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

[Laszlo Ersek <lersek@redhat.com>]

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. 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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>>> edk2-devel mailing list
>>> edk2-devel@lists.01.org
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