Re: [PATCH bpf-next 1/2] bpf: Add dentry kfuncs for BPF LSM programs

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

On Wed, Sep 24, 2025 at 08:08:03PM -0400, David Windsor wrote:
> On Wed, Sep 24, 2025 at 7:55 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Wed, Sep 24, 2025 at 07:24:33PM -0400, David Windsor wrote:
> > > Add six new BPF kfuncs that enable BPF LSM programs to safely interact
> > > with dentry objects:
> > >
> > > - bpf_dget(): Acquire reference on dentry
> > > - bpf_dput(): Release reference on dentry
> > > - bpf_dget_parent(): Get referenced parent dentry
> > > - bpf_d_find_alias(): Find referenced alias dentry for inode
> > > - bpf_file_dentry(): Get dentry from file
> > > - bpf_file_vfsmount(): Get vfsmount from file
> > >
> > > All kfuncs are currently restricted to BPF_PROG_TYPE_LSM programs.
> >
> > You have an interesting definition of safety.
> >
> > We are *NOT* letting random out-of-tree code play around with the
> > lifetime rules for core objects.
> >
> 
> File references are already exposed to bpf (bpf_get_task_exe_file,
> bpf_put_file) with the same KF_ACQUIRE|KF_RELEASE semantics. These
> follow the same pattern and are also LSM-only.

You can safely clone and retain file references.  You can't do that
to dentries unless you are guaranteed an active reference to superblock
to stay around for as long as you are retaining those.  Note that
LSM hooks might be called with ->s_umount held by caller, so the locking
environment for superblocks depends upon the hook in question.