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Date:   Wed, 2 Dec 2020 11:19:02 -0800
From:   Martin KaFai Lau <kafai@fb.com>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Kuniyuki Iwashima <kuniyu@amazon.co.jp>
CC:     "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Benjamin Herrenschmidt <benh@amazon.com>,
        Kuniyuki Iwashima <kuni1840@gmail.com>,
        <osa-contribution-log@amazon.com>, bpf <bpf@vger.kernel.org>,
        Networking <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v1 bpf-next 06/11] bpf: Introduce two attach types for
 BPF_PROG_TYPE_SK_REUSEPORT.
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On Tue, Dec 01, 2020 at 06:04:50PM -0800, Andrii Nakryiko wrote:
> On Tue, Dec 1, 2020 at 6:49 AM Kuniyuki Iwashima <kuniyu@amazon.co.jp> wrote:
> >
> > This commit adds new bpf_attach_type for BPF_PROG_TYPE_SK_REUSEPORT to
> > check if the attached eBPF program is capable of migrating sockets.
> >
> > When the eBPF program is attached, the kernel runs it for socket migration
> > only if the expected_attach_type is BPF_SK_REUSEPORT_SELECT_OR_MIGRATE.
> > The kernel will change the behaviour depending on the returned value:
> >
> >   - SK_PASS with selected_sk, select it as a new listener
> >   - SK_PASS with selected_sk NULL, fall back to the random selection
> >   - SK_DROP, cancel the migration
> >
> > Link: https://lore.kernel.org/netdev/20201123003828.xjpjdtk4ygl6tg6h@kafai-mbp.dhcp.thefacebook.com/
> > Suggested-by: Martin KaFai Lau <kafai@fb.com>
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
> > ---
> >  include/uapi/linux/bpf.h       | 2 ++
> >  kernel/bpf/syscall.c           | 8 ++++++++
> >  tools/include/uapi/linux/bpf.h | 2 ++
> >  3 files changed, 12 insertions(+)
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 85278deff439..cfc207ae7782 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -241,6 +241,8 @@ enum bpf_attach_type {
> >         BPF_XDP_CPUMAP,
> >         BPF_SK_LOOKUP,
> >         BPF_XDP,
> > +       BPF_SK_REUSEPORT_SELECT,
> > +       BPF_SK_REUSEPORT_SELECT_OR_MIGRATE,
> >         __MAX_BPF_ATTACH_TYPE
> >  };
> >
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index f3fe9f53f93c..a0796a8de5ea 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -2036,6 +2036,14 @@ bpf_prog_load_check_attach(enum bpf_prog_type prog_type,
> >                 if (expected_attach_type == BPF_SK_LOOKUP)
> >                         return 0;
> >                 return -EINVAL;
> > +       case BPF_PROG_TYPE_SK_REUSEPORT:
> > +               switch (expected_attach_type) {
> > +               case BPF_SK_REUSEPORT_SELECT:
> > +               case BPF_SK_REUSEPORT_SELECT_OR_MIGRATE:
> > +                       return 0;
> > +               default:
> > +                       return -EINVAL;
> > +               }
> 
> this is a kernel regression, previously expected_attach_type wasn't
> enforced, so user-space could have provided any number without an
> error.
I also think this change alone will break things like when the usual
attr->expected_attach_type == 0 case.  At least changes is needed in
bpf_prog_load_fixup_attach_type() which is also handling a
similar situation for BPF_PROG_TYPE_CGROUP_SOCK.

I now think there is no need to expose new bpf_attach_type to the UAPI.
Since the prog->expected_attach_type is not used, it can be cleared at load time
and then only set to BPF_SK_REUSEPORT_SELECT_OR_MIGRATE (probably defined
internally at filter.[c|h]) in the is_valid_access() when "migration"
is accessed.  When "migration" is accessed, the bpf prog can handle
migration (and the original not-migration) case.

> 
> >         case BPF_PROG_TYPE_EXT:
> >                 if (expected_attach_type)
> >                         return -EINVAL;
> > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > index 85278deff439..cfc207ae7782 100644
> > --- a/tools/include/uapi/linux/bpf.h
> > +++ b/tools/include/uapi/linux/bpf.h
> > @@ -241,6 +241,8 @@ enum bpf_attach_type {
> >         BPF_XDP_CPUMAP,
> >         BPF_SK_LOOKUP,
> >         BPF_XDP,
> > +       BPF_SK_REUSEPORT_SELECT,
> > +       BPF_SK_REUSEPORT_SELECT_OR_MIGRATE,
> >         __MAX_BPF_ATTACH_TYPE
> >  };
> >
> > --
> > 2.17.2 (Apple Git-113)
> >