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
|
//== CStringSyntaxChecker.cpp - CoreFoundation containers API *- C++ -*-==//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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 "ClangSACheckers.h"
#include "clang/Analysis/AnalysisContext.h"
#include "clang/AST/Expr.h"
#include "clang/AST/OperationKinds.h"
#include "clang/AST/StmtVisitor.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/Basic/TypeTraits.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.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> {
BugReporter &BR;
AnalysisDeclContext* AC;
ASTContext &ASTC;
/// Check if two expressions refer to the same declaration.
inline bool sameDecl(const Expr *A1, const Expr *A2) {
if (const DeclRefExpr *D1 = dyn_cast<DeclRefExpr>(A1->IgnoreParenCasts()))
if (const DeclRefExpr *D2 = dyn_cast<DeclRefExpr>(A2->IgnoreParenCasts()))
return D1->getDecl() == D2->getDecl();
return false;
}
/// Check if the expression E is a sizeof(WithArg).
inline bool isSizeof(const Expr *E, const Expr *WithArg) {
if (const UnaryExprOrTypeTraitExpr *UE =
dyn_cast<UnaryExprOrTypeTraitExpr>(E))
if (UE->getKind() == UETT_SizeOf)
return sameDecl(UE->getArgumentExpr(), WithArg);
return false;
}
/// Check if the expression E is a strlen(WithArg).
inline bool isStrlen(const Expr *E, const Expr *WithArg) {
if (const CallExpr *CE = dyn_cast<CallExpr>(E)) {
const FunctionDecl *FD = CE->getDirectCallee();
if (!FD)
return false;
return (CheckerContext::isCLibraryFunction(FD, "strlen", ASTC)
&& sameDecl(CE->getArg(0), WithArg));
}
return false;
}
/// Check if the expression is an integer literal with value 1.
inline bool isOne(const Expr *E) {
if (const IntegerLiteral *IL = dyn_cast<IntegerLiteral>(E))
return (IL->getValue().isIntN(1));
return false;
}
inline StringRef getPrintableName(const Expr *E) {
if (const DeclRefExpr *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);
public:
WalkAST(BugReporter &br, AnalysisDeclContext* ac) :
BR(br), AC(ac), ASTC(AC->getASTContext()) {
}
// 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) {
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 BinaryOperator *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;
}
void WalkAST::VisitCallExpr(CallExpr *CE) {
const FunctionDecl *FD = CE->getDirectCallee();
if (!FD)
return;
if (CheckerContext::isCLibraryFunction(FD, "strncat", ASTC)) {
if (containsBadStrncatPattern(CE)) {
const Expr *DstArg = CE->getArg(0);
const Expr *LenArg = CE->getArg(2);
SourceRange R = LenArg->getSourceRange();
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, "Anti-pattern in the argument", "C String API",
os.str(), Loc, &R, 1);
}
}
// Recurse and check children.
VisitChildren(CE);
}
void WalkAST::VisitChildren(Stmt *S) {
for (Stmt::child_iterator I = S->child_begin(), E = S->child_end(); I != E;
++I)
if (Stmt *child = *I)
Visit(child);
}
namespace {
class CStringSyntaxChecker: public Checker<check::ASTCodeBody> {
public:
void checkASTCodeBody(const Decl *D, AnalysisManager& Mgr,
BugReporter &BR) const {
WalkAST walker(BR, Mgr.getAnalysisDeclContext(D));
walker.Visit(D->getBody());
}
};
}
void ento::registerCStringSyntaxChecker(CheckerManager &mgr) {
mgr.registerChecker<CStringSyntaxChecker>();
}
|
