[edk2-devel] [PATCH EDK2 v1 1/1] MdeModulePkg/FileExplorerLib: remove redundant null pointer check

[wenyi,xie via groups.io]

I see. I will correct commit message soon.

Thanks
Wenyi

On 2020/11/25 23:01, Laszlo Ersek wrote:
> On 11/25/20 02:52, wenyi,xie via groups.io wrote:
>> Since Info is a pointer of struct EFI_FILE_SYSTEM_VOLUME_LABEL,
>> and this struct has only one member VolumeLabel which is char
>> array. So Info and Info->VolumeLabel are point to the same
>> address, and if Info != NULL, Info->VolumeLabel == NULL is
>> always false, it's no necessary to do null pointer check again.
> 
> I agree with the code change, and I agree with Liming that this is
> material for *after* the stable tag.
> 
> However, I disagree with the wording of the commit message. I tried to
> explain why, on edk2-discuss. Let me try again, here.
> 
> It is *irrelevant* that "Info", and "Info->VolumeLabel", evaluate to the
> same pointer values (same addresses). Namely, if "VolumeLabel" were
> *not* the first field in EFI_FILE_SYSTEM_VOLUME_LABEL, then this
> equality would cease to hold, but the code change would *still* be
> correct. Which means that the equality ("same address") is *totally
> irrelevant* -- and so it should not be included in the commit message.
> 
> Instead, what matters is that, if "Info" is a *valid* pointer (it points
> to a valid EFI_FILE_SYSTEM_VOLUME_LABEL object), then
> "Info->VolumeLabel" is a valid *array object*. The expression
> "Info->VolumeLabel" has type "array of CHAR16". When evaluating an
> expression with array type, the array is implicitly converted to a
> pointer to the first element in the array. See the spec quote below:
> 
> ISO C99,
> - 6.3 Conversions
> - 6.3.2 Other operands
> - 6.3.2.1 Lvalues, arrays, and function designators
> - paragprah 3:
> 
>     Except when it is the operand of the sizeof operator or the unary &
>     operator, or is a string literal used to initialize an array, an
>     expression that has type "array of type" is converted to an
>     expression with type "pointer to type" that points to the initial
>     element of the array object and is not an lvalue. [...]
> 
> *That* is why (Info != NULL) guarantees that (Info->VolumeLabel !=
> NULL). Because, we take (Info != NULL) to mean (*Info) is a valid
> EFI_FILE_SYSTEM_VOLUME_LABEL object. And so (Info->VolumeLabel) is an
> array object, Info->VolumeLabel[0] is a valid CHAR16 object, and the
> address of a valid CHAR16 object *cannot be* NULL. For the latter, see
> the following spec excerpt:
> 
> ISO C99,
> - 6.3 Conversions
> - 6.3.2 Other operands
> - 6.3.2.3 Pointers
> - paragprah 3:
> 
>     An integer constant expression with the value 0, or such an
>     expression cast to type void*, is called a null pointer constant.
>     [55] If a null pointer constant is converted to a pointer type, the
>     resulting pointer, called a null pointer, is guaranteed to compare
>     unequal to a pointer to any object or function.
> 
>     Footnote [55]: The macro NULL is defined in <stddef.h> (and other
>     headers) as a null pointer constant; see 7.17.
> 
> 
> Please replace the commit message with the following:
> 
> """
> If "Info" is a valid pointer to an EFI_FILE_SYSTEM_VOLUME_LABEL
> structure, then "Info->VolumeLabel" denotes a valid array object. When
> the "Info->VolumeLabel" expression is evaluated, as seen in the
> LibFindFileSystem(), it is implicitly converted to
> (&Info->VolumeLabel[0]). Because the object described by the expression
> (Info->VolumeLabel[0]) is a valid CHAR16 object, its address can never
> compare equal to NULL. Therefore, the condition (Info->VolumeLabel ==
> NULL) will always evaluate to FALSE. Substitute the constant FALSE into
> the "if" statement, and simplify the resultant code (eliminate the dead
> branch).
> """
> 
> Thanks
> Laszlo
> 
>>
>> Cc: Jian J Wang <jian.j.wang at intel.com>
>> Cc: Hao A Wu <hao.a.wu at intel.com>
>> Cc: Dandan Bi <dandan.bi at intel.com>
>> Cc: Eric Dong <eric.dong at intel.com>
>> Signed-off-by: Wenyi Xie <xiewenyi2 at huawei.com>
>> ---
>>  MdeModulePkg/Library/FileExplorerLib/FileExplorer.c | 10 +++-------
>>  1 file changed, 3 insertions(+), 7 deletions(-)
>>
>> diff --git a/MdeModulePkg/Library/FileExplorerLib/FileExplorer.c b/MdeModulePkg/Library/FileExplorerLib/FileExplorer.c
>> index 58e49102593c..13a214b06af9 100644
>> --- a/MdeModulePkg/Library/FileExplorerLib/FileExplorer.c
>> +++ b/MdeModulePkg/Library/FileExplorerLib/FileExplorer.c
>> @@ -821,13 +821,9 @@ LibFindFileSystem (
>>        if (Info == NULL) {
>>          VolumeLabel = L"NO FILE SYSTEM INFO";
>>        } else {
>> -        if (Info->VolumeLabel == NULL) {
>> -          VolumeLabel = L"NULL VOLUME LABEL";
>> -        } else {
>> -          VolumeLabel = Info->VolumeLabel;
>> -          if (*VolumeLabel == 0x0000) {
>> -            VolumeLabel = L"NO VOLUME LABEL";
>> -          }
>> +        VolumeLabel = Info->VolumeLabel;
>> +        if (*VolumeLabel == 0x0000) {
>> +          VolumeLabel = L"NO VOLUME LABEL";
>>          }
>>        }
>>        MenuEntry->DisplayString  = AllocateZeroPool (MAX_CHAR);
>>
> 
> .
> 


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