linux-kernel - Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

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Message-ID: <20200102090920.gmvq45gqopbzmrgk@yavin.dot.cyphar.com>
Date:   Thu, 2 Jan 2020 20:09:20 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        stable <stable@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Serge Hallyn <serge@...lyn.com>,
        "dev@...ncontainers.org" <dev@...ncontainers.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over
 symlinks

On 2020-01-02, David Laight <David.Laight@...LAB.COM> wrote:
> From: Aleksa Sarai
> > Sent: 30 December 2019 08:32
> ...
> > I'm not sure I agree -- as I mentioned in my other mail, re-opening
> > through /proc/self/fd/$n works *very* well and has for a long time (in
> > fact, both LXC and runc depend on this working).
> 
> I thought it was marginally broken because it is followed as a symlink?
> On, for example, NetBSD /proc/<n>/fd/<n> is a real reference to the
> filesystem inode and can be used to link the file back into the filesystem
> if all the directory entries have been removed.

That is also the case on Linux. It (strictly speaking) isn't a symlink
in the normal sense of the word, it's a magic-link (nd_jump_link
switches the nd->path to the actual 'struct file' in the case of
/proc/self/fd/$n).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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