linux-kernel - Re: [PATCH v2] vfs: Tighten up linkat(..., AT_EMPTY

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Date:	Thu, 22 Aug 2013 22:15:30 +0200
From:	Willy Tarreau <w@....eu>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"security@...nel.org" <security@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Brad Spengler <spender@...ecurity.net>
Subject: Re: [PATCH v2] vfs: Tighten up linkat(..., AT_EMPTY_PATH)

On Thu, Aug 22, 2013 at 01:10:43PM -0700, Andy Lutomirski wrote:
> What's the point of nd_jump_link anyway?  The only way I can think of
> for a magic symlink in /proc to point to another symlink is to open a
> symlink with O_PATH | O_NOFOLLOW.  Actually trying to use the
> resulting link in /proc results in -ELOOP.  (Even just trying to open
> a normal symlink with O_NOFOLLOW and without O_PATH results in
> -ELOOP.)

It's not only that, it also supports sockets and pipes that you can access
via /proc/pid/fd and not via a real symlink which would try to open eg
"pipe:[23456]" instead of the real file. So you can't get rid of it
without breaking existing apps (starting with your shell for which
/dev/stdin is a link to /proc/self/fd/0 for example).

Willy

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