Re: [PATCH v1 bpf-next 06/11] bpf: Introduce two attach types for BPF_PROG_TYPE_SK_REUSEPORT.

[Martin KaFai Lau <kafai@fb.com>]

On Wed, Dec 02, 2020 at 11:19:02AM -0800, Martin KaFai Lau wrote:
> 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.
Scrap this internal only BPF_SK_REUSEPORT_SELECT_OR_MIGRATE idea.
I think there will be cases that bpf prog wants to do both
without accessing any field from sk_reuseport_md.

Lets go back to the discussion on using a similar
idea as BPF_PROG_TYPE_CGROUP_SOCK in bpf_prog_load_fixup_attach_type().
I am not aware there is loader setting a random number
in expected_attach_type, so the chance of breaking
is very low.  There was a similar discussion earlier [0].

[0]: https://lore.kernel.org/netdev/20200126045443.f47dzxdglazzchfm@ast-mbp/

> 
> > 
> > >         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)
> > >