[PATCH v5 4/5] bpf: Only enable BPF LSM hooks when an LSM program is attached

[Jiri Olsa]

On Fri, Oct 06, 2023 at 09:27:57AM +0200, Jiri Olsa wrote:

SNIP

> >  static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
> > struct bpf_trampoline *tr)
> >  {
> >         enum bpf_tramp_prog_type kind;
> >         struct bpf_tramp_link *link_exiting;
> > -       int err = 0, num_lsm_progs = 0;
> > +       int err = 0;
> >         int cnt = 0, i;
> > 
> >         kind = bpf_attach_type_to_tramp(link->link.prog);
> > @@ -547,15 +566,14 @@ static int __bpf_trampoline_link_prog(struct
> > bpf_tramp_link *link, struct bpf_tr
> >                 /* prog already linked */
> >                 return -EBUSY;
> > 
> > -               if (link_exiting->link.prog->type == BPF_PROG_TYPE_LSM)
> > -                       num_lsm_progs++;
> >         }
> > 
> > -       if (!num_lsm_progs && link->link.prog->type == BPF_PROG_TYPE_LSM)
> > -               bpf_lsm_toggle_hook(tr->func.addr, true);
> > -
> >         hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
> >         tr->progs_cnt[kind]++;
> > +
> > +       if (link->link.prog->type == BPF_PROG_TYPE_LSM)
> > +               bpf_trampoline_toggle_lsm(tr, kind);
> 
> how about keeping BPF_PROG_TYPE_LSM progs type count of attached programs
> in bpf_trampoline and toggle lsm on first coming in and last going out?

hm we actually allow other tracing program types to attach to bpf_lsm_*
functions, so I wonder we should toggle the lsm hook for each program
type (for bpf_lsm_* trampolines) because they'd expect the hook is called

but I'm not sure it's a valid use case to have like normal fentry program
attached to bpf_lsm_XXX function

jirka

> 
> also the trampoline attach is actually made in bpf_trampoline_update,
> so I wonder it'd make more sense to put it in there, but it's already
> complicated, so it actually might be easier in here
> 
> jirka
> 
> > +
> >         err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> >         if (err) {
> >                 hlist_del_init(&link->tramp_hlist);
> > @@ -578,7 +596,6 @@ static int __bpf_trampoline_unlink_prog(struct
> > bpf_tramp_link *link, struct bpf_
> >  {
> >         struct bpf_tramp_link *link_exiting;
> >         enum bpf_tramp_prog_type kind;
> > -       bool lsm_link_found = false;
> >         int err, num_lsm_progs = 0;
> > 
> >         kind = bpf_attach_type_to_tramp(link->link.prog);
> > @@ -595,18 +612,14 @@ static int __bpf_trampoline_unlink_prog(struct
> > bpf_tramp_link *link, struct bpf_
> >                                      tramp_hlist) {
> >                         if (link_exiting->link.prog->type == BPF_PROG_TYPE_LSM)
> >                                 num_lsm_progs++;
> > -
> > -                       if (link_exiting->link.prog == link->link.prog)
> > -                               lsm_link_found = true;
> >                 }
> >         }
> > 
> >         hlist_del_init(&link->tramp_hlist);
> >         tr->progs_cnt[kind]--;
> > 
> > -       if (lsm_link_found && num_lsm_progs == 1)
> > -               bpf_lsm_toggle_hook(tr->func.addr, false);
> > -
> > +       if (link->link.prog->type == BPF_PROG_TYPE_LSM)
> > +               bpf_trampoline_toggle_lsm(tr, kind);
> >         return bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> >  }
> > 
> > 
> > - KP