Sema: Warn on sizeof on binary ops on decayed arrays.

The array will decay into a pointer, creating an unexpected result.
sizeof(array + int) is an easy to make typo for sizeof(array) + int.

This was motivated by a NetBSD security bug, used sizeof(key - r) instead of
sizeof(key) - r, reducing entropy in a random number generator.
http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/kern/subr_cprng.c.diff?r1=1.14&r2=1.15&only_with_tag=MAIN&f=h

Differential Revision: http://llvm-reviews.chandlerc.com/D571

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178371 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 28 insertions, 0 deletions

diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index c6882908c2..62bfa3c709 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -3089,6 +3089,24 @@ static bool CheckObjCTraitOperandConstraints(Sema &S, QualType T,
   return false;
 }
 
+/// \brief Check whether E is a pointer from a decayed array type (the decayed
+/// pointer type is equal to T) and emit a warning if it is.
+static void warnOnSizeofOnArrayDecay(Sema &S, SourceLocation Loc, QualType T,
+                                     Expr *E) {
+  // Don't warn if the operation changed the type.
+  if (T != E->getType())
+    return;
+
+  // Now look for array decays.
+  ImplicitCastExpr *ICE = dyn_cast<ImplicitCastExpr>(E);
+  if (!ICE || ICE->getCastKind() != CK_ArrayToPointerDecay)
+    return;
+
+  S.Diag(Loc, diag::warn_sizeof_array_decay) << ICE->getSourceRange()
+                                             << ICE->getType()
+                                             << ICE->getSubExpr()->getType();
+}
+
 /// \brief Check the constrains on expression operands to unary type expression
 /// and type traits.
 ///
@@ -3142,6 +3160,16 @@ bool Sema::CheckUnaryExprOrTypeTraitOperand(Expr *E,
         }
       }
     }
+
+    // Warn on "sizeof(array op x)" and "sizeof(x op array)", where the array
+    // decays into a pointer and returns an unintended result. This is most
+    // likely a typo for "sizeof(array) op x".
+    if (BinaryOperator *BO = dyn_cast<BinaryOperator>(E->IgnoreParens())) {
+      warnOnSizeofOnArrayDecay(*this, BO->getOperatorLoc(), BO->getType(),
+                               BO->getLHS());
+      warnOnSizeofOnArrayDecay(*this, BO->getOperatorLoc(), BO->getType(),
+                               BO->getRHS());
+    }
   }
 
   return false;