Re: [PATCH bpf-next 2/2] selftests/bpf: Add tests for bpf_get_dentry_xattr

[Al Viro <viro@zeniv.linux.org.uk>]

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.