[PATCH v3 2/6] CryptoPkg: Refine type cast for pointer subtraction

[Hao Wu <hao.a.wu@intel.com>]

For pointer subtraction, the result is of type "ptrdiff_t". According to
the C11 standard (Committee Draft - April 12, 2011):

"When two pointers are subtracted, both shall point to elements of the
same array object, or one past the last element of the array object; the
result is the difference of the subscripts of the two array elements. The
size of the result is implementation-defined, and its type (a signed
integer type) is ptrdiff_t defined in the <stddef.h> header. If the result
is not representable in an object of that type, the behavior is
undefined."

In our codes, there are cases that the pointer subtraction is not
performed by pointers to elements of the same array object. This might
lead to potential issues, since the behavior is undefined according to C11
standard.

Also, since the size of type "ptrdiff_t" is implementation-defined. Some
static code checkers may warn that the pointer subtraction might underflow
first and then being cast to a bigger size. For example:

UINT8  *Ptr1, *Ptr2;
UINTN  PtrDiff;
...
PtrDiff = (UINTN) (Ptr1 - Ptr2);

The commit will refine the pointer subtraction expressions by casting each
pointer to UINTN first and then perform the subtraction:

PtrDiff = (UINTN) Ptr1 - (UINTN) Ptr2;

Cc: Qin Long <qin.long@intel.com>
Cc: Ting Ye <ting.ye@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
---
 CryptoPkg/Library/BaseCryptLib/SysCall/RuntimeMemAllocation.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/CryptoPkg/Library/BaseCryptLib/SysCall/RuntimeMemAllocation.c b/CryptoPkg/Library/BaseCryptLib/SysCall/RuntimeMemAllocation.c
index 19c30dc..feaa371 100644
--- a/CryptoPkg/Library/BaseCryptLib/SysCall/RuntimeMemAllocation.c
+++ b/CryptoPkg/Library/BaseCryptLib/SysCall/RuntimeMemAllocation.c
@@ -2,7 +2,7 @@
   Light-weight Memory Management Routines for OpenSSL-based Crypto
   Library at Runtime Phase.
 
-Copyright (c) 2009 - 2016, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2009 - 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
@@ -282,7 +282,7 @@ RuntimeFreeMem (
   UINTN  StartOffset;
   UINTN  StartPageIndex;
 
-  StartOffset    = (UINTN) ((UINT8 *)Buffer - mRTPageTable->DataAreaBase);
+  StartOffset    = (UINTN)Buffer - (UINTN)mRTPageTable->DataAreaBase;
   StartPageIndex = RT_SIZE_TO_PAGES (mRTPageTable->Pages[RT_SIZE_TO_PAGES(StartOffset)].StartPageOffset);
 
   while (StartPageIndex < mRTPageTable->PageCount) {
@@ -403,7 +403,7 @@ void *realloc (void *ptr, size_t size)
   //
   // Get Original Size of ptr
   //
-  StartOffset    = (UINTN) ((UINT8 *)ptr - mRTPageTable->DataAreaBase);
+  StartOffset    = (UINTN)ptr - (UINTN)mRTPageTable->DataAreaBase;
   StartPageIndex = RT_SIZE_TO_PAGES (mRTPageTable->Pages[RT_SIZE_TO_PAGES (StartOffset)].StartPageOffset);
   PageCount      = 0;
   while (StartPageIndex < mRTPageTable->PageCount) {
-- 
1.9.5.msysgit.0