Re: [PATCH 3/5] MdeModulePkg/UdfDxe: replace zero-init of local variables with ZeroMem()

[Laszlo Ersek <lersek@redhat.com>]

On 09/12/17 10:55, Zeng, Star wrote:
> Reviewed-by: Star Zeng <star.zeng@intel.com>
> 
> BTW: How about to use "sizeof ()" instead of "sizeof"?

"sizeof" is a unary operator. The parentheses are mandatory when sizeof
is used with a type name, but when the operand of sizeof is an
expression, the parentheses frequently make the wrong impression.

Consider for example

  UINTN          Var1, Var2, Var3;
  STRUCTURE_TYPE VarStruct;

  Var1 = 5;
  Var2 = sizeof Var3 + Var1;        // [1]

Most people dislike the assignment to Var2, and will rewrite it as:

  Var2 = sizeof (Var3) + Var1;      // [2]

However, this is totally irrelevant, and does not reflect the binding
strengths between the "sizeof" and "+" operators. In order to reflect
that relationship correctly, the following parentheses should be added:

  Var2 = (sizeof Var3) + Var1;      // [3]

None of [2] or [3] make any difference relative to [1], of course, in
behavior.

I just dislike [2] because it *suggests* that putting Var3 in parens
"protects" Var1 from falling under "sizeof".

That's not the case. "sizeof" is not a function, it is a unary operator
like "*", "&", "!", etc.

If we want to add parens for emphasizing the actual operator binding,
then [3] is the way to go. For example, if "Var3" was a pointer, you
wouldn't write

  *(Var3) + Var1

in order to emphasize that "*" takes precedence over "+";  you would write

  (*Var3) + Var1


TL;DR: "sizeof" is a unary, prefix operator, not a function designator.

... In order not to delay this any longer, I will add the parens before
I push.

But, we should all know that those parens don't mean what most people
think they mean :)

Laszlo


> 
> 
> Thanks,
> Star
> -----Original Message-----
> From: Laszlo Ersek [mailto:lersek@redhat.com] 
> Sent: Sunday, September 10, 2017 8:13 AM
> To: edk2-devel-01 <edk2-devel@lists.01.org>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>; Dong, Eric <eric.dong@intel.com>; Paulo Alcantara <pcacjr@zytor.com>; Ni, Ruiyu <ruiyu.ni@intel.com>; Zeng, Star <star.zeng@intel.com>
> Subject: [PATCH 3/5] MdeModulePkg/UdfDxe: replace zero-init of local variables with ZeroMem()
> 
> In edk2, initialization of local variables is forbidden, both for stylistic reasons and because such initialization may generate calls to compiler intrinsics.
> 
> For the following initialization in UdfRead():
> 
>   CHAR16                          FileName[UDF_FILENAME_LENGTH] = { 0 };
> 
> clang-3.8 generates a memset() call, when building UdfDxe for IA32, which then fails to link.
> 
> Replace the initialization with ZeroMem().
> 
> Do the same to "FilePath" in UdfOpen().
> 
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Paulo Alcantara <pcacjr@zytor.com>
> Cc: Ruiyu Ni <ruiyu.ni@intel.com>
> Cc: Star Zeng <star.zeng@intel.com>
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> ---
>  MdeModulePkg/Universal/Disk/UdfDxe/File.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/MdeModulePkg/Universal/Disk/UdfDxe/File.c b/MdeModulePkg/Universal/Disk/UdfDxe/File.c
> index 8b9339567f8e..e7159ff861f7 100644
> --- a/MdeModulePkg/Universal/Disk/UdfDxe/File.c
> +++ b/MdeModulePkg/Universal/Disk/UdfDxe/File.c
> @@ -174,15 +174,16 @@ UdfOpen (
>  {
>    EFI_TPL                     OldTpl;
>    EFI_STATUS                  Status;
>    PRIVATE_UDF_FILE_DATA       *PrivFileData;
>    PRIVATE_UDF_SIMPLE_FS_DATA  *PrivFsData;
> -  CHAR16                      FilePath[UDF_PATH_LENGTH] = { 0 };
> +  CHAR16                      FilePath[UDF_PATH_LENGTH];
>    UDF_FILE_INFO               File;
>    PRIVATE_UDF_FILE_DATA       *NewPrivFileData;
>    CHAR16                      *TempFileName;
>  
> +  ZeroMem (FilePath, sizeof FilePath);
>    OldTpl = gBS->RaiseTPL (TPL_CALLBACK);
>  
>    if (This == NULL || NewHandle == NULL || FileName == NULL) {
>      Status = EFI_INVALID_PARAMETER;
>      goto Error_Invalid_Params;
> @@ -322,14 +323,15 @@ UdfRead (
>    EFI_BLOCK_IO_PROTOCOL           *BlockIo;
>    EFI_DISK_IO_PROTOCOL            *DiskIo;
>    UDF_FILE_INFO                   FoundFile;
>    UDF_FILE_IDENTIFIER_DESCRIPTOR  *NewFileIdentifierDesc;
>    VOID                            *NewFileEntryData;
> -  CHAR16                          FileName[UDF_FILENAME_LENGTH] = { 0 };
> +  CHAR16                          FileName[UDF_FILENAME_LENGTH];
>    UINT64                          FileSize;
>    UINT64                          BufferSizeUint64;
>  
> +  ZeroMem (FileName, sizeof FileName);
>    OldTpl = gBS->RaiseTPL (TPL_CALLBACK);
>  
>    if (This == NULL || BufferSize == NULL || (*BufferSize != 0 &&
>                                               Buffer == NULL)) {
>      Status = EFI_INVALID_PARAMETER;
> --
> 2.14.1.3.gb7cf6e02401b
> 
>