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From: Florian Weimer <fw@deneb.enyo.de>
To: "Cufi, Carles" <Carles.Cufi@nordicsemi.no>
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: Sat, 09 Oct 2021 08:56:03 +0200	[thread overview]
Message-ID: <875yu6bsak.fsf@mid.deneb.enyo.de> (raw)
In-Reply-To: <DB9PR05MB7898339C06B9317EBAB13437E7AE9@DB9PR05MB7898.eurprd05.prod.outlook.com> (Carles Cufi's message of "Mon, 4 Oct 2021 10:30:34 +0000")

* Carles Cufi:

>> * 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?

For structure layout in memory, there is a large overlap between ABIs.
There is divergence around long long (which is easily avoided by
adding padding manually), and potentially bit fileds (but I haven't
looked at that).

Things only get weird for pass-by-value structs and unions and return
types.

      reply	other threads:[~2021-10-09  7:05 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
2021-10-09  6:56     ` Florian Weimer [this message]

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