linux-kernel - Re: compat: autofs v5 packet size ambiguity

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Date:	Wed, 22 Feb 2012 09:43:24 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ian Kent <raven@...maw.net>, David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, autofs@...r.kernel.org,
	Thomas Meyer <thomas@...3r.de>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: compat: autofs v5 packet size ambiguity - update

On 02/22/2012 08:10 AM, Linus Torvalds wrote:
> 
> Well, the kernel gives the right semantics for pipes too - writes are
> guaranteed to be "atomic", so even in the presence of multiple writers
> you can trivially do packetized data.
> 
> You just have to (a) add the length to the packet and (b) do the
> length+packet write as a single write (which is limited to PIPE_SIZE -
> 4kB - for the atomicity guarantee).
> 
> If you don't have multiple concurrent writers without locking, the (b)
> part falls away entirely, of course.
> 
> Yes, for the reader side you need to be able to handle the fact that
> you can get more than one packet in one read() call, but sorting that
> out isn't hard either.
> 

What you describe above is pretty much how autofs 3 used to work; except
it would do one read() for the header including length and then another
read() for the body.  Of course, it could just have read ahead -- if you
read part of the next packet, it wouldn't really matter since at least
at that time the daemon was single-threaded and would have to loop back
anyway.

The PIPE_SIZE guarantee took care of the fact that this was a multiple
writer/single reader situation (since the writes happens in the context
of the process requesting a mount.)  Either way, SOCK_DGRAM and
SOCK_SEQPACKET would solve all of the problems and would Just Work, and
packet boundaries would then be explicit.

> But yeah, writing fixed-size data and then having a reader that reads
> fixed-size data is just not a very good approach. It's doubly bad when
> the "fixed size" isn't an explicit size that is documented in the
> protocol, but depends on data structures.

Indeed.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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