netdev - RE: setns() affecting other threads in 5.10.132 and 6.0

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Message-ID: <6204a74ef41a4463a790962d0409d0bc@AcuMS.aculab.com>
Date:   Tue, 6 Sep 2022 10:48:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexey Dobriyan' <adobriyan@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: RE: setns() affecting other threads in 5.10.132 and 6.0

From: Alexey Dobriyan
> Sent: 05 September 2022 18:33
> > > -----Original Message-----
> > > From: David Laight <David.Laight@...LAB.COM>
> > > Sent: 04 September 2022 15:05
> > >
> > > Sometime after 5.10.105 (5.10.132 and 6.0) there is a change that
> > > makes setns(open("/proc/1/ns/net")) in the main process changes
> > > the behaviour of other process threads.
> 
> Not again...

I've realised what is going on.
It really isn't obvious at all.
Quite possibly the last change did fix it - even though
it broke our code.

/proc/net is a symlink to /proc/self/net.
But that isn't what the code wants to open.
What it needs is /proc/self/task/self/net.
But there isn't a 'self' in /proc/self/task.
Which makes it all a bit tedious (especially without gettid() in glibc).
(This is a busybox/buildroot system, maybe I could add it!)

I'd probably have noticed earlier if the /proc/net
symlink didn't exist.
I guess that is for compatibility with pre-netns kernels.

	David

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