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

[Kuniyuki Iwashima <kuniyu@amazon.co.jp>]

I'm sending this mail just for logging because I failed to send mails only
to LKML, netdev, and bpf yesterday.


From:   Martin KaFai Lau <kafai@fb.com>
Date:   Thu, 3 Dec 2020 21:56:53 -0800
> On Thu, Dec 03, 2020 at 11:16:08PM +0900, Kuniyuki Iwashima wrote:
> > From:   Martin KaFai Lau <kafai@fb.com>
> > Date:   Wed, 2 Dec 2020 20:24:02 -0800
> > > 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/
> > 
> > Thank you for the idea and reference.
> > 
> > I will remove the change in bpf_prog_load_check_attach() and set the
> > default value (BPF_SK_REUSEPORT_SELECT) in bpf_prog_load_fixup_attach_type()
> > for backward compatibility if expected_attach_type is 0.
> check_attach_type() can be kept.  You can refer to
> commit aac3fc320d94 for a similar situation.

I confirmed bpf_prog_load_fixup_attach_type() is called just before
bpf_prog_load_check_attach(), so I will add the fixup code to this patch.
Thank you.