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

[Sergei Temerkhanov <s.temerkhanov@gmail.com>]

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.

> 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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