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Date:   Fri, 28 Aug 2020 22:34:45 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Florian Margaine <florian@platform.sh>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs: allow do_renameat2() over bind mounts of the same
 filesystem.
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On Fri, Aug 28, 2020 at 10:40:35PM +0200, Florian Margaine wrote:
> There's currently this seemingly unnecessary limitation that rename()
> cannot work over bind mounts of the same filesystem,

... is absolutely deliberate - that's how you set a boundary in the
tree, preventing both links and renames across it.

Incidentally, doing that would have fun effects for anyone with current
directory inside the subtree you'd moved - try and see.