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[clang][bytecode] Bail out from type-punning casts #163809

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[clang][bytecode] Bail out from type-punning casts #163809

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44 changes: 44 additions & 0 deletions clang/lib/AST/ByteCode/Compiler.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -212,6 +212,10 @@ template <class Emitter>
bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
const Expr *SubExpr = CE->getSubExpr();

if (isPunningDereference(SubExpr))
return this->emitInvalidCast(CastKind::Reinterpret, /*Fatal=*/true,
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I think it would be better to call this when we already switch'ed for the cast kind later.

CE);

if (DiscardResult)
return this->delegate(SubExpr);

Expand Down Expand Up @@ -5511,6 +5515,46 @@ bool Compiler<Emitter>::maybeEmitDeferredVarInit(const VarDecl *VD) {
return true;
}

template <class Emitter>
bool Compiler<Emitter>::isPunningDereference(const Expr *E)
{
E = E->IgnoreParenImpCasts();

const auto *UO = dyn_cast<UnaryOperator>(E);
if (!UO || UO->getOpcode() != UO_Deref)
return false;

const Expr *Base = UO->getSubExpr()->IgnoreParenImpCasts();
const auto *Cast = dyn_cast<CastExpr>(Base);
if (!Cast)
return false;

// Only consider reinterpret-ish casts
switch (Cast->getCastKind()) {
case CK_BitCast:
case CK_PointerToIntegral:
case CK_IntegralToPointer:
case CK_AddressSpaceConversion:
break;
default:
return false; // CK_NoOp etc. are fine
}
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How did you come up with this list?

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It's tentative. I think we need to check for actual UBness of the cast, as there are cases where some casts are legal and should be allowed in CE

For a POC to see if this fixes the issue I linked, I just put everything that remotely resembles a fishy cast


QualType DestPtrTy = Cast->getType();
QualType SrcPtrTy = Cast->getSubExpr()->getType();
if (!DestPtrTy->isPointerType() || !SrcPtrTy->isPointerType())
return true; // super fishy, treat it as a pun

QualType DestPointee = DestPtrTy->getPointeeType();
QualType SrcPointee = SrcPtrTy->getPointeeType();

// If pointee types differ (ignoring qualifiers), its a pun
if (!Ctx.getASTContext().hasSameUnqualifiedType(DestPointee, SrcPointee))
return true;

return false;
}

static bool hasTrivialDefaultCtorParent(const FieldDecl *FD) {
assert(FD);
assert(FD->getParent()->isUnion());
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2 changes: 2 additions & 0 deletions clang/lib/AST/ByteCode/Compiler.h
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Expand Up @@ -423,6 +423,8 @@ class Compiler : public ConstStmtVisitor<Compiler<Emitter>, bool>,
bool checkLiteralType(const Expr *E);
bool maybeEmitDeferredVarInit(const VarDecl *VD);

bool isPunningDereference(const Expr *E);

bool refersToUnion(const Expr *E);

protected:
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