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

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Date:   Thu, 29 Sep 2022 20:34:52 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 12:05:32PM -0700, Linus Torvalds wrote:
> On Thu, Sep 29, 2022 at 12:00 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > Which is insane, especially since the entire problem is due to wanting
> > that directory to be different for different threads...
> 
> Absolutely. This is all due to Apparmor (a) basing things on pathnames
> and (b) then getting those pathnames wrong.
> 
> Which is why I'm just suggesting we short-circuit the path-name part,
> and not make this be a real symlink that actually walks a real path.
> 
> The proc <pid> handling uses "readlink" to make it *look* like a
> symlink, but then "get_link" to actually look it up (and never walk it
> as a path).
> 
> Something similar?

Apparmor takes mount+dentry and turns that into pathname.  Then acts
upon the resulting string.  *AFTER* the original had been resolved.
IOW, it doesn't see the symlink contents - only the location where the
entire thing ends up.

AFAICS, the only way to make it STFU is either
	* fix the idiotic policy
or
	* make the per-thread directory show up as /proc/<something>/net

As in "../.. from there lands you in /proc".  Because that's what
apparmor does to generate the string it treats as the pathname...