From: "Mattias Rönnblom" <hofors@lysator.liu.se>
To: Tyler Retzlaff <roretzla@linux.microsoft.com>,
Stephen Hemminger <stephen@networkplumber.org>,
techboard@dpdk.org
Cc: "Morten Brørup" <mb@smartsharesystems.com>,
dev@dpdk.org, "Bruce Richardson" <bruce.richardson@intel.com>,
"Thomas Monjalon" <thomas@monjalon.net>
Subject: Re: [PATCH 0/4] RFC samples converting VLA to alloca
Date: Wed, 10 Apr 2024 09:27:10 +0200 [thread overview]
Message-ID: <a2bc370d-bf0b-4234-87c4-4b2fb7ae1025@lysator.liu.se> (raw)
In-Reply-To: <20240408152703.GA25804@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
On 2024-04-08 17:27, Tyler Retzlaff wrote:
> 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@smartsharesystems.com> wrote:
>>
>>>> From: Mattias Rönnblom [mailto:hofors@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).
This is due to dogmatism, or what? Surely, a lot of Open Source projects
written for C99 will use VLAs.
> * alloca() is an alternative that is available on all platforms/toolchain
> combinations.
alloca() is a poor alternative. The use of alloca() should be restricted
to situations where statically sized arrays can't do the job.
> * 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.
> * there is resistance to using alloca() vs VLA so my proposal is to
> change only the code that is built to target windows.
next prev parent reply other threads:[~2024-04-10 7:27 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-07 19:32 RFC acceptable handling of VLAs across toolchains Tyler Retzlaff
2023-11-08 2:31 ` Stephen Hemminger
2023-11-08 3:25 ` Tyler Retzlaff
2023-11-08 8:19 ` Morten Brørup
2023-11-08 16:51 ` Stephen Hemminger
2023-11-08 17:48 ` Morten Brørup
2023-11-09 10:25 ` RFC: default burst sizes in rte_config Morten Brørup
2023-11-09 20:26 ` RFC acceptable handling of VLAs across toolchains Tyler Retzlaff
2024-03-21 0:12 ` Tyler Retzlaff
2024-04-04 17:15 ` [PATCH 0/4] RFC samples converting VLA to alloca Tyler Retzlaff
2024-04-04 17:15 ` [PATCH 1/4] latencystats: use alloca instead of vla trivial Tyler Retzlaff
2024-04-06 15:28 ` Morten Brørup
2024-04-07 9:36 ` Mattias Rönnblom
2024-04-07 17:00 ` Stephen Hemminger
2024-04-04 17:15 ` [PATCH 2/4] hash: " Tyler Retzlaff
2024-04-06 16:01 ` Morten Brørup
2024-04-04 17:15 ` [PATCH 3/4] vhost: use alloca instead of vla sizeof Tyler Retzlaff
2024-04-06 22:30 ` Morten Brørup
2024-04-04 17:15 ` [PATCH 4/4] dispatcher: use alloca instead of vla multi dimensional Tyler Retzlaff
2024-04-06 15:49 ` Morten Brørup
2024-04-07 9:31 ` [PATCH 0/4] RFC samples converting VLA to alloca Mattias Rönnblom
2024-04-07 11:07 ` Morten Brørup
2024-04-07 17:03 ` Stephen Hemminger
2024-04-08 15:27 ` Tyler Retzlaff
2024-04-08 15:53 ` Morten Brørup
2024-04-09 8:28 ` Konstantin Ananyev
2024-04-09 15:08 ` Tyler Retzlaff
2024-04-10 9:58 ` Konstantin Ananyev
2024-04-10 17:03 ` Tyler Retzlaff
2024-04-10 7:32 ` Mattias Rönnblom
2024-04-10 7:52 ` Morten Brørup
2024-04-10 17:04 ` Tyler Retzlaff
2024-04-10 7:27 ` Mattias Rönnblom [this message]
2024-04-10 17:10 ` Tyler Retzlaff
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