linux-kernel - Re: [PATCH] fs: allow do_renameat2() over bind mounts of the same filesystem.

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Message-ID: <20200829221204.GV14765@casper.infradead.org>
Date:   Sat, 29 Aug 2020 23:12:04 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Florian Margaine <florian@...tform.sh>
Cc:     Al Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: allow do_renameat2() over bind mounts of the same
 filesystem.

On Sat, Aug 29, 2020 at 11:23:34PM +0200, Florian Margaine wrote:
> Al Viro <viro@...iv.linux.org.uk> writes:
> 
> > 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.
> 
> Sorry, I'm not not sure I understand what you're saying.

Al's saying this is the way an administrator can intentionally prevent
renames.

>     /*
>      * FICLONE/FICLONERANGE ioctls enforce that src and dest files are on
>      * the same mount. Practically, they only need to be on the same file
>      * system.
>      */
>     if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
>         return -EXDEV;

clone doesn't change the contents of a file, merely how they're laid out
on storage.  There's no particular reason for an administrator to
prohibit clone across mount points.