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

[Laszlo Ersek <lersek@redhat.com>]

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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>>