linux-kernel - Re: autofs: make the autofsv5 packet file descriptor use a packetized pipe

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Date:	Mon, 30 Apr 2012 09:29:58 +0800
From:	Ian Kent <raven@...maw.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Michael Tokarev <mjt@....msk.ru>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Thomas Meyer <thomas@...3r.de>, autofs@...r.kernel.org
Subject: Re: autofs: make the autofsv5 packet file descriptor use a
 packetized pipe

On Sun, 2012-04-29 at 17:43 -0700, Linus Torvalds wrote:
> On Sun, Apr 29, 2012 at 5:35 PM, H. Peter Anvin <hpa@...or.com> wrote:
> >
> > Which is funny/sad, because that's actually the intended way the
> > interface is meant to work.
> 
> Well, the autofs packet model is actually horribly badly misdesigned
> even for that: the header doesn't contain the size of the packet. It
> contains the packet *type*, and from that you can then determine the
> size (of course, every other program would determine it *wrongly* for
> this whole x86-64 alignment reason), but that is actually a horrible
> model because it assumes you know all the packet types.
> 
> (There's a "len" field in the v5 packet, but that's not the packet
> length, that's the length of the name component)
> 
> And the reason nobody does that is that in practice there is only ever
> one single type of packet that is possible anyway, so there's no point
> in even reading the header to find the type.
> 
> So a much nicer model is one where the actual *size* of the packet is
> in the header. That would have allowed for not having that fixed
> maximum size of a name etc, and would have avoided the whole problem
> to begin with.
> 
> Of course, the nicest model of all is to just use a packetized
> interface to begin with, so that none of these issues exist. Which is
> what we're now effectively moving to, unless we can find some horrible
> program that makes that impossible due to it playing games and knowing
> it's a "stream".
> 
> Looking at am-utils, I think we're ok so far. But maybe you know of
> yet another crazy user of the autofs interfaces.

autodir looks ok as well.

http://sourceforge.net/projects/intraperson/files/latest/download

> 
>                       Linus


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