netdev - Re: [patch] Fix handling of overlength pathname in AF_UNIX sun

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Message-ID: <CAKgNAkjZ31JAs0XFutnAozCAcHHAq6pcCAKeDNHccKQ+6uTP6g@mail.gmail.com>
Date:	Thu, 19 Apr 2012 22:33:03 +1200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	David Miller <davem@...emloft.net>, carlos@...temhalted.org,
	netdev@...r.kernel.org, penguin-kernel@...ove.sakura.ne.jp,
	linux-api@...r.kernel.org, yoshfuji@...ux-ipv6.org,
	jengelh@...ozas.de, w@....eu
Subject: Re: [patch] Fix handling of overlength pathname in AF_UNIX sun_path

On Thu, Apr 19, 2012 at 10:19 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>> 3. Considering these two sets:
>>
>>   (a) [applications that rely on the assumption that there
>>        is a null terminator inside sizeof(sun_path) bytes]
>>   (b) [applications that would break if the kernel behavior changed]
>>
>>   I suspect that set (a) is rather larger than set (b)--or, more
>>   likely still, applications ensure they go for the lowest common
>>   denominator limit of 92 (HP-UX) or 104 (historical BSD limit)
>>   bytes, and so avoid this issue completely.
>
> Or another way of putting it
>
> 3(a)    Sloppy coding that may have lots of other bugs
>
> 3(b)    Interfaces and code we promised not to break.

Yes, it's another way of putting it. (Though regarding 3(b), part of
the problem is that there never was a clearly specified contract.)

Anyway, I've dug deeper, looking at hat happens on other platforms.
It's a mess: the BSDs don't even guarantee  that sun_path is
null_terminated. So, here's how one has to portably deal with the
variations:

addrlen = sizeof(struct sockaddr_un);
cfd = accept(lfd, &addr, &addlen);

printf("%.*s", addrlen - offsetof(struct sockaddr_un, sun_path), addr.sun_path);

That's pretty hideous!

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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