[PATCH V2 3/3] DuetPkg FsVariable: Update GetNextVariableName to follow UEFI 2.7

[Star Zeng <star.zeng@intel.com>]

"The size must be large enough to fit input string supplied in
VariableName buffer" is added in the description for VariableNameSize.
And two cases of EFI_INVALID_PARAMETER are added.
1. The input values of VariableName and VendorGuid are not a name and
   GUID of an existing variable.
2. Null-terminator is not found in the first VariableNameSize bytes of
   the input VariableName buffer.

This patch is to update code to follow them.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
---
 DuetPkg/FSVariable/FSVariable.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/DuetPkg/FSVariable/FSVariable.c b/DuetPkg/FSVariable/FSVariable.c
index 34b79305c871..6069cfa8fb98 100644
--- a/DuetPkg/FSVariable/FSVariable.c
+++ b/DuetPkg/FSVariable/FSVariable.c
@@ -6,7 +6,7 @@ disk. They can be changed by user. BIOS is not able to protoect those.
 Duet trusts all meta data from disk. If variable code, variable metadata and variable
 data is modified in inproper way, the behavior is undefined.
 
-Copyright (c) 2006 - 2016, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2006 - 2017, Intel Corporation. All rights reserved.<BR>
 This program and the accompanying materials
 are licensed and made available under the terms and conditions of the BSD License
 which accompanies this distribution.  The full text of the license may be found at
@@ -1400,14 +1400,33 @@ Returns:
   VARIABLE_POINTER_TRACK  Variable;
   UINTN                   VarNameSize;
   EFI_STATUS              Status;
+  UINTN                   MaxLen;
 
   if (VariableNameSize == NULL || VariableName == NULL || VendorGuid == NULL) {
     return EFI_INVALID_PARAMETER;
   }
 
+  //
+  // Calculate the possible maximum length of name string, including the Null terminator.
+  //
+  MaxLen = *VariableNameSize / sizeof (CHAR16);
+  if ((MaxLen == 0) ||
+      ((VariableName[MaxLen - 1] != 0) && (StrnLenS (VariableName, MaxLen) == MaxLen))) {
+    //
+    // Null-terminator is not found in the first VariableNameSize bytes of the input VariableName buffer.
+    //
+    return EFI_INVALID_PARAMETER;
+  }
+
   Status = FindVariable (VariableName, VendorGuid, &Variable);
 
   if (Variable.CurrPtr == NULL || EFI_ERROR (Status)) {
+    if (VariableName[0] != 0) {
+      //
+      // The input values of VariableName and VendorGuid are not a name and GUID of an existing variable.
+      //
+      Status = EFI_INVALID_PARAMETER;
+    }
     return Status;
   }
 
-- 
2.7.0.windows.1