[clang][UBSan] Add implicit conversion check for bitfields (#75481)

This patch implements the implicit truncation and implicit sign change
checks for bitfields using UBSan. E.g.,
`-fsanitize=implicit-bitfield-truncation` and
`-fsanitize=implicit-bitfield-sign-change`.

Author: Zonotora

clang/docs/ReleaseNotes.rst

@@ -195,6 +195,10 @@ Non-comprehensive list of changes in this release
 
 New Compiler Flags
 ------------------
+- ``-fsanitize=implicit-bitfield-conversion`` checks implicit truncation and
+  sign change.
+- ``-fsanitize=implicit-integer-conversion`` a group that replaces the previous
+  group ``-fsanitize=implicit-conversion``.
 
 - ``-Wmissing-designated-field-initializers``, grouped under ``-Wmissing-field-initializers``.
   This diagnostic can be disabled to make ``-Wmissing-field-initializers`` behave
@@ -208,6 +212,9 @@ Modified Compiler Flags
 - Added a new diagnostic flag ``-Wreturn-mismatch`` which is grouped under
   ``-Wreturn-type``, and moved some of the diagnostics previously controlled by
   ``-Wreturn-type`` under this new flag. Fixes #GH72116.
+- ``-fsanitize=implicit-conversion`` is now a group for both
+  ``-fsanitize=implicit-integer-conversion`` and
+  ``-fsanitize=implicit-bitfield-conversion``.
 
 - Added ``-Wcast-function-type-mismatch`` under the ``-Wcast-function-type``
   warning group. Moved the diagnostic previously controlled by

clang/docs/UndefinedBehaviorSanitizer.rst

@@ -148,6 +148,11 @@ Available checks are:
      Issues caught by this sanitizer are not undefined behavior,
      but are often unintentional.
   -  ``-fsanitize=integer-divide-by-zero``: Integer division by zero.
+  -  ``-fsanitize=implicit-bitfield-conversion``: Implicit conversion from
+     integer of larger bit width to smaller bitfield, if that results in data
+     loss. This includes unsigned/signed truncations and sign changes, similarly
+     to how the ``-fsanitize=implicit-integer-conversion`` group works, but
+     explicitly for bitfields.
   -  ``-fsanitize=nonnull-attribute``: Passing null pointer as a function
      parameter which is declared to never be null.
   -  ``-fsanitize=null``: Use of a null pointer or creation of a null
@@ -193,16 +198,16 @@ Available checks are:
      signed division overflow (``INT_MIN/-1``). Note that checks are still
      added even when ``-fwrapv`` is enabled. This sanitizer does not check for
      lossy implicit conversions performed before the computation (see
-     ``-fsanitize=implicit-conversion``). Both of these two issues are handled
-     by ``-fsanitize=implicit-conversion`` group of checks.
+     ``-fsanitize=implicit-integer-conversion``). Both of these two issues are handled
+     by ``-fsanitize=implicit-integer-conversion`` group of checks.
   -  ``-fsanitize=unreachable``: If control flow reaches an unreachable
      program point.
   -  ``-fsanitize=unsigned-integer-overflow``: Unsigned integer overflow, where
      the result of an unsigned integer computation cannot be represented in its
      type. Unlike signed integer overflow, this is not undefined behavior, but
      it is often unintentional. This sanitizer does not check for lossy implicit
      conversions performed before such a computation
-     (see ``-fsanitize=implicit-conversion``).
+     (see ``-fsanitize=implicit-integer-conversion``).
   -  ``-fsanitize=vla-bound``: A variable-length array whose bound
      does not evaluate to a positive value.
   -  ``-fsanitize=vptr``: Use of an object whose vptr indicates that it is of
@@ -224,11 +229,15 @@ You can also use the following check groups:
   -  ``-fsanitize=implicit-integer-arithmetic-value-change``: Catches implicit
      conversions that change the arithmetic value of the integer. Enables
      ``implicit-signed-integer-truncation`` and ``implicit-integer-sign-change``.
-  -  ``-fsanitize=implicit-conversion``: Checks for suspicious
-     behavior of implicit conversions. Enables
+  -  ``-fsanitize=implicit-integer-conversion``: Checks for suspicious
+     behavior of implicit integer conversions. Enables
      ``implicit-unsigned-integer-truncation``,
      ``implicit-signed-integer-truncation``, and
      ``implicit-integer-sign-change``.
+  -  ``-fsanitize=implicit-conversion``: Checks for suspicious
+     behavior of implicit conversions. Enables
+     ``implicit-integer-conversion``, and
+     ``implicit-bitfield-conversion``.
   -  ``-fsanitize=integer``: Checks for undefined or suspicious integer
      behavior (e.g. unsigned integer overflow).
      Enables ``signed-integer-overflow``, ``unsigned-integer-overflow``,

clang/include/clang/Basic/Sanitizers.def

@@ -163,24 +163,24 @@ SANITIZER_GROUP("implicit-integer-arithmetic-value-change",
                 ImplicitIntegerArithmeticValueChange,
                 ImplicitIntegerSignChange | ImplicitSignedIntegerTruncation)
 
-SANITIZER("objc-cast", ObjCCast)
+SANITIZER_GROUP("implicit-integer-conversion", ImplicitIntegerConversion,
+                ImplicitIntegerArithmeticValueChange |
+                    ImplicitUnsignedIntegerTruncation)
 
-// FIXME:
-//SANITIZER_GROUP("implicit-integer-conversion", ImplicitIntegerConversion,
-//                ImplicitIntegerArithmeticValueChange |
-//                    ImplicitUnsignedIntegerTruncation)
-//SANITIZER_GROUP("implicit-conversion", ImplicitConversion,
-//                ImplicitIntegerConversion)
+// Implicit bitfield sanitizers
+SANITIZER("implicit-bitfield-conversion", ImplicitBitfieldConversion)
 
 SANITIZER_GROUP("implicit-conversion", ImplicitConversion,
-                ImplicitIntegerArithmeticValueChange |
-                    ImplicitUnsignedIntegerTruncation)
+                ImplicitIntegerConversion |
+                    ImplicitBitfieldConversion)
 
 SANITIZER_GROUP("integer", Integer,
-                ImplicitConversion | IntegerDivideByZero | Shift |
+                ImplicitIntegerConversion | IntegerDivideByZero | Shift |
                     SignedIntegerOverflow | UnsignedIntegerOverflow |
                     UnsignedShiftBase)
 
+SANITIZER("objc-cast", ObjCCast)
+
 SANITIZER("local-bounds", LocalBounds)
 SANITIZER_GROUP("bounds", Bounds, ArrayBounds | LocalBounds)
 

clang/lib/CodeGen/CGExpr.cpp

@@ -5580,11 +5580,44 @@ LValue CodeGenFunction::EmitBinaryOperatorLValue(const BinaryOperator *E) {
       break;
     }
 
-    RValue RV = EmitAnyExpr(E->getRHS());
+    // TODO: Can we de-duplicate this code with the corresponding code in
+    // CGExprScalar, similar to the way EmitCompoundAssignmentLValue works?
+    RValue RV;
+    llvm::Value *Previous = nullptr;
+    QualType SrcType = E->getRHS()->getType();
+    // Check if LHS is a bitfield, if RHS contains an implicit cast expression
+    // we want to extract that value and potentially (if the bitfield sanitizer
+    // is enabled) use it to check for an implicit conversion.
+    if (E->getLHS()->refersToBitField()) {
+      llvm::Value *RHS =
+          EmitWithOriginalRHSBitfieldAssignment(E, Previous, &SrcType);
+      RV = RValue::get(RHS);
+    } else
+      RV = EmitAnyExpr(E->getRHS());
+
     LValue LV = EmitCheckedLValue(E->getLHS(), TCK_Store);
+
     if (RV.isScalar())
       EmitNullabilityCheck(LV, RV.getScalarVal(), E->getExprLoc());
-    EmitStoreThroughLValue(RV, LV);
+
+    if (LV.isBitField()) {
+      llvm::Value *Result;
+      // If bitfield sanitizers are enabled we want to use the result
+      // to check whether a truncation or sign change has occurred.
+      if (SanOpts.has(SanitizerKind::ImplicitBitfieldConversion))
+        EmitStoreThroughBitfieldLValue(RV, LV, &Result);
+      else
+        EmitStoreThroughBitfieldLValue(RV, LV);
+
+      // If the expression contained an implicit conversion, make sure
+      // to use the value before the scalar conversion.
+      llvm::Value *Src = Previous ? Previous : RV.getScalarVal();
+      QualType DstType = E->getLHS()->getType();
+      EmitBitfieldConversionCheck(Src, SrcType, Result, DstType,
+                                  LV.getBitFieldInfo(), E->getExprLoc());
+    } else
+      EmitStoreThroughLValue(RV, LV);
+
     if (getLangOpts().OpenMP)
       CGM.getOpenMPRuntime().checkAndEmitLastprivateConditional(*this,
                                                                 E->getLHS());