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

[Song Liu <liu.song.a23@gmail.com>]

On Wed, Oct 2, 2019 at 10:36 AM Stanislav Fomichev <sdf@google.com> 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 <ppenkov@google.com>
> Signed-off-by: Stanislav Fomichev <sdf@google.com>
> ---
>  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?

Thanks,
Song