[PATCH v2] bpf: restore the ebpf program ID for BPF_AUDIT_UNLOAD and PERF_BPF_EVENT_PROG_UNLOAD

[Stanislav Fomichev]

> On Fri, Dec 23, 2022 at 5:49 PM Stanislav Fomichev <sdf at google.com> wrote:
> get_func_ip() */
> > > -                               tstamp_type_access:1; /* Accessed __sk_buff->tstamp_type */
> > > +                               tstamp_type_access:1, /* Accessed __sk_buff->tstamp_type */
> > > +                               valid_id:1; /* Is bpf_prog::aux::__id valid? */
> > >         enum bpf_prog_type      type;           /* Type of BPF program */
> > >         enum bpf_attach_type    expected_attach_type; /* For some prog types */
> > >         u32                     len;            /* Number of filter blocks */
> > > @@ -1688,6 +1689,12 @@ void bpf_prog_inc(struct bpf_prog *prog);
> > >  struct bpf_prog * __must_check bpf_prog_inc_not_zero(struct bpf_prog *prog);
> > >  void bpf_prog_put(struct bpf_prog *prog);
> > >
> > > +static inline u32 bpf_prog_get_id(const struct bpf_prog *prog)
> > > +{
> > > +       if (WARN(!prog->valid_id, "Attempting to use an invalid eBPF program"))
> > > +               return 0;
> > > +       return prog->aux->__id;
> > > +}
> >
> > I'm still missing why we need to have this WARN and have a check at all.
> > IIUC, we're actually too eager in resetting the id to 0, and need to
> > keep that stale id around at least for perf/audit.
> > Why not have a flag only to protect against double-idr_remove
> > bpf_prog_free_id and keep the rest as is?
> > Which places are we concerned about that used to report id=0 but now
> > would report stale id?
> 
> What double-idr_remove are you concerned about?
> bpf_prog_by_id() is doing bpf_prog_inc_not_zero
> while __bpf_prog_put just dropped it to zero.

(traveling, sending from an untested setup, hope it reaches everyone)

There is a call to bpf_prog_free_id from __bpf_prog_offload_destroy which
tries to make offloaded program disappear from the idr when the netdev
goes offline. So I'm assuming that '!prog->aux->id' check in bpf_prog_free_id
is to handle that case where we do bpf_prog_free_id much earlier than the
rest of the __bpf_prog_put stuff.

> Maybe just move bpf_prog_free_id() into bpf_prog_put_deferred()
> after perf_event_bpf_event and bpf_audit_prog ?
> Probably can remove the obsolete do_idr_lock bool flag as
> separate patch?

+1 on removing do_idr_lock separately.

> Much simpler fix and no code churn.
> Both valid_id and saved_id approaches have flaws.

Given the __bpf_prog_offload_destroy path above, we still probably need
some flag to indicate that the id has been already removed from the idr?