On Mon, Aug 19, 2024 at 08:25:38PM +0000, Song Liu wrote:

int bpf_get_parent_path(struct path *p) {
again:
    if (p->dentry == p->mnt.mnt_root) {
        follow_up(p);
        goto again;
    }
    if (unlikely(IS_ROOT(p->dentry))) {
        return PARENT_WALK_DONE;  
    }
    parent_dentry = dget_parent(p->dentry);
    dput(p->dentry);
    p->dentry = parent_dentry;
    return PARENT_WALK_NEXT; 
}

This will handle the mount. However, we cannot guarantee deny-by-default
policies like LandLock does, because this is just a building block of 
some security policies. 

You do realize that above is racy as hell, right?

Filesystem objects do get moved around.  You can, theoretically, play with
rename_lock, but that is highly antisocial.

What's more, _mounts_ can get moved around.  That is to say, there is no
such thing as stable canonical pathname of a file.