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Date: Sun, 20 Mar 2016 18:22:51 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Jordi Pujol Palomer <jordipujolp@gmail.com>
Cc: Linux-Fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH] fs/namei.c: Hard link and rename should check same
 device instead of same mount
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On Sun, Mar 20, 2016 at 07:06:25PM +0100, Jordi Pujol Palomer wrote:
> Therefore we can develop a new option for binding to let the superuser
> choose what is needed for every mount-bind, example:
> 
> mount --bind -o boundary $DIR $DIR
> 
> mount --bind -o noboundary $DIR1 $DIR2
> 
> and allow remount also,
> 
> mount -o remount,boundary $DIR
> 
> mount -o remount,noboundary $DIR
> 
> That seems feasible,

... until you consider the fun of moving subtrees between different mounts.
IMO it's a bloody bad idea; semantics would be hard to define, not to mention
that things that used to be on the same fs might eventually get moved to
different ones, and for those there's obviously no way to do link(2) or
rename(2) at all.

What are you actually trying to achieve?  There might be saner ways to do
it...