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//== 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/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);
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;
}
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());
}
}
// 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>();
}
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