From: Tyler Retzlaff <roretzla@linux.microsoft.com>
To: Stephen Hemminger <stephen@networkplumber.org>, techboard@dpdk.org
Cc: "Morten Brørup" <mb@smartsharesystems.com>,
"Mattias Rönnblom" <hofors@lysator.liu.se>,
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: Mon, 8 Apr 2024 08:27:03 -0700 [thread overview]
Message-ID: <20240408152703.GA25804@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> (raw)
In-Reply-To: <20240407100306.36c9688f@hermes.local>
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).
* 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.
* 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:[~2024-04-08 15:27 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 [this message]
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
2024-04-10 17:10 ` Tyler Retzlaff
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20240408152703.GA25804@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net \
--to=roretzla@linux.microsoft.com \
--cc=bruce.richardson@intel.com \
--cc=dev@dpdk.org \
--cc=hofors@lysator.liu.se \
--cc=mb@smartsharesystems.com \
--cc=stephen@networkplumber.org \
--cc=techboard@dpdk.org \
--cc=thomas@monjalon.net \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).