Re: [PATCH] MdePkg: Fix undefined behavior on variadic parameters

["Gao, Liming" <liming.gao@intel.com>]

Sergey:
  This patch updates API interface. I still need to verify its functionality on other tool chain. I will give you feedback after I am done. 

Thanks
Liming
>-----Original Message-----
>From: Laszlo Ersek [mailto:lersek@redhat.com]
>Sent: Friday, May 19, 2017 4:16 PM
>To: Sergei Temerkhanov <s.temerkhanov@gmail.com>
>Cc: Gao, Liming <liming.gao@intel.com>; edk2-devel@lists.01.org
>Subject: Re: [edk2] [PATCH] MdePkg: Fix undefined behavior on variadic
>parameters
>
>On 05/19/17 04:45, Sergei Temerkhanov wrote:
>> On Thu, May 18, 2017 at 1:19 PM, Laszlo Ersek <lersek@redhat.com> wrote:
>>> On 05/16/17 14:10, Sergei Temerkhanov wrote:
>>>> On Tue, May 16, 2017 at 8:10 AM, Gao, Liming <liming.gao@intel.com>
>wrote:
>>>>> Sergey:
>>>>>   Could you give more detail on the undefined behavior on variadic
>parameters?
>>>>>
>>>>>   I see https://bugzilla.tianocore.org/show_bug.cgi?id=410 describe this
>issues found in the latest CLANG tool chain. Do you find other tool chain
>reports it?
>>>>
>>>> Yes, this is exactly the bug this patch fixes.
>>>>
>>>> As per the C99 standard:
>>>> "The parameter parmN is the identifier of the rightmost parameter in
>>>> the variable parameter list in the function definition (the one just
>>>> before the , ...). If the parameter parmN is declared with the
>>>> register storage class, with a function or array type, or with a type
>>>> that is not compatible with the type that results after application of
>>>> the default argument promotions, the behavior is undefined."
>>>>
>>>> That's exactly the case here since BOOLEAN is a typedef for unsigned
>>>> char. It undergoes a promotion to an unsigned int
>>>
>>> Side topic:
>>>
>>> It is promoted, but not to "unsigned int".
>>>
>>> The standard says, in "6.3.1.1 Boolean, characters, and integers",
>>> paragraph 2,
>>>
>>>     The following may be used in an expression wherever an /int/ or
>>>     /unsigned int/ may be used:
>>>
>>>     — An object or expression with an integer type whose integer
>>>       conversion rank is less than or equal to the rank of /int/ and
>>>       /unsigned int/.
>>>     — A bit-field of type /_Bool/, /int/, /signed int/, or
>>>       /unsigned int/.
>>>
>>>     If an /int/ can represent all values of the original type, the value
>>>     is converted to an /int/; otherwise, it is converted to an
>>>     /unsigned int/. These are called the /integer promotions/. [...]
>>>
>>> On all supported edk2 platforms, "unsigned char"'s range is 0..255
>>> inclusive, which can be represented by "int" (again on all supported
>>> edk2 platforms). So the promotion occurs to "int", not "unsigned int"
>>>
>>>
>>> Furthermore, in place of the suggested UINTN type (which is fine), the
>>> following further types would be correct: INT32, UINT32, INT64, UINT64,
>>> INTN.
>>
>> On 32-bit architectures, using 64-bit types here may change the ABI. Which
>might
>> affect some corner cases like linking precompiled object files to the
>> library in question.
>
>True.
>
>I missed the fact that in edk2 you can have binary-only library
>instances. I should have remembered, after all I had filed
><https://bugzilla.tianocore.org/show_bug.cgi?id=463> :)
>
>So yes, UINTN is the best choice; it keeps binary compat beyond
>everything else.
>
>Thanks!
>Laszlo
>
>>
>>> The reason is that all of these map to standard C types, on all
>>> edk2 platforms, whose integer conversion ranks are not less than that of
>>> "int" and "unsigned int". Hence they are all unaffected by the integer
>>> promotions.
>>>
>>> (This digression does not affect your main point, which remains correct;
>>> I just wanted to be precise here, since we're quoting the standard.)
>>>
>>> Thanks
>>> Laszlo
>>>
>>>> which is not a
>>>> compatible type for unsigned char. Correct me if I'm wrong.
>>>>
>>>> Regards,
>>>> Sergey
>>>>
>>>>>
>>>>> Thanks
>>>>> Liming
>>>>>> -----Original Message-----
>>>>>> From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf
>Of Sergey Temerkhanov
>>>>>> Sent: Tuesday, May 16, 2017 10:57 AM
>>>>>> To: edk2-devel@lists.01.org
>>>>>> Subject: [edk2] [PATCH] MdePkg: Fix undefined behavior on variadic
>parameters
>>>>>>
>>>>>> Fix undefined behavior by avoiding parameter type promotion
>>>>>>
>>>>>> Signed-off-by: Sergey Temerkhanov <s.temerkhanov@gmail.com>
>>>>>> ---
>>>>>>  MdePkg/Include/Library/UefiLib.h | 2 +-
>>>>>>  MdePkg/Library/UefiLib/UefiLib.c | 2 +-
>>>>>>  2 files changed, 2 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/MdePkg/Include/Library/UefiLib.h
>b/MdePkg/Include/Library/UefiLib.h
>>>>>> index 0b14792..4e4697c 100644
>>>>>> --- a/MdePkg/Include/Library/UefiLib.h
>>>>>> +++ b/MdePkg/Include/Library/UefiLib.h
>>>>>> @@ -818,7 +818,7 @@ CHAR8 *
>>>>>>  EFIAPI
>>>>>>  GetBestLanguage (
>>>>>>    IN CONST CHAR8  *SupportedLanguages,
>>>>>> -  IN BOOLEAN      Iso639Language,
>>>>>> +  IN UINTN      Iso639Language,
>>>>>>    ...
>>>>>>    );
>>>>>>
>>>>>> diff --git a/MdePkg/Library/UefiLib/UefiLib.c
>b/MdePkg/Library/UefiLib/UefiLib.c
>>>>>> index a7eee01..74528ec 100644
>>>>>> --- a/MdePkg/Library/UefiLib/UefiLib.c
>>>>>> +++ b/MdePkg/Library/UefiLib/UefiLib.c
>>>>>> @@ -1514,7 +1514,7 @@ CHAR8 *
>>>>>>  EFIAPI
>>>>>>  GetBestLanguage (
>>>>>>    IN CONST CHAR8  *SupportedLanguages,
>>>>>> -  IN BOOLEAN      Iso639Language,
>>>>>> +  IN UINTN      Iso639Language,
>>>>>>    ...
>>>>>>    )
>>>>>>  {
>>>>>> --
>>>>>> 2.7.4
>>>>>>
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