MdeModulePkg/Tpl: Fix negative value left shift

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=695

Within function CoreRestoreTpl(), left shift a negative value -2 is used
in:
"while (((-2 << NewTpl) & gEventPending) != 0) {"

which involves undefined behavior.

According to the C11 spec, Section 6.5.7:
> 4 The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated
>   bits are filled with zeros. If E1 has an unsigned type, the value
>   of the result is E1 * 2^E2 , reduced modulo one more than the
>   maximum value representable in the result type. If E1 has a signed
>   type and nonnegative value, and E1 * 2^E2 is representable in the
>   result type, then that is the resulting value; otherwise, the
>   behavior is undefined.

This commit refines the code logic to avoid left shifting the negative
value.

Cc: Steven Shi <steven.shi@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>

diff --git a/MdeModulePkg/Core/Dxe/Event/Tpl.c b/MdeModulePkg/Core/Dxe/Event/Tpl.c
index 8ad0a337015600614492d0094b87ddfb36c05e0c..e3caf832b8638124ace38d68f88e290160e7901c 100644 (file)
--- a/MdeModulePkg/Core/Dxe/Event/Tpl.c
+++ b/MdeModulePkg/Core/Dxe/Event/Tpl.c
@@ -1,7 +1,7 @@
 /** @file\r
   Task priority (TPL) functions.\r
 \r
-Copyright (c) 2006 - 2015, Intel Corporation. All rights reserved.<BR>\r
+Copyright (c) 2006 - 2017, Intel Corporation. All rights reserved.<BR>\r
 This program and the accompanying materials\r
 are licensed and made available under the terms and conditions of the BSD License\r
 which accompanies this distribution.  The full text of the license may be found at\r
@@ -103,6 +103,7 @@ CoreRestoreTpl (
   )\r
 {\r
   EFI_TPL     OldTpl;\r
+  EFI_TPL     PendingTpl;\r
 \r
   OldTpl = gEfiCurrentTpl;\r
   if (NewTpl > OldTpl) {\r
@@ -123,8 +124,13 @@ CoreRestoreTpl (
   //\r
   // Dispatch any pending events\r
   //\r
-  while (((-2 << NewTpl) & gEventPending) != 0) {\r
-    gEfiCurrentTpl = (UINTN) HighBitSet64 (gEventPending);\r
+  while (gEventPending != 0) {\r
+    PendingTpl = (UINTN) HighBitSet64 (gEventPending);\r
+    if (PendingTpl <= NewTpl) {\r
+      break;\r
+    }\r
+\r
+    gEfiCurrentTpl = PendingTpl;\r
     if (gEfiCurrentTpl < TPL_HIGH_LEVEL) {\r
       CoreSetInterruptState (TRUE);\r
     }\r