On 2019-11-13, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Tue, Nov 05, 2019 at 08:05:47PM +1100, Aleksa Sarai wrote:
> 
> > @@ -862,6 +870,8 @@ static int nd_jump_root(struct nameidata *nd)
> >  void nd_jump_link(struct path *path)
> >  {
> >  	struct nameidata *nd = current->nameidata;
> > +
> > +	nd->last_magiclink.same_mnt = (nd->path.mnt == path->mnt);
> >  	path_put(&nd->path);
> >  
> >  	nd->path = *path;
> > @@ -1082,6 +1092,10 @@ const char *get_link(struct nameidata *nd)
> >  		if (nd->flags & LOOKUP_MAGICLINK_JUMPED) {
> >  			if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS))
> >  				return ERR_PTR(-ELOOP);
> > +			if (unlikely(nd->flags & LOOKUP_NO_XDEV)) {
> > +				if (!nd->last_magiclink.same_mnt)
> > +					return ERR_PTR(-EXDEV);
> > +			}
> >  		}
> 
> Ugh...  Wouldn't it be better to take that logics (some equivalent thereof)
> into nd_jump_link()?  Or just have nd_jump_link() return an error...

This could be done, but the reason for stashing it away in
last_magiclink is because of the future magic-link re-opening patches
which can't be implemented like that without putting the open_flags
inside nameidata (which was decided to be too ugly a while ago).

My point being that I could implement it this way for this series, but
I'd have to implement something like last_magiclink when I end up
re-posting the magic-link stuff in a few weeks.

Looking at all the nd_jump_link() users, the other option is to just
disallow magic-link crossings entirely for LOOKUP_NO_XDEV. The only
thing allowing them permits is to resolve file descriptors that are
pointing to the same procfs mount -- and it's unclear to me how useful
that really is (apparmorfs and nsfs will always give -EXDEV because
aafs_mnt and nsfs_mnt are internal kernel vfsmounts).

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