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

[Matthew Wilcox]

On Sat, Aug 29, 2020 at 11:23:34PM +0200, Florian Margaine wrote:
> Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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.