[PATCH 0/4] RFC samples converting VLA to alloca

[Mattias Rönnblom]

On 2024-04-08 17:53, Morten Brørup wrote:
>> From: Tyler Retzlaff [mailto:roretzla at linux.microsoft.com]
>> Sent: Monday, 8 April 2024 17.27
>>
>> For next technboard meeting.
>>
>> On Sun, Apr 07, 2024 at 10:03:06AM -0700, Stephen Hemminger wrote:
>>> On Sun, 7 Apr 2024 13:07:06 +0200
>>> Morten Brørup <mb at smartsharesystems.com> wrote:
>>>
>>>>> From: Mattias Rönnblom [mailto:hofors at lysator.liu.se]
>>>>> Sent: Sunday, 7 April 2024 11.32
>>>>>
>>>>> On 2024-04-04 19:15, Tyler Retzlaff wrote:
>>>>>> This series is not intended for merge.  It insteat provides examples
>>>>> of
>>>>>> converting use of VLAs to alloca() would look like.
>>>>>>
>>>>>> what's the advantages of VLA over alloca()?
>>>>>>
>>>>>> * sizeof(array) works as expected.
>>>>>>
>>>>>> * multi-dimensional arrays are still arrays instead of pointers to
>>>>>>     dynamically allocated space. this means multiple subscript syntax
>>>>>>     works (unlike on a pointer) and calculation of addresses into
>>>>> allocated
>>>>>>     space in ascending order is performed by the compiler instead of
>>>>> manually.
>>>>>>
>>>>>
>>>>> alloca() is a pretty obscure mechanism, and also not a part of the C
>>>>> standard. VLAs are C99, and well-known and understood, and very
>>>>> efficient.
>>>>
>>>> The RFC fails to mention why we need to replace VLAs with something else:
>>>>
>>>> VLAs are C99, but not C++; VLAs were made optional in C11.
>>>>
>>>> MSVC doesn't support VLAs, and is not going to:
>>>> https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-support-
>> arriving-in-msvc/#variable-length-arrays
>>>>
>>>>
>>>> I dislike alloca() too, and the notes section in the alloca(3) man page
>> even discourages the use of alloca():
>>>> https://man7.org/linux/man-pages/man3/alloca.3.html
>>>>
>>>> But I guess alloca() is the simplest replacement for VLAs.
>>>> This RFC patch series opens the discussion for alternatives in different
>> use cases.
>>>>
>>>
>>> The other issue with VLA's is that if the number is something that can be
>> externally
>>> input, then it can be a source of stack overflow bugs. That is why the Linux
>> kernel
>>> has stopped using them; for security reasons. DPDK has much less of a
>> security
>>> trust domain. Mostly need to make sure that no data from network is being
>>> used to compute VLA size.
>>>
>>
>> Looks like we need to discuss this at the next techboard meeting.
>>
>> * MSVC doesn't support C11 optional VLAs (and never will).
>> * alloca() is an alternative that is available on all platforms/toolchain
>>    combinations.
>> * it's reasonable for some VLAs to be turned into regular arrays but it
>>    would be unsatisfactory to be stuck waiting discussions of defining new
>>    constant expression macros on a per-use basis.
> 
> We must generally stop using VLAs, for many reasons.

What reasons would that be? And which of those reasons are not also 
reasons to stop using alloca().

> The only available 1:1 replacement is alloca(), so we have to accept that.
> 
> If anyone still cares about improvements, we can turn alloca()'d arrays into regular arrays after this patch series.
> 
> Alternatives to VLAs are very interesting discussions, but let's not stall MSVC progress because of it!
> 

What is this supposed to mean? Finding alternatives to VLAs are required 
to make progress of MSVC support in DPDK.

>> * there is resistance to using alloca() vs VLA so my proposal is to
>>    change only the code that is built to target windows.
> 
> I would prefer to get rid of them all, so the CI can build with -Wvla to prevent them from being introduced again.
> Not a strong preference.
> On the other hand, the CI's MSVC builds will catch them if used for a Windows target.
> And limiting to Windows code reduces the amount of work, so that's probably the most realistic solution.
>