cppcheck/lib/checkautovariables.cpp

/*
 * Cppcheck - A tool for static C/C++ code analysis
 * Copyright (C) 2007-2016 Cppcheck team.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

//---------------------------------------------------------------------------
// Auto variables checks
//---------------------------------------------------------------------------

#include "checkautovariables.h"
#include "symboldatabase.h"

#include <string>

//---------------------------------------------------------------------------


// Register this check class into cppcheck by creating a static instance of it..
namespace {
    static CheckAutoVariables instance;
}

static const CWE CWE398(398U);  // Indicator of Poor Code Quality
static const CWE CWE562(562U);  // Return of Stack Variable Address
static const CWE CWE590(590U);  // Free of Memory not on the Heap

bool CheckAutoVariables::isPtrArg(const Token *tok)
{
    const Variable *var = tok->variable();

    return (var && var->isArgument() && var->isPointer());
}

bool CheckAutoVariables::isArrayArg(const Token *tok)
{
    const Variable *var = tok->variable();

    return (var && var->isArgument() && var->isArray());
}

bool CheckAutoVariables::isRefPtrArg(const Token *tok)
{
    const Variable *var = tok->variable();

    return (var && var->isArgument() && var->isReference() && var->isPointer());
}

bool CheckAutoVariables::isNonReferenceArg(const Token *tok)
{
    const Variable *var = tok->variable();

    return (var && var->isArgument() && !var->isReference() && (var->isPointer() || var->typeStartToken()->isStandardType() || var->type()));
}

bool CheckAutoVariables::isAutoVar(const Token *tok)
{
    const Variable *var = tok->variable();

    if (!var || !var->isLocal() || var->isStatic())
        return false;

    if (var->isReference()) {
        // address of reference variable can be taken if the address
        // of the variable it points at is not a auto-var
        // TODO: check what the reference variable references.
        return false;
    }

    if (Token::Match(tok, "%name% .|::")) {
        do {
            tok = tok->tokAt(2);
        } while (Token::Match(tok, "%name% .|::"));
        if (Token::Match(tok, "%name% ("))
            return false;
    }
    return true;
}

bool CheckAutoVariables::isAutoVarArray(const Token *tok)
{
    if (!tok)
        return false;

    // &x[..]
    if (tok->str() == "&" && Token::simpleMatch(tok->astOperand1(), "[") && !tok->astOperand2())
        return isAutoVarArray(tok->astOperand1()->astOperand1());

    // x+y
    if (tok->str() == "+")
        return isAutoVarArray(tok->astOperand1()) || isAutoVarArray(tok->astOperand2());

    // x-intexpr
    if (tok->str() == "-")
        return isAutoVarArray(tok->astOperand1()) &&
               tok->astOperand2() &&
               tok->astOperand2()->valueType() &&
               tok->astOperand2()->valueType()->isIntegral();

    const Variable *var = tok->variable();
    if (!var)
        return false;

    // Variable
    if (var->isLocal() && !var->isStatic() && var->isArray() && !var->isPointer())
        return true;

    // ValueFlow
    if (var->isPointer() && !var->isArgument()) {
        for (std::list<ValueFlow::Value>::const_iterator it = tok->values.begin(); it != tok->values.end(); ++it) {
            const ValueFlow::Value &val = *it;
            if (val.isTokValue() && isAutoVarArray(val.tokvalue))
                return true;
        }
    }

    return false;
}

// Verification that we really take the address of a local variable
static bool checkRvalueExpression(const Token * const vartok)
{
    const Variable * const var = vartok->variable();
    if (var == nullptr)
        return false;

    if (Token::Match(vartok->previous(), "& %name% [") && var->isPointer())
        return false;

    const Token * const next = vartok->next();
    // &a.b[0]
    if (Token::Match(vartok, "%name% . %var% [") && !var->isPointer()) {
        const Variable *var2 = next->next()->variable();
        return var2 && !var2->isPointer();
    }

    return ((next->str() != "." || (!var->isPointer() && (!var->isClass() || var->type()))) && next->strAt(2) != ".");
}

static bool variableIsUsedInScope(const Token* start, unsigned int varId, const Scope *scope)
{
    if (!start) // Ticket #5024
        return false;

    for (const Token *tok = start; tok && tok != scope->classEnd; tok = tok->next()) {
        if (tok->varId() == varId)
            return true;
        const Scope::ScopeType scopeType = tok->scope()->type;
        if (scopeType == Scope::eFor || scopeType == Scope::eDo || scopeType == Scope::eWhile) // In case of loops, better checking would be necessary
            return true;
        if (Token::simpleMatch(tok, "asm ("))
            return true;
    }
    return false;
}

void CheckAutoVariables::assignFunctionArg()
{
    const bool printStyle = _settings->isEnabled("style");
    const bool printWarning = _settings->isEnabled("warning");
    if (!printStyle && !printWarning)
        return;

    const SymbolDatabase *symbolDatabase = _tokenizer->getSymbolDatabase();
    const std::size_t functions = symbolDatabase->functionScopes.size();
    for (std::size_t i = 0; i < functions; ++i) {
        const Scope * scope = symbolDatabase->functionScopes[i];
        for (const Token *tok = scope->classStart; tok && tok != scope->classEnd; tok = tok->next()) {
            // TODO: What happens if this is removed?
            if (tok->astParent())
                continue;
            if (!Token::Match(tok, "=|++|--") || !Token::Match(tok->astOperand1(), "%var%"))
                continue;
            const Token* const vartok = tok->astOperand1();
            if (isNonReferenceArg(vartok) &&
                !Token::Match(vartok->next(), "= %varid% ;", vartok->varId()) &&
                !variableIsUsedInScope(Token::findsimplematch(vartok->next(), ";"), vartok->varId(), scope) &&
                !Token::findsimplematch(vartok, "goto", scope->classEnd)) {
                if (vartok->variable()->isPointer() && printWarning)
                    errorUselessAssignmentPtrArg(vartok);
                else if (printStyle)
                    errorUselessAssignmentArg(vartok);
            }
        }
    }
}

void CheckAutoVariables::autoVariables()
{
    const bool printInconclusive = _settings->inconclusive;
    const SymbolDatabase *symbolDatabase = _tokenizer->getSymbolDatabase();
    const std::size_t functions = symbolDatabase->functionScopes.size();
    for (std::size_t i = 0; i < functions; ++i) {
        const Scope * scope = symbolDatabase->functionScopes[i];
        for (const Token *tok = scope->classStart; tok && tok != scope->classEnd; tok = tok->next()) {
            // Critical assignment
            if (Token::Match(tok, "[;{}] %var% = & %var%") && isRefPtrArg(tok->next()) && isAutoVar(tok->tokAt(4))) {
                if (checkRvalueExpression(tok->tokAt(4)))
                    errorAutoVariableAssignment(tok->next(), false);
            } else if (Token::Match(tok, "[;{}] * %var% = & %var%") && isPtrArg(tok->tokAt(2)) && isAutoVar(tok->tokAt(5))) {
                if (checkRvalueExpression(tok->tokAt(5)))
                    errorAutoVariableAssignment(tok->next(), false);
            } else if (Token::Match(tok, "[;{}] %var% . %var% = & %var%")) {
                // TODO: check if the parameter is only changed temporarily (#2969)
                if (printInconclusive && isPtrArg(tok->next())) {
                    const Token * const var2tok = tok->tokAt(6);
                    if (isAutoVar(var2tok) && checkRvalueExpression(var2tok))
                        errorAutoVariableAssignment(tok->next(), true);
                }
                tok = tok->tokAt(6);
            } else if (Token::Match(tok, "[;{}] %var% . %var% = %var% ;")) {
                // TODO: check if the parameter is only changed temporarily (#2969)
                if (printInconclusive && isPtrArg(tok->next())) {
                    if (isAutoVarArray(tok->tokAt(5)))
                        errorAutoVariableAssignment(tok->next(), true);
                }
                tok = tok->tokAt(5);
            } else if (Token::Match(tok, "[;{}] * %var% = %var% ;")) {
                const Variable * var1 = tok->tokAt(2)->variable();
                if (var1 && var1->isArgument() && Token::Match(var1->nameToken()->tokAt(-3), "%type% * *")) {
                    if (isAutoVarArray(tok->tokAt(4)))
                        errorAutoVariableAssignment(tok->next(), false);
                }
                tok = tok->tokAt(4);
            } else if (Token::Match(tok, "[;{}] %var% [") && Token::Match(tok->linkAt(2), "] = & %var%") &&
                       (isPtrArg(tok->next()) || isArrayArg(tok->next())) && isAutoVar(tok->linkAt(2)->tokAt(3))) {
                const Token* const varTok = tok->linkAt(2)->tokAt(3);
                if (checkRvalueExpression(varTok))
                    errorAutoVariableAssignment(tok->next(), false);
            }
            // Critical return
            else if (Token::Match(tok, "return & %var% ;")) {
                const Token* varTok = tok->tokAt(2);
                if (isAutoVar(varTok))
                    errorReturnAddressToAutoVariable(tok);
                else if (varTok->varId()) {
                    const Variable * var1 = varTok->variable();
                    if (var1 && var1->isArgument() && var1->typeEndToken()->str() != "&")
                        errorReturnAddressOfFunctionParameter(tok, varTok->str());
                }
            }
            // Invalid pointer deallocation
            else if ((Token::Match(tok, "%name% ( %var% ) ;") && _settings->library.dealloc(tok)) ||
                     (_tokenizer->isCPP() && Token::Match(tok, "delete [| ]| (| %var% !!["))) {
                tok = Token::findmatch(tok->next(), "%var%");
                if (isAutoVarArray(tok))
                    errorInvalidDeallocation(tok);
            } else if ((Token::Match(tok, "%name% ( & %var% ) ;") && _settings->library.dealloc(tok)) ||
                       (_tokenizer->isCPP() && Token::Match(tok, "delete [| ]| (| & %var% !!["))) {
                tok = Token::findmatch(tok->next(), "%var%");
                if (isAutoVar(tok))
                    errorInvalidDeallocation(tok);
            }
        }
    }
}

//---------------------------------------------------------------------------

void CheckAutoVariables::returnPointerToLocalArray()
{
    const SymbolDatabase *symbolDatabase = _tokenizer->getSymbolDatabase();

    const std::size_t functions = symbolDatabase->functionScopes.size();
    for (std::size_t i = 0; i < functions; ++i) {
        const Scope * scope = symbolDatabase->functionScopes[i];
        if (!scope->function)
            continue;

        const Token *tok = scope->function->tokenDef;

        // have we reached a function that returns a pointer
        if (tok->previous() && tok->previous()->str() == "*") {
            for (const Token *tok2 = scope->classStart->next(); tok2 && tok2 != scope->classEnd; tok2 = tok2->next()) {
                // Return pointer to local array variable..
                if (tok2 ->str() == "return" && isAutoVarArray(tok2->astOperand1())) {
                    errorReturnPointerToLocalArray(tok2);
                }
            }
        }
    }
}

void CheckAutoVariables::errorReturnAddressToAutoVariable(const Token *tok)
{
    reportError(tok, Severity::error, "returnAddressOfAutoVariable", "Address of an auto-variable returned.", CWE562, false);
}

void CheckAutoVariables::errorReturnPointerToLocalArray(const Token *tok)
{
    reportError(tok, Severity::error, "returnLocalVariable", "Pointer to local array variable returned.", CWE562, false);
}

void CheckAutoVariables::errorAutoVariableAssignment(const Token *tok, bool inconclusive)
{
    if (!inconclusive) {
        reportError(tok, Severity::error, "autoVariables",
                    "Address of local auto-variable assigned to a function parameter.\n"
                    "Dangerous assignment - the function parameter is assigned the address of a local "
                    "auto-variable. Local auto-variables are reserved from the stack which "
                    "is freed when the function ends. So the pointer to a local variable "
                    "is invalid after the function ends.", CWE562, false);
    } else {
        reportError(tok, Severity::error, "autoVariables",
                    "Address of local auto-variable assigned to a function parameter.\n"
                    "Function parameter is assigned the address of a local auto-variable. "
                    "Local auto-variables are reserved from the stack which is freed when "
                    "the function ends. The address is invalid after the function ends and it "
                    "might 'leak' from the function through the parameter.",
                    CWE562,
                    true);
    }
}

void CheckAutoVariables::errorReturnAddressOfFunctionParameter(const Token *tok, const std::string &varname)
{
    reportError(tok, Severity::error, "returnAddressOfFunctionParameter",
                "Address of function parameter '" + varname + "' returned.\n"
                "Address of the function parameter '" + varname + "' becomes invalid after the function exits because "
                "function parameters are stored on the stack which is freed when the function exits. Thus the returned "
                "value is invalid.", CWE562, false);
}

void CheckAutoVariables::errorUselessAssignmentArg(const Token *tok)
{
    reportError(tok,
                Severity::style,
                "uselessAssignmentArg",
                "Assignment of function parameter has no effect outside the function.", CWE398, false);
}

void CheckAutoVariables::errorUselessAssignmentPtrArg(const Token *tok)
{
    reportError(tok,
                Severity::warning,
                "uselessAssignmentPtrArg",
                "Assignment of function parameter has no effect outside the function. Did you forget dereferencing it?", CWE398, false);
}

//---------------------------------------------------------------------------

// return temporary?
bool CheckAutoVariables::returnTemporary(const Token *tok)
{
    bool func = false;     // Might it be a function call?
    bool retref = false;   // is there such a function that returns a reference?
    bool retvalue = false; // is there such a function that returns a value?

    const Function *function = tok->function();
    if (function) {
        // Ticket #5478: Only functions or operator equal might return a temporary
        if (function->type != Function::eOperatorEqual && function->type != Function::eFunction)
            return false;
        retref = function->tokenDef->strAt(-1) == "&";
        if (!retref) {
            const Token *start = function->retDef;
            if (start->str() == "const")
                start = start->next();
            if (start->str() == "::")
                start = start->next();

            if (Token::simpleMatch(start, "std ::")) {
                if (start->strAt(3) != "<" || !Token::simpleMatch(start->linkAt(3), "> ::"))
                    retvalue = true;
                else
                    retref = true; // Assume that a reference is returned
            } else {
                if (start->type())
                    retvalue = true;
                else
                    retref = true;
            }
        }
        func = true;
    }
    if (!func && tok->type())
        return true;

    return bool(!retref && retvalue);
}

//---------------------------------------------------------------------------

static bool astHasAutoResult(const Token *tok)
{
    if (tok->astOperand1() && !astHasAutoResult(tok->astOperand1()))
        return false;
    if (tok->astOperand2() && !astHasAutoResult(tok->astOperand2()))
        return false;

    if (tok->isOp()) {
        if (tok->tokType() == Token::eIncDecOp)
            return false;
        if ((tok->str() == "<<" || tok->str() == ">>") && tok->astOperand1()) {
            const Token* tok2 = tok->astOperand1();
            while (tok2 && tok2->str() == "*" && !tok2->astOperand2())
                tok2 = tok2->astOperand1();
            return tok2 && tok2->variable() && !tok2->variable()->isClass() && !tok2->variable()->isStlType(); // Class or unknown type on LHS: Assume it is a stream
        }
        return true;
    }

    if (tok->isLiteral())
        return true;

    if (tok->isName()) {
        // TODO: check function calls, struct members, arrays, etc also
        if (!tok->variable())
            return false;
        if (tok->variable()->isStlType())
            return true;
        if (tok->variable()->isClass() || tok->variable()->isPointer() || tok->variable()->isReference()) // TODO: Properly handle pointers/references to classes in symbol database
            return false;

        return true;
    }

    return false;
}

/*!
 * Skip over a lambda expression
 * \return next token - or next token beyond lambda
 * \todo handle explicit return type
 */
static const Token* skipLambda(const Token* first)
{
    if (!first)
        return nullptr;
    if (first->str() != "[")
        return first;
    const Token* tok = first->link()->next();
    if (!tok)
        return nullptr;
    if (tok->str() == "(") {
        tok = tok->link()->next();
    }
    if (tok->str() == "constexpr")
        tok = tok->next();
    if (tok->str() == "mutable")
        tok = tok->next();
    if (tok->str() == "{") {
        tok = tok->link()->next();
    }
    return tok;
}

void CheckAutoVariables::returnReference()
{
    if (_tokenizer->isC())
        return;

    const SymbolDatabase *symbolDatabase = _tokenizer->getSymbolDatabase();

    const std::size_t functions = symbolDatabase->functionScopes.size();
    for (std::size_t i = 0; i < functions; ++i) {
        const Scope * scope = symbolDatabase->functionScopes[i];
        if (!scope->function)
            continue;

        const Token *tok = scope->function->tokenDef;

        // have we reached a function that returns a reference?
        if (tok->previous() && tok->previous()->str() == "&") {
            for (const Token *tok2 = scope->classStart->next(); tok2 && tok2 != scope->classEnd; tok2 = tok2->next()) {
                if (!tok2->scope()->isExecutable()) {
                    tok2 = tok2->scope()->classEnd;
                    continue;
                }

                // Skip over lambdas
                tok2 = skipLambda(tok2);
                if (!tok2)
                    break;

                if (tok2->str() == "(")
                    tok2 = tok2->link();

                if (tok2->str() != "return")
                    continue;

                // return..
                if (Token::Match(tok2, "return %var% ;")) {
                    // is the returned variable a local variable?
                    if (isAutoVar(tok2->next())) {
                        const Variable *var1 = tok2->next()->variable();
                        // If reference variable is used, check what it references
                        if (Token::Match(var1->nameToken(), "%var% [=(]")) {
                            const Token *tok3 = var1->nameToken()->tokAt(2);
                            if (!Token::Match(tok3, "%var% [);.]"))
                                continue;

                            // Only report error if variable that is referenced is
                            // a auto variable
                            if (!isAutoVar(tok3))
                                continue;
                        }

                        // report error..
                        errorReturnReference(tok2);
                    }
                }

                // return reference to temporary..
                else if (Token::Match(tok2, "return %name% (") &&
                         Token::simpleMatch(tok2->linkAt(2), ") ;")) {
                    if (returnTemporary(tok2->next())) {
                        // report error..
                        errorReturnTempReference(tok2);
                    }
                }

                // Return reference to a literal or the result of a calculation
                else if (tok2->astOperand1() && (tok2->astOperand1()->isCalculation() || tok2->next()->isLiteral()) && astHasAutoResult(tok2->astOperand1())) {
                    errorReturnTempReference(tok2);
                }
            }
        }
    }
}

void CheckAutoVariables::errorReturnReference(const Token *tok)
{
    reportError(tok, Severity::error, "returnReference", "Reference to auto variable returned.", CWE562, false);
}

void CheckAutoVariables::errorReturnTempReference(const Token *tok)
{
    reportError(tok, Severity::error, "returnTempReference", "Reference to temporary returned.", CWE562, false);
}

void CheckAutoVariables::errorInvalidDeallocation(const Token *tok)
{
    reportError(tok,
                Severity::error,
                "autovarInvalidDeallocation",
                "Deallocation of an auto-variable results in undefined behaviour.\n"
                "The deallocation of an auto-variable results in undefined behaviour. You should only free memory "
                "that has been allocated dynamically.", CWE590, false);
}