On 2019-11-13, Al Viro [off-list ref] wrote:

On Tue, Nov 05, 2019 at 08:05:47PM +1100, Aleksa Sarai wrote:

quoted

@@ -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/>