Spatrick /// Check if the expression E is a strlen(WithArg).
isStrlen(const Expr * E,const Expr * WithArg)54e5dd7070Spatrick bool isStrlen(const Expr *E, const Expr *WithArg) {
55e5dd7070Spatrick if (const auto *CE = dyn_cast<CallExpr>(E)) {
56e5dd7070Spatrick const FunctionDecl *FD = CE->getDirectCallee();
57e5dd7070Spatrick if (!FD)
58e5dd7070Spatrick return false;
59e5dd7070Spatrick return (CheckerContext::isCLibraryFunction(FD, "strlen") &&
60e5dd7070Spatrick sameDecl(CE->getArg(0), WithArg));
61e5dd7070Spatrick }
62e5dd7070Spatrick return false;
63e5dd7070Spatrick }
64e5dd7070Spatrick
65e5dd7070Spatrick /// Check if the expression is an integer literal with value 1.
isOne(const Expr * E)66e5dd7070Spatrick bool isOne(const Expr *E) {
67e5dd7070Spatrick if (const auto *IL = dyn_cast<IntegerLiteral>(E))
68e5dd7070Spatrick return (IL->getValue().isIntN(1));
69e5dd7070Spatrick return false;
70e5dd7070Spatrick }
71e5dd7070Spatrick
getPrintableName(const Expr * E)72e5dd7070Spatrick StringRef getPrintableName(const Expr *E) {
73e5dd7070Spatrick if (const auto *D = dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
74e5dd7070Spatrick return D->getDecl()->getName();
75e5dd7070Spatrick return StringRef();
76e5dd7070Spatrick }
77e5dd7070Spatrick
78e5dd7070Spatrick /// Identify erroneous patterns in the last argument to strncat - the number
79e5dd7070Spatrick /// of bytes to copy.
80e5dd7070Spatrick bool containsBadStrncatPattern(const CallExpr *CE);
81e5dd7070Spatrick
82e5dd7070Spatrick /// Identify erroneous patterns in the last argument to strlcpy - the number
83e5dd7070Spatrick /// of bytes to copy.
84e5dd7070Spatrick /// The bad pattern checked is when the size is known
85e5dd7070Spatrick /// to be larger than the destination can handle.
86e5dd7070Spatrick /// char dst[2];
87e5dd7070Spatrick /// size_t cpy = 4;
88e5dd7070Spatrick /// strlcpy(dst, "abcd", sizeof("abcd") - 1);
89e5dd7070Spatrick /// strlcpy(dst, "abcd", 4);
90e5dd7070Spatrick /// strlcpy(dst + 3, "abcd", 2);
91e5dd7070Spatrick /// strlcpy(dst, "abcd", cpy);
92e5dd7070Spatrick /// Identify erroneous patterns in the last argument to strlcat - the number
93e5dd7070Spatrick /// of bytes to copy.
94e5dd7070Spatrick /// The bad pattern checked is when the last argument is basically
95e5dd7070Spatrick /// pointing to the destination buffer size or argument larger or
96e5dd7070Spatrick /// equal to.
97e5dd7070Spatrick /// char dst[2];
98e5dd7070Spatrick /// strlcat(dst, src2, sizeof(dst));
99e5dd7070Spatrick /// strlcat(dst, src2, 2);
100e5dd7070Spatrick /// strlcat(dst, src2, 10);
101e5dd7070Spatrick bool containsBadStrlcpyStrlcatPattern(const CallExpr *CE);
102e5dd7070Spatrick
103e5dd7070Spatrick public:
WalkAST(const CheckerBase * Checker,BugReporter & BR,AnalysisDeclContext * AC)104e5dd7070Spatrick WalkAST(const CheckerBase *Checker, BugReporter &BR, AnalysisDeclContext *AC)
105e5dd7070Spatrick : Checker(Checker), BR(BR), AC(AC) {}
106e5dd7070Spatrick
107e5dd7070Spatrick // Statement visitor methods.
108e5dd7070Spatrick void VisitChildren(Stmt *S);
VisitStmt(Stmt * S)109e5dd7070Spatrick void VisitStmt(Stmt *S) {
110e5dd7070Spatrick VisitChildren(S);
111e5dd7070Spatrick }
112e5dd7070Spatrick void VisitCallExpr(CallExpr *CE);
113e5dd7070Spatrick };
114e5dd7070Spatrick } // end anonymous namespace
115e5dd7070Spatrick
116e5dd7070Spatrick // The correct size argument should look like following:
117e5dd7070Spatrick // strncat(dst, src, sizeof(dst) - strlen(dest) - 1);
118e5dd7070Spatrick // We look for the following anti-patterns:
119e5dd7070Spatrick // - strncat(dst, src, sizeof(dst) - strlen(dst));
120e5dd7070Spatrick // - strncat(dst, src, sizeof(dst) - 1);
121e5dd7070Spatrick // - strncat(dst, src, sizeof(dst));
containsBadStrncatPattern(const CallExpr * CE)122e5dd7070Spatrick bool WalkAST::containsBadStrncatPattern(const CallExpr *CE) {
123e5dd7070Spatrick if (CE->getNumArgs() != 3)
124e5dd7070Spatrick return false;
125e5dd7070Spatrick const Expr *DstArg = CE->getArg(0);
126e5dd7070Spatrick const Expr *SrcArg = CE->getArg(1);
127e5dd7070Spatrick const Expr *LenArg = CE->getArg(2);
128e5dd7070Spatrick
129e5dd7070Spatrick // Identify wrong size expressions, which are commonly used instead.
130e5dd7070Spatrick if (const auto *BE = dyn_cast<BinaryOperator>(LenArg->IgnoreParenCasts())) {
131e5dd7070Spatrick // - sizeof(dst) - strlen(dst)
132e5dd7070Spatrick if (BE->getOpcode() == BO_Sub) {
133e5dd7070Spatrick const Expr *L = BE->getLHS();
134e5dd7070Spatrick const Expr *R = BE->getRHS();
135e5dd7070Spatrick if (isSizeof(L, DstArg) && isStrlen(R, DstArg))
136e5dd7070Spatrick return true;
137e5dd7070Spatrick
138e5dd7070Spatrick // - sizeof(dst) - 1
139e5dd7070Spatrick if (isSizeof(L, DstArg) && isOne(R->IgnoreParenCasts()))
140e5dd7070Spatrick return true;
141e5dd7070Spatrick }
142e5dd7070Spatrick }
143e5dd7070Spatrick // - sizeof(dst)
144e5dd7070Spatrick if (isSizeof(LenArg, DstArg))
145e5dd7070Spatrick return true;
146e5dd7070Spatrick
147e5dd7070Spatrick // - sizeof(src)
148e5dd7070Spatrick if (isSizeof(LenArg, SrcArg))
149e5dd7070Spatrick return true;
150e5dd7070Spatrick return false;
151e5dd7070Spatrick }
152e5dd7070Spatrick
containsBadStrlcpyStrlcatPattern(const CallExpr * CE)153e5dd7070Spatrick bool WalkAST::containsBadStrlcpyStrlcatPattern(const CallExpr *CE) {
154e5dd7070Spatrick if (CE->getNumArgs() != 3)
155e5dd7070Spatrick return false;
156e5dd7070Spatrick const Expr *DstArg = CE->getArg(0);
157e5dd7070Spatrick const Expr *LenArg = CE->getArg(2);
158e5dd7070Spatrick
159e5dd7070Spatrick const auto *DstArgDRE = dyn_cast<DeclRefExpr>(DstArg->IgnoreParenImpCasts());
160e5dd7070Spatrick const auto *LenArgDRE =
161e5dd7070Spatrick dyn_cast<DeclRefExpr>(LenArg->IgnoreParenLValueCasts());
162e5dd7070Spatrick uint64_t DstOff = 0;
163e5dd7070Spatrick if (isSizeof(LenArg, DstArg))
164e5dd7070Spatrick return false;
165e5dd7070Spatrick
166e5dd7070Spatrick // - size_t dstlen = sizeof(dst)
167e5dd7070Spatrick if (LenArgDRE) {
168e5dd7070Spatrick const auto *LenArgVal = dyn_cast<VarDecl>(LenArgDRE->getDecl());
169e5dd7070Spatrick // If it's an EnumConstantDecl instead, then we're missing out on something.
170e5dd7070Spatrick if (!LenArgVal) {
171e5dd7070Spatrick assert(isa<EnumConstantDecl>(LenArgDRE->getDecl()));
172e5dd7070Spatrick return false;
173e5dd7070Spatrick }
174e5dd7070Spatrick if (LenArgVal->getInit())
175e5dd7070Spatrick LenArg = LenArgVal->getInit();
176e5dd7070Spatrick }
177e5dd7070Spatrick
178e5dd7070Spatrick // - integral value
179e5dd7070Spatrick // We try to figure out if the last argument is possibly longer
180e5dd7070Spatrick // than the destination can possibly handle if its size can be defined.
181e5dd7070Spatrick if (const auto *IL = dyn_cast<IntegerLiteral>(LenArg->IgnoreParenImpCasts())) {
182e5dd7070Spatrick uint64_t ILRawVal = IL->getValue().getZExtValue();
183e5dd7070Spatrick
184e5dd7070Spatrick // Case when there is pointer arithmetic on the destination buffer
185e5dd7070Spatrick // especially when we offset from the base decreasing the
186e5dd7070Spatrick // buffer length accordingly.
187e5dd7070Spatrick if (!DstArgDRE) {
188e5dd7070Spatrick if (const auto *BE =
189e5dd7070Spatrick dyn_cast<BinaryOperator>(DstArg->IgnoreParenImpCasts())) {
190e5dd7070Spatrick DstArgDRE = dyn_cast<DeclRefExpr>(BE->getLHS()->IgnoreParenImpCasts());
191e5dd7070Spatrick if (BE->getOpcode() == BO_Add) {
192e5dd7070Spatrick if ((IL = dyn_cast<IntegerLiteral>(BE->getRHS()->IgnoreParenImpCasts()))) {
193e5dd7070Spatrick DstOff = IL->getValue().getZExtValue();
194e5dd7070Spatrick }
195e5dd7070Spatrick }
196e5dd7070Spatrick }
197e5dd7070Spatrick }
198e5dd7070Spatrick if (DstArgDRE) {
199e5dd7070Spatrick if (const auto *Buffer =
200e5dd7070Spatrick dyn_cast<ConstantArrayType>(DstArgDRE->getType())) {
201e5dd7070Spatrick ASTContext &C = BR.getContext();
202e5dd7070Spatrick uint64_t BufferLen = C.getTypeSize(Buffer) / 8;
203e5dd7070Spatrick auto RemainingBufferLen = BufferLen - DstOff;
204e5dd7070Spatrick if (RemainingBufferLen < ILRawVal)
205e5dd7070Spatrick return true;
206e5dd7070Spatrick }
207e5dd7070Spatrick }
208e5dd7070Spatrick }
209e5dd7070Spatrick
210e5dd7070Spatrick return false;
211e5dd7070Spatrick }
212e5dd7070Spatrick
VisitCallExpr(CallExpr * CE)213e5dd7070Spatrick void WalkAST::VisitCallExpr(CallExpr *CE) {
214e5dd7070Spatrick const FunctionDecl *FD = CE->getDirectCallee();
215e5dd7070Spatrick if (!FD)
216e5dd7070Spatrick return;
217e5dd7070Spatrick
218e5dd7070Spatrick if (CheckerContext::isCLibraryFunction(FD, "strncat")) {
219e5dd7070Spatrick if (containsBadStrncatPattern(CE)) {
220e5dd7070Spatrick const Expr *DstArg = CE->getArg(0);
221e5dd7070Spatrick const Expr *LenArg = CE->getArg(2);
222e5dd7070Spatrick PathDiagnosticLocation Loc =
223e5dd7070Spatrick PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
224e5dd7070Spatrick
225e5dd7070Spatrick StringRef DstName = getPrintableName(DstArg);
226e5dd7070Spatrick
227e5dd7070Spatrick SmallString<256> S;
228e5dd7070Spatrick llvm::raw_svector_ostream os(S);
229e5dd7070Spatrick os << "Potential buffer overflow. ";
230e5dd7070Spatrick if (!DstName.empty()) {
231e5dd7070Spatrick os << "Replace with 'sizeof(" << DstName << ") "
232e5dd7070Spatrick "- strlen(" << DstName <<") - 1'";
233e5dd7070Spatrick os << " or u";
234e5dd7070Spatrick } else
235e5dd7070Spatrick os << "U";
236e5dd7070Spatrick os << "se a safer 'strlcat' API";
237e5dd7070Spatrick
238e5dd7070Spatrick BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
239e5dd7070Spatrick "C String API", os.str(), Loc,
240e5dd7070Spatrick LenArg->getSourceRange());
241e5dd7070Spatrick }
242e5dd7070Spatrick } else if (CheckerContext::isCLibraryFunction(FD, "strlcpy") ||
243e5dd7070Spatrick CheckerContext::isCLibraryFunction(FD, "strlcat")) {
244e5dd7070Spatrick if (containsBadStrlcpyStrlcatPattern(CE)) {
245e5dd7070Spatrick const Expr *DstArg = CE->getArg(0);
246e5dd7070Spatrick const Expr *LenArg = CE->getArg(2);
247e5dd7070Spatrick PathDiagnosticLocation Loc =
248e5dd7070Spatrick PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
249e5dd7070Spatrick
250e5dd7070Spatrick StringRef DstName = getPrintableName(DstArg);
251e5dd7070Spatrick
252e5dd7070Spatrick SmallString<256> S;
253e5dd7070Spatrick llvm::raw_svector_ostream os(S);
254e5dd7070Spatrick os << "The third argument allows to potentially copy more bytes than it should. ";
255e5dd7070Spatrick os << "Replace with the value ";
256e5dd7070Spatrick if (!DstName.empty())
257e5dd7070Spatrick os << "sizeof(" << DstName << ")";
258e5dd7070Spatrick else
259e5dd7070Spatrick os << "sizeof(<destination buffer>)";
260e5dd7070Spatrick os << " or lower";
261e5dd7070Spatrick
262e5dd7070Spatrick BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
263e5dd7070Spatrick "C String API", os.str(), Loc,
264e5dd7070Spatrick LenArg->getSourceRange());
265e5dd7070Spatrick }
266e5dd7070Spatrick }
267e5dd7070Spatrick
268e5dd7070Spatrick // Recurse and check children.
269e5dd7070Spatrick VisitChildren(CE);
270e5dd7070Spatrick }
271e5dd7070Spatrick
VisitChildren(Stmt * S)272e5dd7070Spatrick void WalkAST::VisitChildren(Stmt *S) {
273e5dd7070Spatrick for (Stmt *Child : S->children())
274e5dd7070Spatrick if (Child)
275e5dd7070Spatrick Visit(Child);
276e5dd7070Spatrick }
277e5dd7070Spatrick
278e5dd7070Spatrick namespace {
279e5dd7070Spatrick class CStringSyntaxChecker: public Checker<check::ASTCodeBody> {
280e5dd7070Spatrick public:
281e5dd7070Spatrick
checkASTCodeBody(const Decl * D,AnalysisManager & Mgr,BugReporter & BR) const282e5dd7070Spatrick void checkASTCodeBody(const Decl *D, AnalysisManager& Mgr,
283e5dd7070Spatrick BugReporter &BR) const {
284e5dd7070Spatrick WalkAST walker(this, BR, Mgr.getAnalysisDeclContext(D));
285e5dd7070Spatrick walker.Visit(D->getBody());
286e5dd7070Spatrick }
287e5dd7070Spatrick };
288e5dd7070Spatrick }
289e5dd7070Spatrick
registerCStringSyntaxChecker(CheckerManager & mgr)290e5dd7070Spatrick void ento::registerCStringSyntaxChecker(CheckerManager &mgr) {
291e5dd7070Spatrick mgr.registerChecker<CStringSyntaxChecker>();
292e5dd7070Spatrick }
293e5dd7070Spatrick
shouldRegisterCStringSyntaxChecker(const CheckerManager & mgr)294*ec727ea7Spatrick bool ento::shouldRegisterCStringSyntaxChecker(const CheckerManager &mgr) {
295e5dd7070Spatrick return true;
296e5dd7070Spatrick }
297