From: Stephen Hemminger <stephen@networkplumber.org>
To: "Morten Brørup" <mb@smartsharesystems.com>
Cc: "Mattias Rönnblom" <hofors@lysator.liu.se>,
"Tyler Retzlaff" <roretzla@linux.microsoft.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: Sun, 7 Apr 2024 10:03:06 -0700 [thread overview]
Message-ID: <20240407100306.36c9688f@hermes.local> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35E9F379@smartserver.smartshare.dk>
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.
next prev parent reply other threads:[~2024-04-07 17:03 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 [this message]
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
2024-04-10 17:10 ` Tyler Retzlaff
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