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

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

> -----Original Message-----
> From: Ni, Ruiyu
> Sent: Monday, February 27, 2017 1:10 PM
> To: Wu, Hao A; edk2-devel@lists.01.org
> Cc: Carsey, Jaben
> Subject: RE: [PATCH v3 6/6] ShellPkg: Refine type cast for pointer subtraction
> 
> 
> 
> Thanks/Ray
> 
> > -----Original Message-----
> > From: Wu, Hao A
> > Sent: Saturday, February 25, 2017 12:05 PM
> > To: edk2-devel@lists.01.org
> > Cc: Wu, Hao A <hao.a.wu@intel.com>; Carsey, Jaben
> > <jaben.carsey@intel.com>; Ni, Ruiyu <ruiyu.ni@intel.com>
> > Subject: [PATCH v3 6/6] ShellPkg: 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;
> 
> Is this the new coding rule? I think the below code is not very readable:
> Diff = ((UINTN) Ptr1 - (UINTN) Ptr2) / sizeof (STRUCTURE))
> Do we have better fix?
> 

Hi Ray,

Most of the pointer subtraction expressions in the code base are not
performed between two pointers to elements of the same array object, or
one past the last element of the array object (according to C11 standard).
Although the refined code is less readable, but I think the refine stick
with the standard better.

The number of pointer subtractions used in the edk2 code base is not
very large. So the series does not touch lots of files. Also, we may take
this as part of the coding style when dealing with pointer subtractions.

Best Regards,
Hao Wu

> >
> > Cc: Jaben Carsey <jaben.carsey@intel.com>
> > Cc: Ruiyu Ni <ruiyu.ni@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>
> > ---
> >  ShellPkg/Application/Shell/ShellParametersProtocol.c                | 6 +++---
> >  ShellPkg/Library/UefiShellDebug1CommandsLib/DmpStore.c              | 4 ++--
> >  ShellPkg/Library/UefiShellDebug1CommandsLib/SmbiosView/SmbiosView.c
> > | 4 ++--
> >  ShellPkg/Library/UefiShellDriver1CommandsLib/Dh.c                   | 4 ++--
> >  ShellPkg/Library/UefiShellLevel2CommandsLib/Cd.c                    | 4 ++--
> >  5 files changed, 11 insertions(+), 11 deletions(-)
> >
> > diff --git a/ShellPkg/Application/Shell/ShellParametersProtocol.c
> > b/ShellPkg/Application/Shell/ShellParametersProtocol.c
> > index 4999155..8d76fb4 100644
> > --- a/ShellPkg/Application/Shell/ShellParametersProtocol.c
> > +++ b/ShellPkg/Application/Shell/ShellParametersProtocol.c
> > @@ -5,7 +5,7 @@
> >    (C) Copyright 2016 Hewlett Packard Enterprise Development LP<BR>
> >    Copyright (C) 2014, Red Hat, Inc.
> >    (C) Copyright 2013 Hewlett-Packard Development Company, L.P.<BR>
> > -  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
> > @@ -1034,9 +1034,9 @@ UpdateStdInStdOutStdErr(
> >    StrnCpyS(CommandLineCopy, StrSize(CommandLineCopy)/sizeof(CHAR16),
> > NewCommandLine, StrLen(NewCommandLine));
> >
> >    if (FirstLocation != CommandLineCopy + StrLen(CommandLineCopy)
> > -    && ((UINTN)(FirstLocation - CommandLineCopy) <
> > StrLen(NewCommandLine))
> > +    && (((UINTN)FirstLocation - (UINTN)CommandLineCopy)/sizeof(CHAR16)
> > < StrLen(NewCommandLine))
> >      ){
> > -    *(NewCommandLine + (UINTN)(FirstLocation - CommandLineCopy)) =
> > CHAR_NULL;
> > +    *(NewCommandLine + ((UINTN)FirstLocation -
> > (UINTN)CommandLineCopy)/sizeof(CHAR16)) = CHAR_NULL;
> >    }
> >
> >    if (!EFI_ERROR(Status)) {
> > diff --git a/ShellPkg/Library/UefiShellDebug1CommandsLib/DmpStore.c
> > b/ShellPkg/Library/UefiShellDebug1CommandsLib/DmpStore.c
> > index bb2c0b9..18f15fa 100644
> > --- a/ShellPkg/Library/UefiShellDebug1CommandsLib/DmpStore.c
> > +++ b/ShellPkg/Library/UefiShellDebug1CommandsLib/DmpStore.c
> > @@ -2,7 +2,7 @@
> >    Main file for DmpStore shell Debug1 function.
> >
> >    (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.<BR>
> > -  Copyright (c) 2005 - 2016, Intel Corporation. All rights reserved.<BR>
> > +  Copyright (c) 2005 - 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
> > @@ -364,7 +364,7 @@ AppendSingleVariableToFile (
> >    //
> >    // Crc32
> >    //
> > -  gBS->CalculateCrc32 (Buffer, (UINTN) (Ptr - Buffer), (UINT32 *) Ptr);
> > +  gBS->CalculateCrc32 (Buffer, (UINTN) Ptr - (UINTN) Buffer, (UINT32 *) Ptr);
> >
> >    Status = ShellWriteFile (FileHandle, &BufferSize, Buffer);
> >    FreePool (Buffer);
> > diff --git
> > a/ShellPkg/Library/UefiShellDebug1CommandsLib/SmbiosView/SmbiosView
> > .c
> > b/ShellPkg/Library/UefiShellDebug1CommandsLib/SmbiosView/SmbiosView
> > .c
> > index 56b682a..a5b16fe 100644
> > ---
> > a/ShellPkg/Library/UefiShellDebug1CommandsLib/SmbiosView/SmbiosView
> > .c
> > +++
> > b/ShellPkg/Library/UefiShellDebug1CommandsLib/SmbiosView/SmbiosView
> > .c
> > @@ -2,7 +2,7 @@
> >    Tools of clarify the content of the smbios table.
> >
> >    (C) Copyright 2015 Hewlett-Packard Development Company, L.P.<BR>
> > -  Copyright (c) 2005 - 2012, Intel Corporation. All rights reserved.<BR>
> > +  Copyright (c) 2005 - 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
> > @@ -700,7 +700,7 @@ CalculateSmbios64BitStructureCountAndLength (
> >      //
> >      // Length = Next structure head - this structure head
> >      //
> > -    (*Smbios64TableLength) += (UINTN) (Smbios.Raw - Raw);
> > +    (*Smbios64TableLength) += ((UINTN) Smbios.Raw - (UINTN) Raw);
> >      if ((*Smbios64TableLength) > Smbios64EntryPoint->TableMaximumSize) {
> >      	//
> >      	// The actual table length exceeds maximum table size,
> > diff --git a/ShellPkg/Library/UefiShellDriver1CommandsLib/Dh.c
> > b/ShellPkg/Library/UefiShellDriver1CommandsLib/Dh.c
> > index d17a29d..0bcee92 100644
> > --- a/ShellPkg/Library/UefiShellDriver1CommandsLib/Dh.c
> > +++ b/ShellPkg/Library/UefiShellDriver1CommandsLib/Dh.c
> > @@ -75,7 +75,7 @@ IsValidGuidString(
> >         ) {
> >        Walker++;
> >      } else {
> > -      if (*Walker == L'-' && (UINTN)(Walker - PrevWalker) ==
> > mGuidDataLen[Index]) {
> > +      if (*Walker == L'-' && (((UINTN)Walker - (UINTN)PrevWalker) / sizeof
> > (CHAR16)) == mGuidDataLen[Index]) {
> >          Walker++;
> >          PrevWalker = Walker;
> >          Index++;
> > @@ -85,7 +85,7 @@ IsValidGuidString(
> >      }
> >    }
> >
> > -  if ((UINTN)(Walker - PrevWalker) == mGuidDataLen[Index]) {
> > +  if ((((UINTN)Walker - (UINTN)PrevWalker) / sizeof (CHAR16)) ==
> > mGuidDataLen[Index]) {
> >      return TRUE;
> >    } else {
> >      return FALSE;
> > diff --git a/ShellPkg/Library/UefiShellLevel2CommandsLib/Cd.c
> > b/ShellPkg/Library/UefiShellLevel2CommandsLib/Cd.c
> > index dd4a740..9ae8176 100644
> > --- a/ShellPkg/Library/UefiShellLevel2CommandsLib/Cd.c
> > +++ b/ShellPkg/Library/UefiShellLevel2CommandsLib/Cd.c
> > @@ -99,10 +99,10 @@ IsCurrentFileSystem (
> >
> >    Splitter2 = StrStr (Cwd, L":");
> >
> > -  if ((UINTN) (Splitter1 - FullPath) != (UINTN) (Splitter2 - Cwd)) {
> > +  if (((UINTN) Splitter1 - (UINTN) FullPath) != ((UINTN) Splitter2 - (UINTN)
> > Cwd)) {
> >      return FALSE;
> >    } else {
> > -    if (StrniCmp (FullPath, Cwd, (UINTN) (Splitter1 - FullPath)) == NULL) {
> > +    if (StrniCmp (FullPath, Cwd, ((UINTN) Splitter1 - (UINTN) FullPath) / sizeof
> > (CHAR16)) == NULL) {
> >        return TRUE;
> >      } else {
> >        return FALSE;
> > --
> > 1.9.5.msysgit.0