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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 https://llvm.org/LICENSE.txt 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 "clang/AST/CharUnits.h"
#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
#include "clang/StaticAnalyzer/Checkers/Taint.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 "clang/StaticAnalyzer/Core/PathSensitive/DynamicExtent.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/Support/raw_ostream.h"
#include <optional>
using namespace clang;
using namespace ento;
using namespace taint;
namespace {
class VLASizeChecker
: public Checker<check::PreStmt<DeclStmt>,
check::PreStmt<UnaryExprOrTypeTraitExpr>> {
const BugType BT{this, "Dangerous variable-length array (VLA) declaration"};
const BugType TaintBT{this,
"Dangerous variable-length array (VLA) declaration",
categories::TaintedData};
enum VLASize_Kind { VLA_Garbage, VLA_Zero, VLA_Negative, VLA_Overflow };
/// Check a VLA for validity.
/// Every dimension of the array and the total size is checked for validity.
/// Returns null or a new state where the size is validated.
/// 'ArraySize' will contain SVal that refers to the total size (in char)
/// of the array.
ProgramStateRef checkVLA(CheckerContext &C, ProgramStateRef State,
const VariableArrayType *VLA, SVal &ArraySize) const;
/// Check a single VLA index size expression for validity.
ProgramStateRef checkVLAIndexSize(CheckerContext &C, ProgramStateRef State,
const Expr *SizeE) const;
void reportBug(VLASize_Kind Kind, const Expr *SizeE, ProgramStateRef State,
CheckerContext &C) const;
void reportTaintBug(const Expr *SizeE, ProgramStateRef State,
CheckerContext &C, SVal TaintedSVal) const;
public:
void checkPreStmt(const DeclStmt *DS, CheckerContext &C) const;
void checkPreStmt(const UnaryExprOrTypeTraitExpr *UETTE,
CheckerContext &C) const;
};
} // end anonymous namespace
ProgramStateRef VLASizeChecker::checkVLA(CheckerContext &C,
ProgramStateRef State,
const VariableArrayType *VLA,
SVal &ArraySize) const {
assert(VLA && "Function should be called with non-null VLA argument.");
const VariableArrayType *VLALast = nullptr;
llvm::SmallVector<const Expr *, 2> VLASizes;
// Walk over the VLAs for every dimension until a non-VLA is found.
// There is a VariableArrayType for every dimension (fixed or variable) until
// the most inner array that is variably modified.
// Dimension sizes are collected into 'VLASizes'. 'VLALast' is set to the
// innermost VLA that was encountered.
// In "int vla[x][2][y][3]" this will be the array for index "y" (with type
// int[3]). 'VLASizes' contains 'x', '2', and 'y'.
while (VLA) {
const Expr *SizeE = VLA->getSizeExpr();
State = checkVLAIndexSize(C, State, SizeE);
if (!State)
return nullptr;
VLASizes.push_back(SizeE);
VLALast = VLA;
VLA = C.getASTContext().getAsVariableArrayType(VLA->getElementType());
};
assert(VLALast &&
"Array should have at least one variably-modified dimension.");
ASTContext &Ctx = C.getASTContext();
SValBuilder &SVB = C.getSValBuilder();
CanQualType SizeTy = Ctx.getSizeType();
uint64_t SizeMax =
SVB.getBasicValueFactory().getMaxValue(SizeTy).getZExtValue();
// Get the element size.
CharUnits EleSize = Ctx.getTypeSizeInChars(VLALast->getElementType());
NonLoc ArrSize =
SVB.makeIntVal(EleSize.getQuantity(), SizeTy).castAs<NonLoc>();
// Try to calculate the known real size of the array in KnownSize.
uint64_t KnownSize = 0;
if (const llvm::APSInt *KV = SVB.getKnownValue(State, ArrSize))
KnownSize = KV->getZExtValue();
for (const Expr *SizeE : VLASizes) {
auto SizeD = C.getSVal(SizeE).castAs<DefinedSVal>();
// Convert the array length to size_t.
NonLoc IndexLength =
SVB.evalCast(SizeD, SizeTy, SizeE->getType()).castAs<NonLoc>();
// Multiply the array length by the element size.
SVal Mul = SVB.evalBinOpNN(State, BO_Mul, ArrSize, IndexLength, SizeTy);
if (auto MulNonLoc = Mul.getAs<NonLoc>())
ArrSize = *MulNonLoc;
else
// Extent could not be determined.
return State;
if (const llvm::APSInt *IndexLVal = SVB.getKnownValue(State, IndexLength)) {
// Check if the array size will overflow.
// Size overflow check does not work with symbolic expressions because a
// overflow situation can not be detected easily.
uint64_t IndexL = IndexLVal->getZExtValue();
// FIXME: See https://reviews.llvm.org/D80903 for discussion of
// some difference in assume and getKnownValue that leads to
// unexpected behavior. Just bail on IndexL == 0 at this point.
if (IndexL == 0)
return nullptr;
if (KnownSize <= SizeMax / IndexL) {
KnownSize *= IndexL;
} else {
// Array size does not fit into size_t.
reportBug(VLA_Overflow, SizeE, State, C);
return nullptr;
}
} else {
KnownSize = 0;
}
}
ArraySize = ArrSize;
return State;
}
ProgramStateRef VLASizeChecker::checkVLAIndexSize(CheckerContext &C,
ProgramStateRef State,
const Expr *SizeE) const {
SVal SizeV = C.getSVal(SizeE);
if (SizeV.isUndef()) {
reportBug(VLA_Garbage, SizeE, State, C);
return nullptr;
}
// See if the size value is known. It can't be undefined because we would have
// warned about that already.
if (SizeV.isUnknown())
return nullptr;
// Check if the size is tainted.
if (isTainted(State, SizeV)) {
reportTaintBug(SizeE, State, C, SizeV);
return nullptr;
}
// 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, SizeE, StateZero, C);
return nullptr;
}
// From this point on, assume that the size is not zero.
State = StateNotZero;
// Check if the size is negative.
SValBuilder &SVB = C.getSValBuilder();
QualType SizeTy = SizeE->getType();
DefinedOrUnknownSVal Zero = SVB.makeZeroVal(SizeTy);
SVal LessThanZeroVal =
SVB.evalBinOp(State, BO_LT, SizeD, Zero, SVB.getConditionType());
if (std::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, SizeE, State, C);
return nullptr;
}
State = StatePos;
}
return State;
}
void VLASizeChecker::reportTaintBug(const Expr *SizeE, ProgramStateRef State,
CheckerContext &C, SVal TaintedSVal) const {
// Generate an error node.
ExplodedNode *N = C.generateErrorNode(State);
if (!N)
return;
SmallString<256> buf;
llvm::raw_svector_ostream os(buf);
os << "Declared variable-length array (VLA) ";
os << "has tainted size";
auto report = std::make_unique<PathSensitiveBugReport>(TaintBT, os.str(), N);
report->addRange(SizeE->getSourceRange());
bugreporter::trackExpressionValue(N, SizeE, *report);
// The vla size may be a complex expression where multiple memory locations
// are tainted.
for (auto Sym : getTaintedSymbols(State, TaintedSVal))
report->markInteresting(Sym);
C.emitReport(std::move(report));
}
void VLASizeChecker::reportBug(VLASize_Kind Kind, const Expr *SizeE,
ProgramStateRef State, CheckerContext &C) const {
// Generate an error node.
ExplodedNode *N = C.generateErrorNode(State);
if (!N)
return;
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";
break;
case VLA_Zero:
os << "has zero size";
break;
case VLA_Negative:
os << "has negative size";
break;
case VLA_Overflow:
os << "has too large size";
break;
}
auto report = std::make_unique<PathSensitiveBugReport>(BT, os.str(), N);
report->addRange(SizeE->getSourceRange());
bugreporter::trackExpressionValue(N, SizeE, *report);
C.emitReport(std::move(report));
}
void VLASizeChecker::checkPreStmt(const DeclStmt *DS, CheckerContext &C) const {
if (!DS->isSingleDecl())
return;
ASTContext &Ctx = C.getASTContext();
SValBuilder &SVB = C.getSValBuilder();
ProgramStateRef State = C.getState();
QualType TypeToCheck;
const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl());
if (VD)
TypeToCheck = VD->getType().getCanonicalType();
else if (const auto *TND = dyn_cast<TypedefNameDecl>(DS->getSingleDecl()))
TypeToCheck = TND->getUnderlyingType().getCanonicalType();
else
return;
const VariableArrayType *VLA = Ctx.getAsVariableArrayType(TypeToCheck);
if (!VLA)
return;
// Check the VLA sizes for validity.
SVal ArraySize;
State = checkVLA(C, State, VLA, ArraySize);
if (!State)
return;
if (!isa<NonLoc>(ArraySize)) {
// Array size could not be determined but state may contain new assumptions.
C.addTransition(State);
return;
}
// VLASizeChecker is responsible for defining the extent of the array.
if (VD) {
State =
setDynamicExtent(State, State->getRegion(VD, C.getLocationContext()),
ArraySize.castAs<NonLoc>(), SVB);
}
// Remember our assumptions!
C.addTransition(State);
}
void VLASizeChecker::checkPreStmt(const UnaryExprOrTypeTraitExpr *UETTE,
CheckerContext &C) const {
// Want to check for sizeof.
if (UETTE->getKind() != UETT_SizeOf)
return;
// Ensure a type argument.
if (!UETTE->isArgumentType())
return;
const VariableArrayType *VLA = C.getASTContext().getAsVariableArrayType(
UETTE->getTypeOfArgument().getCanonicalType());
// Ensure that the type is a VLA.
if (!VLA)
return;
ProgramStateRef State = C.getState();
SVal ArraySize;
State = checkVLA(C, State, VLA, ArraySize);
if (!State)
return;
C.addTransition(State);
}
void ento::registerVLASizeChecker(CheckerManager &mgr) {
mgr.registerChecker<VLASizeChecker>();
}
bool ento::shouldRegisterVLASizeChecker(const CheckerManager &mgr) {
return true;
}
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