linux-toolchains.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: "Cufi, Carles" <Carles.Cufi@nordicsemi.no>
To: Florian Weimer <fweimer@redhat.com>
Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"jukka.rissanen@linux.intel.com" <jukka.rissanen@linux.intel.com>,
	"johan.hedberg@intel.com" <johan.hedberg@intel.com>,
	"Lubos, Robert" <Robert.Lubos@nordicsemi.no>,
	"Bursztyka, Tomasz" <tomasz.bursztyka@intel.com>,
	"linux-toolchains@vger.kernel.org"
	<linux-toolchains@vger.kernel.org>
Subject: RE: Non-packed structures in IP headers
Date: Mon, 4 Oct 2021 10:30:34 +0000	[thread overview]
Message-ID: <DB9PR05MB7898339C06B9317EBAB13437E7AE9@DB9PR05MB7898.eurprd05.prod.outlook.com> (raw)
In-Reply-To: <87bl48v74v.fsf@oldenburg.str.redhat.com>

Hi Florian,

Thanks for your response.

> 
> * Carles Cufi:
> 
> > I was looking through the structures for IPv{4,6} packet headers and
> > noticed that several of those that seem to be used to parse a packet
> > directly from the wire are not declared as packed. This surprised me
> > because, although I did find that provisions are made so that the
> > alignment of the structure, it is still technically possible for the
> > compiler to inject padding bytes inside those structures, since AFAIK
> > the C standard makes no guarantees about padding unless it's
> > instructed to pack the structure.
> 
> The C standards do not make such guarantees, but the platform ABI
> standards describe struct layout and ensure that there is no padding.
> Linux relies on that not just for networking, but also for the userspace
> ABI, support for separately compiled kernel modules, and in other places.

That makes sense, but aren't ABI standards different for every architecture? For example, I checked the Arm AAPCS[1] and it states:

"The size of an aggregate shall be the smallest multiple of its alignment that is sufficient to hold all of its
members."

Which, unless I am reading this wrong, means that the compiler would indeed insert padding if the size of the IP headers structs was not a multiple of 4. In this particular case, the struct sizes for the IP headers are 20 and 40 bytes respectively, so there will be no padding inserted. But I only checked a single architecture's ABI (or Procedure Call Standard) documentation, is this true for all archs? 

> Sometimes there are alignment concerns in the way these structs are used,
> but I believe the kernel generally controls placement of the data that is
> being worked on, so that does not matter, either.

I did see those when browsing the code, thanks for confirming this. It is really padding that I am concerned about, and not alignment.

> Therefore, I do not believe this is an actual problem.

Would the static assert still make sense in order to check this for all architectures?

Thanks,

Carles

[1] https://github.com/ARM-software/abi-aa/blob/2bcab1e3b22d55170c563c3c7940134089176746/aapcs64/aapcs64.rst#aggregates


  parent reply	other threads:[~2021-10-04 10:30 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <AS8PR05MB78952FE7E8D82245D309DEBCE7AA9@AS8PR05MB7895.eurprd05.prod.outlook.com>
2021-10-01 20:10 ` Non-packed structures in IP headers Florian Weimer
2021-10-02 15:54   ` David Laight
2021-10-04 10:41     ` Cufi, Carles
2021-10-04 12:18       ` David Laight
2021-10-04 10:30   ` Cufi, Carles [this message]
2021-10-09  6:56     ` Florian Weimer

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=DB9PR05MB7898339C06B9317EBAB13437E7AE9@DB9PR05MB7898.eurprd05.prod.outlook.com \
    --to=carles.cufi@nordicsemi.no \
    --cc=Robert.Lubos@nordicsemi.no \
    --cc=fweimer@redhat.com \
    --cc=johan.hedberg@intel.com \
    --cc=jukka.rissanen@linux.intel.com \
    --cc=linux-toolchains@vger.kernel.org \
    --cc=netdev@vger.kernel.org \
    --cc=tomasz.bursztyka@intel.com \
    /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).