On 2018-09-29, Aleksa Sarai <cyphar@cyphar.com> wrote:
> * AT_XDEV: Disallow mount-point crossing (both *down* into one, or *up*
>   from one). The primary "scoping" use is to blocking resolution that
>   crosses a bind-mount, which has a similar property to a symlink (in
>   the way that it allows for escape from the starting-point). Since it
>   is not possible to differentiate bind-mounts However since
>   bind-mounting requires privileges (in ways symlinks don't) this has
>   been split from LOOKUP_BENEATH. The naming is based on "find -xdev"
>   (though find(1) doesn't walk upwards, the semantics seem obvious).

I've just noticed that the mountpoint-crossing code for AT_XDEV doesn't
detect things like:

   % ln -s / /tmp/jumpup
   % vfs_helper -o open -F xdev -d /tmp jumpup
   /

I will fix that in v2.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>