Retry: [ubsan] Detect UB loads from bitfields

It's possible to load out-of-range values from bitfields backed by a
boolean or an enum. Check for UB loads from bitfields.

This is the motivating example:

  struct S {
    BOOL b : 1; // Signed ObjC BOOL.
  };

  S s;
  s.b = 1; // This is actually stored as -1.
  if (s.b == 1) // Evaluates to false, -1 != 1.
    ...

Changes since the original commit:

- Single-bit bools are a special case (see CGF::EmitFromMemory), and we
  can't avoid dealing with them when loading from a bitfield. Don't try to
  insert a check in this case.

Differential Revision: https://reviews.llvm.org/D30423

llvm-svn: 297389

1 files changed, 11 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 15cd7ccbaac..ab4448bf3c5 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -1327,6 +1327,13 @@ bool CodeGenFunction::EmitScalarRangeCheck(llvm::Value *Value, QualType Ty,
   if (!NeedsBoolCheck && !NeedsEnumCheck)
     return false;
 
+  // Single-bit booleans don't need to be checked. Special-case this to avoid
+  // a bit width mismatch when handling bitfield values. This is handled by
+  // EmitFromMemory for the non-bitfield case.
+  if (IsBool &&
+      cast<llvm::IntegerType>(Value->getType())->getBitWidth() == 1)
+    return false;
+
   llvm::APInt Min, End;
   if (!getRangeForType(*this, Ty, Min, End, /*StrictEnums=*/true, IsBool))
     return true;
@@ -1549,10 +1556,11 @@ RValue CodeGenFunction::EmitLoadOfLValue(LValue LV, SourceLocation Loc) {
     return EmitLoadOfGlobalRegLValue(LV);
 
   assert(LV.isBitField() && "Unknown LValue type!");
-  return EmitLoadOfBitfieldLValue(LV);
+  return EmitLoadOfBitfieldLValue(LV, Loc);
 }
 
-RValue CodeGenFunction::EmitLoadOfBitfieldLValue(LValue LV) {
+RValue CodeGenFunction::EmitLoadOfBitfieldLValue(LValue LV,
+                                                 SourceLocation Loc) {
   const CGBitFieldInfo &Info = LV.getBitFieldInfo();
 
   // Get the output type.
@@ -1577,7 +1585,7 @@ RValue CodeGenFunction::EmitLoadOfBitfieldLValue(LValue LV) {
                               "bf.clear");
   }
   Val = Builder.CreateIntCast(Val, ResLTy, Info.IsSigned, "bf.cast");
-
+  EmitScalarRangeCheck(Val, LV.getType(), Loc);
   return RValue::get(Val);
 }