linux-kernel - Re: *at syscalls for xattrs?

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Date:	Mon, 16 Jul 2007 16:03:24 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Miklos Szeredi <miklos@...redi.hu>, jeremy@...p.org,
	jengelh@...putergmbh.de, viro@....linux.org.uk, nmiell@...cast.net,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: *at syscalls for xattrs?

H. Peter Anvin wrote:
> Jeff Garzik wrote:
>>> What the *at() interfaces really do is fix/paper over a longstanding
>>> wart in Unix: the cwd really should have been a standard file descriptor
>>> (like stdin/stdout/stderr) instead of a magic piece of state maintained
>>> in kernel space.
>> It's more than a wart, IMO.  *at() allows one to close races (with
>> potential security implications) that are otherwise impossible to close,
>> in directory traversal.
>>
>> *at() permits a userspace program to hold proper references to all
>> objects during a directory traversal, with all that implies.
>>
> 
> Well, as Jeremy pointed out, in the absence of threads you can do the
> same thing with fchdir(), however, that's much more of a hack.

My posixutils project (coreutils replacement) used fchdir(2), but that 
still doesn't get you 100% race-free.  It gets you close, yes.

	Jeff



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