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

On Thu, Dec 29, 2022 at 7:10 PM Alexei Starovoitov
<[email protected]> wrote:
>
> On Thu, Dec 29, 2022 at 6:13 PM Stanislav Fomichev <[email protected]> wrote:
> >
> > On Tue, Dec 27, 2022 at 8:40 AM Paul Moore <[email protected]> wrote:
> > >
> > > On December 26, 2022 10:35:49 PM Stanislav Fomichev <[email protected]>
> > > wrote:
> > > >> On Fri, Dec 23, 2022 at 5:49 PM Stanislav Fomichev <[email protected]> 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?
> > >
> > > So what do you guys want in a patch?  Is there a consensus on what you
> > > would merge to fix this bug/regression?
> >
> > Can we try the following?
> >
> > 1. Remove calls to bpf_prog_free_id (and bpf_map_free_id?) from
> > kernel/bpf/offload.c; that should make it easier to reason about those
> > '!id' checks
>
> calls? you mean a single call, right?

Right, there is a single call to bpf_prog_free_id. But there is also
another single call to bpf_map_free_id with the same "remove it from
idr so it can't be found if GET_NEXT_ID" reasoning.
It's probably worth it to look into whether we can remove it as well
to have consistent id management for progs and maps?

> > 2. Move bpf_prog_free_id (and bpf_map_free_id?) to happen after
> > audit/perf in kernel/bpf/syscall.c (there are comments that say "must
> > be called first", but I don't see why; seems like GET_FD_BY_ID would
> > correctly return -ENOENT; maybe Martin can chime in, CC'ed him
> > explicitly)
>
> The comment says that it should be removed from idr
> before __bpf_prog_put_noref will proceed to clean up.

Which one? I was trying to see if there is any reasoning in the
original commit 34ad5580f8f9 ("bpf: Add BPF_(PROG|MAP)_GET_NEXT_ID
command"), but couldn't find anything useful :-(

> > 3. (optionally) Remove do_idr_lock arguments (all callers are passing 'true')
>
> yes. please.

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