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

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Message-ID: <4F4484F0.9070501@zytor.com>
Date:	Tue, 21 Feb 2012 22:02:24 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ian Kent <raven@...maw.net>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 02/21/2012 09:43 PM, Ian Kent wrote:
> 
> However, with the final "char name[NAME_MAX+1]" array at the end, the
> actual size of the structure ends up being not very well defined:
> because the struct isn't marked 'packed', doing a "sizeof()" on it will
> align the size of the struct up to the biggest alignment of the members
> it has.
> 

Note that it's really rather unfortunate that this transmits the entire
packet no matter what, which also means that the "len" field is actually
completely and totally pointless.

I have mentioned in the past that I consider it a design mistake on my
part to have used a pipe in the first place: it was "so easy", but
required an extra read for no good reason, or padding to fixed size
resulting in this problem.  The right thing really should have been to
use a Unix datagram socket; these days using SOCK_SEQPACKET - that way
the kernel would give you the right semantics by design.

	-hpa

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

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