linux-kernel - Re: [PATCH,TRIVIAL] AF_UNIX, accept() and addrlen

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Message-ID: <20080324104330.GF4434@implementation.uk.xensource.com>
Date:	Mon, 24 Mar 2008 10:43:30 +0000
From:	Samuel Thibault <samuel.thibault@...-lyon.org>
To:	David Miller <davem@...emloft.net>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH,TRIVIAL] AF_UNIX, accept() and addrlen

David Miller, le Sun 23 Mar 2008 21:56:41 -0700, a écrit :
> From: Samuel Thibault <samuel.thibault@...-lyon.org>
> Date: Sat, 8 Mar 2008 02:23:21 +0000
> 
> > Accept and getpeername are supposed to return the amount of bytes
> > written in the returned address.  However, on unnamed sockets, only
> > sizeof(short) is returned, while a 0 is put in the sun_path member.
> > This patch adds 1 for that additional byte.
> > 
> > Signed-off-by: Samuel Thibault <samuel.thibault@...-lyon.org>
> 
> This change isn't correct.  It's the fact that the
> length returned is sizeof(short) that tells the caller
> that the unix socket is unnamed.

Mmm, where that is documented?

I can't find any details about that in SUS, and man 7 unix says

`If sun_path starts with a null byte ('' '), then it refers to the
abstract namespace main- tained by the Unix protocol module.'

It doesn't talk about the size being only sizeof(short) (which I guess
you meant sizeof(sa_family_t) actually).

> We zero out the sun_path[0] member just to be polite and tidy.
> 
> You would break applications if you changed this, so
> marking this patch as "trivial" is extremely premature.

See documentation above.  If applications don't follow documentation,
then they deserve breaking :)

Note also that on some (BSD-ish) systems, sockaddr_un contains a sun_len
field, containing the length of the data, and thus on them accept and
getpeername return more that sizeof(sa_family_t) as length (it actually
returns 16).  So such applications are really broken.

Samuel
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