[EXTERNAL] Re: [edk2-devel] [PATCH v1 0/5] EDK2 Code First: PI Specification: Update EFI_MM_COMMUNICATE_HEADER

Bret Barkelew via groups.io bret.barkelew=microsoft.com at groups.io
Tue Jun 29 06:49:59 UTC 2021

The fact that it may vary per ABI seems like a pretty big gotcha if the SMM/MM Core was compiled at a different time or on a different system than the module that’s invoking the communication.

- Bret

From: Marvin Häuser<mailto:mhaeuser at posteo.de>
Sent: Monday, June 28, 2021 8:43 AM
To: Laszlo Ersek<mailto:lersek at redhat.com>; Kun Qin<mailto:kuqin12 at gmail.com>; Kinney, Michael D<mailto:michael.d.kinney at intel.com>; devel at edk2.groups.io<mailto:devel at edk2.groups.io>
Cc: Wang, Jian J<mailto:jian.j.wang at intel.com>; Wu, Hao A<mailto:hao.a.wu at intel.com>; Dong, Eric<mailto:eric.dong at intel.com>; Ni, Ray<mailto:ray.ni at intel.com>; Liming Gao<mailto:gaoliming at byosoft.com.cn>; Liu, Zhiguang<mailto:zhiguang.liu at intel.com>; Andrew Fish<mailto:afish at apple.com>; Lindholm, Leif<mailto:leif at nuviainc.com>; Bret Barkelew<mailto:Bret.Barkelew at microsoft.com>; Michael Kubacki<mailto:Michael.Kubacki at microsoft.com>
Subject: [EXTERNAL] Re: [edk2-devel] [PATCH v1 0/5] EDK2 Code First: PI Specification: Update EFI_MM_COMMUNICATE_HEADER

On 28.06.21 16:57, Laszlo Ersek wrote:
> On 06/25/21 20:47, Kun Qin wrote:
>> Hi Mike,
>> Thanks for the information. I can update the patch and proposed spec
>> change to use flexible array in v-next if there is no other concerns.
>> After switching to flexible array, OFFSET_OF (Data) should lead to the
>> same result as sizeof.
> This may be true on specific compilers, but it is *not* guaranteed by
> the C language standard.

Sorry, for completeness sake... :)

I don't think it really depends on the compiler (but can vary per ABI),
but it's a conceptual issue with alignment requirements. Assuming
natural alignment and the usual stuff, for "struct s { uint64_t a;
uint32_t b; uint8_t c[]; }" the alignment requirement is 8 Bytes, where
there are 4 Bytes of padding after "b" (as "c" may as well be empty).
"c" however is guaranteed to start after b in the same fashion as if it
was declared with the maximum amount of elements that fit the memory. So
if we take 4 elements for "c", and note that "c" has an alignment
requirement of 1 Byte, c[0 .. 3] will alias the padding after "b". For
"sizeof" this means that the padding is included, whereas for "offsetof"
it is not, yielding "sizeof(struct s) == offsetof(struct s, c) + 4".
That is what I meant by "wasted space" earlier, but this could possibly
be made nicer with macros as necessary.

As for this specific struct, the values should be identical as it is padded.

Best regards,

> Quoting C99 "Structure and union specifiers", paragraph 16:
> "In most situations, the flexible array member is ignored. In
> particular, the size of the structure is as if the flexible array member
> were omitted except that it may have more trailing padding than the
> omission would imply."
> Quoting footnotes 17 and 19,
> (17)  [...]
>        struct s { int n; double d[]; };
>        [...]
> (19)  [...]
>        the expressions:
>        [...]
>        sizeof (struct s) >= offsetof(struct s, d)
>        are always equal to 1.
> Thanks
> Laszlo
>> While sizeof would be a preferred way to move
>> forward.
>> Regards,
>> Kun
>> On 06/24/2021 08:25, Kinney, Michael D wrote:
>>> Hello,
>>> Flexible array members are supported and should be used.  The old style
>>> of adding an array of size [1] at the end of a structure was used at a
>>> time
>>> flexible array members were not supported by all compilers (late 1990's).
>>> The workarounds used to handle the array of size [1] are very
>>> confusing when
>>> reading the C  code and the fact that sizeof() does not produce the
>>> expected
>>> result make it even worse.
>>> If we use flexible array members in this proposed change then there is
>>> no need to use OFFSET_OF().  Correct?
>>> Mike
>>>> -----Original Message-----
>>>> From: Marvin Häuser <mhaeuser at posteo.de>
>>>> Sent: Thursday, June 24, 2021 1:00 AM
>>>> To: Kun Qin <kuqin12 at gmail.com>; Laszlo Ersek <lersek at redhat.com>;
>>>> devel at edk2.groups.io
>>>> Cc: Wang, Jian J <jian.j.wang at intel.com>; Wu, Hao A
>>>> <hao.a.wu at intel.com>; Dong, Eric <eric.dong at intel.com>; Ni, Ray
>>>> <ray.ni at intel.com>; Kinney, Michael D <michael.d.kinney at intel.com>;
>>>> Liming Gao <gaoliming at byosoft.com.cn>; Liu, Zhiguang
>>>> <zhiguang.liu at intel.com>; Andrew Fish <afish at apple.com>; Leif
>>>> Lindholm <leif at nuviainc.com>; Bret Barkelew
>>>> <Bret.Barkelew at microsoft.com>; michael.kubacki at microsoft.com
>>>> Subject: Re: [edk2-devel] [PATCH v1 0/5] EDK2 Code First: PI
>>>> Specification: Update EFI_MM_COMMUNICATE_HEADER
>>>> Hey Kun,
>>>> Why would you rely on undefined behaviours? The OFFSET_OF macro is
>>>> well-defined for GCC and Clang as it's implemented by an intrinsic, and
>>>> while the expression for the MSVC compiler is undefined behaviour as per
>>>> the C standard, it is well-defined for MSVC due to their own
>>>> implementation being identical. From my standpoint, all supported
>>>> compilers will yield well-defined behaviour even this way. OFFSET_OF on
>>>> flexible arrays is not UB in any case to my knowledge.
>>>> However, the same way as your new suggestion, you can replace OFFSET_OF
>>>> with sizeof. While this *can* lead to wasted space with certain
>>>> structure layouts (e.g. when the flexible array overlays padding bytes),
>>>> this is not the case here, and otherwise just loses you a few bytes. I
>>>> think this comes down to preference.
>>>> The pattern you mentioned arguably is less nice syntax when used
>>>> (involves address calculation and casting), but the biggest problem here
>>>> is alignment constraints. For packed structures, you lose the ability of
>>>> automatic unaligned accesses (irrelevant here because the structure is
>>>> manually padded anyway). For non-packed structures, you still need to
>>>> ensure the alignment requirement of the trailing array data is met
>>>> manually. With flexible array members, the compiler takes care of both
>>>> cases automatically.
>>>> Best regards,
>>>> Marvin
>>>> On 24.06.21 02:24, Kun Qin wrote:
>>>>> Hi Marvin,
>>>>> I would prefer not to rely on undefined behaviors from different
>>>>> compilers. Instead of using flexible arrays, is it better to remove
>>>>> the `Data` field, pack the structure and follow
>>>>> In that case, OFFSET_OF will be forced to change to sizeof, and
>>>>> read/write to `Data` will follow the range indicated by MessageLength.
>>>>> But yes, that will enforce developers to update their platform level
>>>>> implementations accordingly.
>>>>> Regards,
>>>>> Kun
>>>>> On 06/23/2021 08:26, Laszlo Ersek wrote:
>>>>>> On 06/23/21 08:54, Marvin Häuser wrote:
>>>>>>> On 22.06.21 17:34, Laszlo Ersek wrote:
>>>>>>>> On 06/18/21 11:37, Marvin Häuser wrote:
>>>>>>>>> On 16.06.21 22:58, Kun Qin wrote:
>>>>>>>>>> On 06/16/2021 00:02, Marvin Häuser wrote:
>>>>>>>>>>> 2) Is it feasible yet with the current set of supported
>>>>>>>>>>> compilers to
>>>>>>>>>>> support flexible arrays?
>>>>>>>>>> My impression is that flexible arrays are already supported (as
>>>>>>>>>> seen
>>>>>>>>>> in UnitTestFrameworkPkg/PrivateInclude/UnitTestFrameworkTypes.h).
>>>>>>>>>> Please correct me if I am wrong.
>>>>>>>>>> Would you mind letting me know why this is applicable here? We are
>>>>>>>>>> trying to seek ideas on how to catch developer mistakes caused by
>>>>>>>>>> this
>>>>>>>>>> change. So any input is appreciated.
>>>>>>>>> Huh, interesting. Last time I tried I was told about
>>>>>>>>> incompatibilities
>>>>>>>>> with MSVC, but I know some have been dropped since then (2005 and
>>>>>>>>> 2008
>>>>>>>>> if I recall correctly?), so that'd be great to allow globally.
>>>>>>>> I too am surprised to see
>>>>>>>> "UnitTestFrameworkPkg/PrivateInclude/UnitTestFrameworkTypes.h". The
>>>>>>>> flexible array member is a C99 feature, and I didn't even know
>>>>>>>> that we
>>>>>>>> disallowed it for the sake of particular VS toolchains -- I
>>>>>>>> thought we
>>>>>>>> had a more general reason than just "not supported by VS versions X
>>>>>>>> and Y".
>>>>>>>> The behavior of OFFSET_OF() would be interesting -- the OFFSET_OF()
>>>>>>>> macro definition for non-gcc / non-clang:
>>>>>>>> #define OFFSET_OF(TYPE, Field) ((UINTN) &(((TYPE *)0)->Field))
>>>>>>>> borders on undefined behavior as far as I can tell, so its
>>>>>>>> behavior is
>>>>>>>> totally up to the compiler. It works thus far okay on Visual
>>>>>>>> Studio, but
>>>>>>>> I couldn't say if it extended correctly to flexible array members.
>>>>>>> Yes, it's UB by the standard, but this is actually how MS implements
>>>>>>> them (or used to anyway?). I don't see why it'd cause issues with
>>>>>>> flexible arrays, as only the start of the array is relevant (which is
>>>>>>> constant for all instances of the structure no matter the amount of
>>>>>>> elements actually stored). Any specific concern? If so, they could be
>>>>>>> addressed by appropriate STATIC_ASSERTs.
>>>>>> No specific concern; my point was that two aspects of the same "class"
>>>>>> of undefined behavior didn't need to be consistent with each other.
>>>>>> Thanks
>>>>>> Laszlo

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