[PATCH] MdePkg/UefiFileHandleLib: Fix the Root directory determination.

["Marvin Häuser" <Marvin.Haeuser@outlook.com>]

The current implementation of the FileHandleGetFileName() function
assumes that the Root directory always has the FileName '\0'.
However, the only requirement the UEFI specification defines is that
a prepended '\\' must be supported to access files and folders
relative to the Root directory.
This patch removes this assumption and supports constructing valid
paths for any value of FileName for the Root Directory.

In practice, this fixes compatibility issues with File System drivers
that report '\\' as the FileName of the Root directory, which
currently is both generating an invalid path ("\\\\") and resulting
in an EFI_NOT_FOUND result from the CurrentHandle->Open() call.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marvin Haeuser <Marvin.Haeuser@outlook.com>
---
 MdePkg/Library/UefiFileHandleLib/UefiFileHandleLib.c | 29 ++++++++++++++------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/MdePkg/Library/UefiFileHandleLib/UefiFileHandleLib.c b/MdePkg/Library/UefiFileHandleLib/UefiFileHandleLib.c
index 57aad77bc135..820801422996 100644
--- a/MdePkg/Library/UefiFileHandleLib/UefiFileHandleLib.c
+++ b/MdePkg/Library/UefiFileHandleLib/UefiFileHandleLib.c
@@ -1,7 +1,7 @@
 /** @file
   Provides interface to EFI_FILE_HANDLE functionality.
 
-  Copyright (c) 2006 - 2017, Intel Corporation. All rights reserved. <BR>
+  Copyright (c) 2006 - 2018, 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
@@ -820,10 +820,25 @@ FileHandleGetFileName (
         Status = EFI_OUT_OF_RESOURCES;
         break;
       } else {
+        //
+        // Prepare to move to the parent directory.
+        // Also determine whether CurrentHandle refers to the Root directory.
+        //
+        Status = CurrentHandle->Open (CurrentHandle, &NextHigherHandle, L"..", EFI_FILE_MODE_READ, 0);
         //
         // We got info... do we have a name? if yes precede the current path with it...
         //
-        if (StrLen (FileInfo->FileName) == 0) {
+        if ((StrLen (FileInfo->FileName) == 0) || EFI_ERROR (Status)) {
+          //
+          // Both FileInfo->FileName being '\0' and EFI_ERROR() suggest that
+          // CurrentHandle refers to the Root directory.  As this loop ensures
+          // FullFileName is starting with '\\' at all times, signal success
+          // and exit the loop.
+          // While FileInfo->FileName could theoretically be a value other than
+          // '\0' or '\\', '\\' is guaranteed to be supported by the
+          // specification and hence its value can safely be ignored.
+          //
+          Status = EFI_SUCCESS;
           if (*FullFileName == NULL) {
             ASSERT((*FullFileName == NULL && Size == 0) || (*FullFileName != NULL));
             *FullFileName = StrnCatGrowLeft(FullFileName, &Size, L"\\", 0);
@@ -841,15 +856,11 @@ FileHandleGetFileName (
           FreePool(FileInfo);
         }
       }
-      //
-      // Move to the parent directory
-      //
-      Status = CurrentHandle->Open (CurrentHandle, &NextHigherHandle, L"..", EFI_FILE_MODE_READ, 0);
-      if (EFI_ERROR (Status)) {
-        break;
-      }
 
       FileHandleClose(CurrentHandle);
+      //
+      // Move to the parent directory
+      //
       CurrentHandle = NextHigherHandle;
     }
   } else if (Status == EFI_NOT_FOUND) {
-- 
2.17.0.windows.1