1 //=== StringChecker.cpp -------------------------------------------*- 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-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements the modeling of the std::basic_string type.
10 // This involves checking preconditions of the operations and applying the
11 // effects of the operations, e.g. their post-conditions.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
16 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
17 #include "clang/StaticAnalyzer/Core/PathSensitive/CallDescription.h"
18 #include "clang/StaticAnalyzer/Core/PathSensitive/CallEvent.h"
19 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
20 
21 using namespace clang;
22 using namespace ento;
23 
24 namespace {
25 class StringChecker : public Checker<check::PreCall> {
26   BugType BT_Null{this, "Dereference of null pointer", categories::LogicError};
27   mutable const FunctionDecl *StringConstCharPtrCtor = nullptr;
28   mutable CanQualType SizeTypeTy;
29   const CallDescription TwoParamStdStringCtor = {
30       {"std", "basic_string", "basic_string"}, 2, 2};
31 
32   bool isCharToStringCtor(const CallEvent &Call, const ASTContext &ACtx) const;
33 
34 public:
35   void checkPreCall(const CallEvent &Call, CheckerContext &C) const;
36 };
37 
38 bool StringChecker::isCharToStringCtor(const CallEvent &Call,
39                                        const ASTContext &ACtx) const {
40   if (!TwoParamStdStringCtor.matches(Call))
41     return false;
42   const auto *FD = dyn_cast<FunctionDecl>(Call.getDecl());
43   assert(FD);
44 
45   // See if we already cached it.
46   if (StringConstCharPtrCtor && StringConstCharPtrCtor == FD)
47     return true;
48 
49   // Verify that the parameters have the expected types:
50   // - arg 1: `const CharT *`
51   // - arg 2: some allocator - which is definately not `size_t`.
52   const QualType Arg1Ty = Call.getArgExpr(0)->getType().getCanonicalType();
53   const QualType Arg2Ty = Call.getArgExpr(1)->getType().getCanonicalType();
54 
55   if (!Arg1Ty->isPointerType())
56     return false;
57 
58   // It makes sure that we don't select the `string(const char* p, size_t len)`
59   // overload accidentally.
60   if (Arg2Ty.getCanonicalType() == ACtx.getSizeType())
61     return false;
62 
63   StringConstCharPtrCtor = FD; // Cache the decl of the right overload.
64   return true;
65 }
66 
67 void StringChecker::checkPreCall(const CallEvent &Call,
68                                  CheckerContext &C) const {
69   if (!isCharToStringCtor(Call, C.getASTContext()))
70     return;
71   const auto Param = Call.getArgSVal(0).getAs<Loc>();
72   if (!Param)
73     return;
74 
75   // We managed to constrain the parameter to non-null.
76   ProgramStateRef NotNull, Null;
77   std::tie(NotNull, Null) = C.getState()->assume(*Param);
78 
79   if (NotNull) {
80     const auto Callback = [Param](PathSensitiveBugReport &BR) -> std::string {
81       return BR.isInteresting(*Param) ? "Assuming the pointer is not null."
82                                       : "";
83     };
84 
85     // Emit note only if this operation constrained the pointer to be null.
86     C.addTransition(NotNull, Null ? C.getNoteTag(Callback) : nullptr);
87     return;
88   }
89 
90   // We found a path on which the parameter is NULL.
91   if (ExplodedNode *N = C.generateErrorNode(C.getState())) {
92     auto R = std::make_unique<PathSensitiveBugReport>(
93         BT_Null, "The parameter must not be null", N);
94     bugreporter::trackExpressionValue(N, Call.getArgExpr(0), *R);
95     C.emitReport(std::move(R));
96   }
97 }
98 
99 } // end anonymous namespace
100 
101 void ento::registerStringChecker(CheckerManager &Mgr) {
102   Mgr.registerChecker<StringChecker>();
103 }
104 
105 bool ento::shouldRegisterStringChecker(const CheckerManager &) { return true; }
106