1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
| //== 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")) {
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 LangOptions &LO) {
return true;
}
|