linux-kernel - Re: [PATCH 3/4] proc: Point /proc/net at /proc/thread-self/net instead of /proc/self/net

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Message-ID: <CAHk-=wizrMKtFxtK-5b-RmC2T562A8iHSYpnAygu__U-HcG_3A@mail.gmail.com>
Date:   Thu, 29 Sep 2022 12:00:59 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     David Laight <David.Laight@...lab.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH 3/4] proc: Point /proc/net at /proc/thread-self/net
 instead of /proc/self/net

On Thu, Sep 29, 2022 at 11:50 AM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> What do you mean?  Lookup on "net" in /proc returning what, exactly?

Returning the same directory as "thread-self/net", just not with a
symlink so that Apparmor doesn't get to mess things up..

> What would that dentry have for ->d_parent?

In a perfect world, I think it should act like a dynamic bind mount
(where the "dynamic" part is that thread-self part, and the parent
would be /proc.

That said, I think this is all a hack to deal with an Apparmor bug, so
I don't think we need perfect. Right now it's a symlink, so the parent
is the thread-self directory. I think that kind of magic jump would be
perfectly acceptable.

We have "magic jump" behavior in other /proc places, where the thing
*looks* like a symlink (ie readlink and friends just work), but the
lookup doesn't *actually* follow the symlink, it just looks things up
directly. IOW, all the /proc/<pid>/fd/<X> stuff.

So I think this would be just another case of that.

                Linus