1 //===--- ThrowByValueCatchByReferenceCheck.cpp - clang-tidy----------------===//
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 #include "ThrowByValueCatchByReferenceCheck.h"
11 #include "clang/AST/ASTContext.h"
12 #include "clang/ASTMatchers/ASTMatchFinder.h"
13 #include "clang/AST/OperationKinds.h"
14 
15 using namespace clang::ast_matchers;
16 
17 namespace clang {
18 namespace tidy {
19 
20 ThrowByValueCatchByReferenceCheck::ThrowByValueCatchByReferenceCheck(
21     StringRef Name, ClangTidyContext *Context)
22     : ClangTidyCheck(Name, Context),
23       CheckAnonymousTemporaries(Options.get("CheckThrowTemporaries", true)) {}
24 
25 void ThrowByValueCatchByReferenceCheck::registerMatchers(MatchFinder *Finder) {
26   // This is a C++ only check thus we register the matchers only for C++
27   if (!getLangOpts().CPlusPlus)
28     return;
29 
30   Finder->addMatcher(cxxThrowExpr().bind("throw"), this);
31   Finder->addMatcher(cxxCatchStmt().bind("catch"), this);
32 }
33 
34 void ThrowByValueCatchByReferenceCheck::storeOptions(
35     ClangTidyOptions::OptionMap &Opts) {
36   Options.store(Opts, "CheckThrowTemporaries", true);
37 }
38 
39 void ThrowByValueCatchByReferenceCheck::check(
40     const MatchFinder::MatchResult &Result) {
41   diagnoseThrowLocations(Result.Nodes.getNodeAs<CXXThrowExpr>("throw"));
42   diagnoseCatchLocations(Result.Nodes.getNodeAs<CXXCatchStmt>("catch"),
43                          *Result.Context);
44 }
45 
46 bool ThrowByValueCatchByReferenceCheck::isFunctionParameter(
47     const DeclRefExpr *declRefExpr) {
48   return isa<ParmVarDecl>(declRefExpr->getDecl());
49 }
50 
51 bool ThrowByValueCatchByReferenceCheck::isCatchVariable(
52     const DeclRefExpr *declRefExpr) {
53   auto *valueDecl = declRefExpr->getDecl();
54   if (auto *varDecl = dyn_cast<VarDecl>(valueDecl))
55     return varDecl->isExceptionVariable();
56   return false;
57 }
58 
59 bool ThrowByValueCatchByReferenceCheck::isFunctionOrCatchVar(
60     const DeclRefExpr *declRefExpr) {
61   return isFunctionParameter(declRefExpr) || isCatchVariable(declRefExpr);
62 }
63 
64 void ThrowByValueCatchByReferenceCheck::diagnoseThrowLocations(
65     const CXXThrowExpr *throwExpr) {
66   if (!throwExpr)
67     return;
68   auto *subExpr = throwExpr->getSubExpr();
69   if (!subExpr)
70     return;
71   auto qualType = subExpr->getType();
72   if (qualType->isPointerType()) {
73     // The code is throwing a pointer.
74     // In case it is strng literal, it is safe and we return.
75     auto *inner = subExpr->IgnoreParenImpCasts();
76     if (isa<StringLiteral>(inner))
77       return;
78     // If it's a variable from a catch statement, we return as well.
79     auto *declRef = dyn_cast<DeclRefExpr>(inner);
80     if (declRef && isCatchVariable(declRef)) {
81       return;
82     }
83     diag(subExpr->getLocStart(), "throw expression throws a pointer; it should "
84                                  "throw a non-pointer value instead");
85   }
86   // If the throw statement does not throw by pointer then it throws by value
87   // which is ok.
88   // There are addition checks that emit diagnosis messages if the thrown value
89   // is not an RValue. See:
90   // https://www.securecoding.cert.org/confluence/display/cplusplus/ERR09-CPP.+Throw+anonymous+temporaries
91   // This behavior can be influenced by an option.
92 
93   // If we encounter a CXXThrowExpr, we move through all casts until you either
94   // encounter a DeclRefExpr or a CXXConstructExpr.
95   // If it's a DeclRefExpr, we emit a message if the referenced variable is not
96   // a catch variable or function parameter.
97   // When encountering a CopyOrMoveConstructor: emit message if after casts,
98   // the expression is a LValue
99   if (CheckAnonymousTemporaries) {
100     bool emit = false;
101     auto *currentSubExpr = subExpr->IgnoreImpCasts();
102     const DeclRefExpr *variableReference =
103         dyn_cast<DeclRefExpr>(currentSubExpr);
104     const CXXConstructExpr *constructorCall =
105         dyn_cast<CXXConstructExpr>(currentSubExpr);
106     // If we have a DeclRefExpr, we flag for emitting a diagnosis message in
107     // case the referenced variable is neither a function parameter nor a
108     // variable declared in the catch statement.
109     if (variableReference)
110       emit = !isFunctionOrCatchVar(variableReference);
111     else if (constructorCall &&
112              constructorCall->getConstructor()->isCopyOrMoveConstructor()) {
113       // If we have a copy / move construction, we emit a diagnosis message if
114       // the object that we copy construct from is neither a function parameter
115       // nor a variable declared in a catch statement
116       auto argIter =
117           constructorCall
118               ->arg_begin(); // there's only one for copy constructors
119       auto *currentSubExpr = (*argIter)->IgnoreImpCasts();
120       if (currentSubExpr->isLValue()) {
121         if (auto *tmp = dyn_cast<DeclRefExpr>(currentSubExpr))
122           emit = !isFunctionOrCatchVar(tmp);
123         else if (isa<CallExpr>(currentSubExpr))
124           emit = true;
125       }
126     }
127     if (emit)
128       diag(subExpr->getLocStart(),
129            "throw expression should throw anonymous temporary values instead");
130   }
131 }
132 
133 void ThrowByValueCatchByReferenceCheck::diagnoseCatchLocations(
134     const CXXCatchStmt *catchStmt, ASTContext &context) {
135   const char *diagMsgCatchReference = "catch handler catches a pointer value; "
136                                       "should throw a non-pointer value and "
137                                       "catch by reference instead";
138   if (!catchStmt)
139     return;
140   auto caughtType = catchStmt->getCaughtType();
141   if (caughtType.isNull())
142     return;
143   auto *varDecl = catchStmt->getExceptionDecl();
144   if (const auto *PT = caughtType.getCanonicalType()->getAs<PointerType>()) {
145     // We do not diagnose when catching pointer to strings since we also allow
146     // throwing string literals.
147     if (!PT->getPointeeType()->isAnyCharacterType())
148       diag(varDecl->getLocStart(), diagMsgCatchReference);
149   } else if (!caughtType->isReferenceType()) {
150     // If it's not a pointer and not a reference then it must be thrown "by
151     // value". In this case we should emit a diagnosis message unless the type
152     // is trivial.
153     if (!caughtType.isTrivialType(context))
154       diag(varDecl->getLocStart(), diagMsgCatchReference);
155   }
156 }
157 
158 } // namespace tidy
159 } // namespace clang
160