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

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 <redacted>
Date:   Thu, 3 Dec 2020 21:56:53 -0800

On Thu, Dec 03, 2020 at 11:16:08PM +0900, Kuniyuki Iwashima wrote:

quoted

From:   Martin KaFai Lau <redacted>
Date:   Wed, 2 Dec 2020 20:24:02 -0800

quoted

On Wed, Dec 02, 2020 at 11:19:02AM -0800, Martin KaFai Lau wrote:

quoted

On Tue, Dec 01, 2020 at 06:04:50PM -0800, Andrii Nakryiko wrote:

quoted

On Tue, Dec 1, 2020 at 6:49 AM Kuniyuki Iwashima [off-list ref] wrote:

quoted

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/ (local)
Suggested-by: Martin KaFai Lau <redacted>
Signed-off-by: Kuniyuki Iwashima <redacted>
---
 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/ (local)

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.