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//=== VLASizeChecker.cpp - Undefined dereference checker --------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This defines VLASizeChecker, a builtin check in ExprEngine that
// performs checks for declaration of VLA of undefined or zero size.
// In addition, VLASizeChecker is responsible for defining the extent
// of the MemRegion that represents a VLA.
#include "Taint.h"
#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
#include "clang/AST/CharUnits.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/CheckerManager.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/Support/raw_ostream.h"
using namespace clang;
using namespace ento;
using namespace taint;
namespace {
class VLASizeChecker : public Checker< check::PreStmt<DeclStmt> > {
mutable std::unique_ptr<BugType> BT;
enum VLASize_Kind { VLA_Garbage, VLA_Zero, VLA_Tainted, VLA_Negative };
void reportBug(VLASize_Kind Kind, const Expr *SizeE, ProgramStateRef State,
CheckerContext &C,
std::unique_ptr<BugReporterVisitor> Visitor = nullptr) const;
void checkPreStmt(const DeclStmt *DS, CheckerContext &C) const;
} // end anonymous namespace
void VLASizeChecker::reportBug(
VLASize_Kind Kind, const Expr *SizeE, ProgramStateRef State,
CheckerContext &C, std::unique_ptr<BugReporterVisitor> Visitor) const {
// Generate an error node.
ExplodedNode *N = C.generateErrorNode(State);
if (!N)
if (!BT)
BT.reset(new BuiltinBug(
this, "Dangerous variable-length array (VLA) declaration"));
SmallString<256> buf;
llvm::raw_svector_ostream os(buf);
os << "Declared variable-length array (VLA) ";
switch (Kind) {
case VLA_Garbage:
os << "uses a garbage value as its size";
case VLA_Zero:
os << "has zero size";
case VLA_Tainted:
os << "has tainted size";
case VLA_Negative:
os << "has negative size";
auto report = std::make_unique<PathSensitiveBugReport>(*BT, os.str(), N);
bugreporter::trackExpressionValue(N, SizeE, *report);
void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const {
if (!DS->isSingleDecl())
const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl());
if (!VD)
ASTContext &Ctx = C.getASTContext();
const VariableArrayType *VLA = Ctx.getAsVariableArrayType(VD->getType());
if (!VLA)
// FIXME: Handle multi-dimensional VLAs.
const Expr *SE = VLA->getSizeExpr();
ProgramStateRef state = C.getState();
SVal sizeV = C.getSVal(SE);
if (sizeV.isUndef()) {
reportBug(VLA_Garbage, SE, state, C);
// See if the size value is known. It can't be undefined because we would have
// warned about that already.
if (sizeV.isUnknown())
// Check if the size is tainted.
if (isTainted(state, sizeV)) {
reportBug(VLA_Tainted, SE, nullptr, C,
// Check if the size is zero.
DefinedSVal sizeD = sizeV.castAs<DefinedSVal>();
ProgramStateRef stateNotZero, stateZero;
std::tie(stateNotZero, stateZero) = state->assume(sizeD);
if (stateZero && !stateNotZero) {
reportBug(VLA_Zero, SE, stateZero, C);
// From this point on, assume that the size is not zero.
state = stateNotZero;
// VLASizeChecker is responsible for defining the extent of the array being
// declared. We do this by multiplying the array length by the element size,
// then matching that with the array region's extent symbol.
// Check if the size is negative.
SValBuilder &svalBuilder = C.getSValBuilder();
QualType Ty = SE->getType();
DefinedOrUnknownSVal Zero = svalBuilder.makeZeroVal(Ty);
SVal LessThanZeroVal = svalBuilder.evalBinOp(state, BO_LT, sizeD, Zero, Ty);
if (Optional<DefinedSVal> LessThanZeroDVal =
LessThanZeroVal.getAs<DefinedSVal>()) {
ConstraintManager &CM = C.getConstraintManager();
ProgramStateRef StatePos, StateNeg;
std::tie(StateNeg, StatePos) = CM.assumeDual(state, *LessThanZeroDVal);
if (StateNeg && !StatePos) {
reportBug(VLA_Negative, SE, state, C);
state = StatePos;
// Convert the array length to size_t.
QualType SizeTy = Ctx.getSizeType();
NonLoc ArrayLength =
svalBuilder.evalCast(sizeD, SizeTy, SE->getType()).castAs<NonLoc>();
// Get the element size.
CharUnits EleSize = Ctx.getTypeSizeInChars(VLA->getElementType());
SVal EleSizeVal = svalBuilder.makeIntVal(EleSize.getQuantity(), SizeTy);
// Multiply the array length by the element size.
SVal ArraySizeVal = svalBuilder.evalBinOpNN(
state, BO_Mul, ArrayLength, EleSizeVal.castAs<NonLoc>(), SizeTy);
// Finally, assume that the array's extent matches the given size.
const LocationContext *LC = C.getLocationContext();
DefinedOrUnknownSVal Extent =
state->getRegion(VD, LC)->getExtent(svalBuilder);
DefinedOrUnknownSVal ArraySize = ArraySizeVal.castAs<DefinedOrUnknownSVal>();
DefinedOrUnknownSVal sizeIsKnown =
svalBuilder.evalEQ(state, Extent, ArraySize);
state = state->assume(sizeIsKnown, true);
// Assume should not fail at this point.
// Remember our assumptions!
void ento::registerVLASizeChecker(CheckerManager &mgr) {
bool ento::shouldRegisterVLASizeChecker(const LangOptions &LO) {
return true;