Re: [PATCH v3 4/6] NetworkPkg: Refine type cast for pointer subtraction

["Fu, Siyuan" <siyuan.fu@intel.com>]

Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>


-----Original Message-----
From: Wu, Hao A 
Sent: 2017年2月25日 12:05
To: edk2-devel@lists.01.org
Cc: Wu, Hao A <hao.a.wu@intel.com>; Fu, Siyuan <siyuan.fu@intel.com>; Wu, Jiaxin <jiaxin.wu@intel.com>
Subject: [PATCH v3 4/6] NetworkPkg: Refine type cast for pointer subtraction

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: Siyuan Fu <siyuan.fu@intel.com>
Cc: Jiaxin Wu <jiaxin.wu@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>
---
 NetworkPkg/HttpDxe/HttpImpl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/NetworkPkg/HttpDxe/HttpImpl.c b/NetworkPkg/HttpDxe/HttpImpl.c index 745832b..1f7a4fa 100644
--- a/NetworkPkg/HttpDxe/HttpImpl.c
+++ b/NetworkPkg/HttpDxe/HttpImpl.c
@@ -1342,7 +1342,7 @@ HttpResponseWorker (
     // We receive part of header of next HTTP msg.
     //
     if (HttpInstance->NextMsg != NULL) {
-      HttpMsg->BodyLength = MIN ((UINTN) (HttpInstance->NextMsg - (CHAR8 *) Fragment.Bulk), HttpMsg->BodyLength);
+      HttpMsg->BodyLength = MIN ((UINTN) HttpInstance->NextMsg - 
+ (UINTN) Fragment.Bulk, HttpMsg->BodyLength);
       CopyMem (HttpMsg->Body, Fragment.Bulk, HttpMsg->BodyLength);
       
       HttpInstance->CacheLen = Fragment.Len - HttpMsg->BodyLength; @@ -1360,7 +1360,7 @@ HttpResponseWorker (
         CopyMem (HttpInstance->CacheBody, Fragment.Bulk + HttpMsg->BodyLength, HttpInstance->CacheLen);
         HttpInstance->CacheOffset = 0;
 
-        HttpInstance->NextMsg = HttpInstance->CacheBody + (UINTN) (HttpInstance->NextMsg - (CHAR8 *) (Fragment.Bulk + HttpMsg->BodyLength));
+        HttpInstance->NextMsg = HttpInstance->CacheBody + ((UINTN) 
+ HttpInstance->NextMsg - (UINTN) (Fragment.Bulk + 
+ HttpMsg->BodyLength));
       }
     } else {
       HttpMsg->BodyLength = MIN (Fragment.Len, (UINT32) HttpMsg->BodyLength);
--
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