Subject: Re: [PATCH bpf-next 1/2] bpf/flow_dissector: add mode to enforce global BPF flow dissector

From: Stanislav Fomichev <[email-masked]>
Date: Wed, 2 Oct 2019 14:31:21 -0700

On 10/02, Song Liu wrote:
> On Wed, Oct 2, 2019 at 10:36 AM Stanislav Fomichev <[email-masked]> wrote:
> >
> > Always use init_net flow dissector BPF program if it's attached and fall
> > back to the per-net namespace one. Also, deny installing new programs if
> > there is already one attached to the root namespace.
> > Users can still detach their BPF programs, but can't attach any
> > new ones (-EPERM).
> >
> > Cc: Petar Penkov <[email-masked]>
> > Signed-off-by: Stanislav Fomichev <[email-masked]>
> > ---
> >  Documentation/bpf/prog_flow_dissector.rst |  3 +++
> >  net/core/flow_dissector.c                 | 11 ++++++++++-
> >  2 files changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/bpf/prog_flow_dissector.rst b/Documentation/bpf/prog_flow_dissector.rst
> > index a78bf036cadd..4d86780ab0f1 100644
> > --- a/Documentation/bpf/prog_flow_dissector.rst
> > +++ b/Documentation/bpf/prog_flow_dissector.rst
> > @@ -142,3 +142,6 @@ BPF flow dissector doesn't support exporting all the metadata that in-kernel
> >  C-based implementation can export. Notable example is single VLAN (802.1Q)
> >  and double VLAN (802.1AD) tags. Please refer to the ``struct bpf_flow_keys``
> >  for a set of information that's currently can be exported from the BPF context.
> > +
> > +When BPF flow dissector is attached to the root network namespace (machine-wide
> > +policy), users can't override it in their child network namespaces.
> > diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> > index 7c09d87d3269..494e2016fe84 100644
> > --- a/net/core/flow_dissector.c
> > +++ b/net/core/flow_dissector.c
> > @@ -115,6 +115,11 @@ int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
> >         struct bpf_prog *attached;
> >         struct net *net;
> >
> > +       if (rcu_access_pointer(init_net.flow_dissector_prog)) {
> > +               /* Can't override root flow dissector program */
> > +               return -EPERM;
> 
> Maybe -EBUSY is more accurate?
I'm not sure, -EBUSY to me means that I can retry and (maybe) eventually
will succeed. Maybe return -EEXIST? At least it gives a hint that BPF
flow dissector is already there and retrying won't help. Thoughts?

> Thanks,
> Song