netdev - RE: [PATCH net-next 0/6] A step closer to RFC 6458 compliancy

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Date:	Wed, 18 Jun 2014 13:16:32 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Michael Tuexen' <Michael.Tuexen@...chi.franken.de>
CC:	Vlad Yasevich <vyasevich@...il.com>,
	Geir Ola Vaagland <geirola@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>
Subject: RE: [PATCH net-next 0/6] A step closer to RFC 6458 compliancy

From: Michael Tuexen [ 
> On 18 Jun 2014, at 10:42, David Laight <David.Laight@...LAB.COM> wrote:
> 
> > From: Vlad Yasevich
> >> On 06/17/2014 11:36 AM, David Laight wrote:
> >>> From: Of Geir Ola Vaagland
> >>>> These patches are part of my master thesis project. I have been searching for discrepancies
> between
> >>>> the socket API specificiation in RFC 6458 and the current Linux SCTP implementation. The
> following
> >>>> patches are my humble attempts at getting somewhat closer to compliancy.
> >>>
> >>> I've just been reading RFC 6458 - HTF did it get past the editors and
> >>> then published in its current form?
> >>> Lots of the structures have implied padding.
> > ...
> >> I've argued the padding issue, but the editor stance is that it's implementation
> >> dependent.
> >
> > It wouldn't be as bad if the RFC said that the structure contained the
> > fields that followed (as is typical of the posix definitions),
> > but instead it gives a definition of the structure.

> That would have been a possibility, but it was never suggested.
> As far as I know, C does not guarantee the memory layout for structs,
> except for the sequence of the components. So a compiler might add
> some padding at any place. When implementing this, you need to take
> care of this (and your job might be simpler, since you might only
> work with a specific set of compilers).
> In FreeBSD we also added some padding to some structures since they
> "evolved" during the lifetime of of the internet draft and we wanted
> to preserve some compatibility.
> I agree, that one must take care of the implied padding and I will double
> check how this is handled in FreeBSD. Not sure...

You need to add explicit named pad fields in order to zero them.
(since you don't really want a memset())
That is against my reading of the RFC.

What does FreeBSD do about the 'sockaddr_storage'?
I'd have thought it had the same rules as NetBSD - where (IIRC) it should never
be instantiated, but only exists as a pointer type for function parameters.

	David




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