3 files changed, 20 insertions, 3 deletions

diff --git a/clang/lib/Sema/Sema.h b/clang/lib/Sema/Sema.h
index 0a4086fe3e4..63ecdcb108d 100644
--- a/clang/lib/Sema/Sema.h
+++ b/clang/lib/Sema/Sema.h
@@ -3308,6 +3308,7 @@ private:
   Action::OwningExprResult CheckBlockCall(NamedDecl *NDecl, CallExpr *TheCall);
   SourceLocation getLocationOfStringLiteralByte(const StringLiteral *SL,
                                                 unsigned ByteNo) const;
+  bool CheckablePrintfAttr(const FormatAttr *Format, CallExpr *TheCall);
   bool CheckObjCString(Expr *Arg);
   bool SemaBuiltinVAStart(CallExpr *TheCall);
   bool SemaBuiltinUnorderedCompare(CallExpr *TheCall);
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index d6095fc2ae5..949c33dfff8 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -84,6 +84,21 @@ SourceLocation Sema::getLocationOfStringLiteralByte(const StringLiteral *SL,
   }
 }
 
+/// CheckablePrintfAttr - does a function call have a "printf" attribute
+/// and arguments that merit checking?
+bool Sema::CheckablePrintfAttr(const FormatAttr *Format, CallExpr *TheCall) {
+  if (Format->getType() == "printf") return true;
+  if (Format->getType() == "printf0") {
+    // printf0 allows null "format" string; if so don't check format/args
+    unsigned format_idx = Format->getFormatIdx() - 1;
+    if (format_idx < TheCall->getNumArgs()) {
+      Expr *Format = TheCall->getArg(format_idx)->IgnoreParenCasts();
+      if (!Format->isNullPointerConstant(Context))
+        return true;
+    }
+  }
+  return false;
+}
 
 /// CheckFunctionCall - Check a direct function call for various correctness
 /// and safety properties not strictly enforced by the C type system.
@@ -167,7 +182,7 @@ Sema::CheckFunctionCall(FunctionDecl *FDecl, CallExpr *TheCall) {
 
   // Printf checking.
   if (const FormatAttr *Format = FDecl->getAttr<FormatAttr>()) {
-    if (Format->getType() == "printf") {
+    if (CheckablePrintfAttr(Format, TheCall)) {
       bool HasVAListArg = Format->getFirstArg() == 0;
       if (!HasVAListArg) {
         if (const FunctionProtoType *Proto 
@@ -201,7 +216,7 @@ Sema::CheckBlockCall(NamedDecl *NDecl, CallExpr *TheCall) {
   QualType Ty = V->getType();
   if (!Ty->isBlockPointerType())
     return move(TheCallResult);
-  if (Format->getType() == "printf") {
+  if (CheckablePrintfAttr(Format, TheCall)) {
       bool HasVAListArg = Format->getFirstArg() == 0;
       if (!HasVAListArg) {
         const FunctionType *FT = 
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 239f9951c51..35230005904 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -1212,7 +1212,8 @@ static void HandleFormatAttr(Decl *d, const AttributeList &Attr, Sema &S) {
   default: break;
   case 5: Supported = !memcmp(Format, "scanf", 5); break;
   case 6: Supported = !memcmp(Format, "printf", 6); break;
-  case 7: Supported = !memcmp(Format, "strfmon", 7); break;
+  case 7: Supported = !memcmp(Format, "printf0", 7) ||
+                      !memcmp(Format, "strfmon", 7); break;
   case 8:
     Supported = (is_strftime = !memcmp(Format, "strftime", 8)) ||
                 (is_NSString = !memcmp(Format, "NSString", 8)) ||