Re: [PATCH bpf-next v2 2/2] bpf: Require only one of cong_avoid() and cong_control() from a TCP CC

On Mon, 2022-06-13 at 10:17 -0700, Martin KaFai Lau wrote:

On Thu, Jun 09, 2022 at 10:47:02PM +0200, Jörn-Thorben Hinz wrote:

quoted

Remove the check for required and optional functions in a struct
tcp_congestion_ops from bpf_tcp_ca.c. Rely on
tcp_register_congestion_control() to reject a BPF CC that does not
implement all required functions, as it will do for a non-BPF CC.

When a CC implements tcp_congestion_ops.cong_control(), the
alternate
cong_avoid() is not in use in the TCP stack. Previously, a BPF CC
was
still forced to implement cong_avoid() as a no-op since it was
non-optional in bpf_tcp_ca.c.

Signed-off-by: Jörn-Thorben Hinz <redacted>
---
 net/ipv4/bpf_tcp_ca.c | 32 --------------------------------
 1 file changed, 32 deletions(-)

diff --git a/net/ipv4/bpf_tcp_ca.c b/net/ipv4/bpf_tcp_ca.c
index 1f5c53ede4e5..39b64f317499 100644
--- a/net/ipv4/bpf_tcp_ca.c
+++ b/net/ipv4/bpf_tcp_ca.c

@@ -14,18 +14,6 @@

 /* "extern" is to avoid sparse warning.  It is only used in
bpf_struct_ops.c. */
 extern struct bpf_struct_ops bpf_tcp_congestion_ops;
 
-static u32 optional_ops[] = {
-       offsetof(struct tcp_congestion_ops, init),
-       offsetof(struct tcp_congestion_ops, release),
-       offsetof(struct tcp_congestion_ops, set_state),
-       offsetof(struct tcp_congestion_ops, cwnd_event),
-       offsetof(struct tcp_congestion_ops, in_ack_event),
-       offsetof(struct tcp_congestion_ops, pkts_acked),
-       offsetof(struct tcp_congestion_ops, min_tso_segs),
-       offsetof(struct tcp_congestion_ops, sndbuf_expand),
-       offsetof(struct tcp_congestion_ops, cong_control),
-};
-
 static u32 unsupported_ops[] = {
        offsetof(struct tcp_congestion_ops, get_info),
 };

@@ -51,18 +39,6 @@ static int bpf_tcp_ca_init(struct btf *btf)

        return 0;
 }
 
-static bool is_optional(u32 member_offset)
-{
-       unsigned int i;
-
-       for (i = 0; i < ARRAY_SIZE(optional_ops); i++) {
-               if (member_offset == optional_ops[i])
-                       return true;
-       }
-
-       return false;
-}
-
 static bool is_unsupported(u32 member_offset)
 {
        unsigned int i;

@@ -268,14 +244,6 @@ static int bpf_tcp_ca_init_member(const struct

btf_type *t,
                return 1;
        }
 
-       if (!btf_type_resolve_func_ptr(btf_vmlinux, member->type,
NULL))
-               return 0;
-
-       /* Ensure bpf_prog is provided for compulsory func ptr */
-       prog_fd = (int)(*(unsigned long *)(udata + moff));
-       if (!prog_fd && !is_optional(moff) &&
!is_unsupported(moff))

!is_unsupported() is still needed.

I don’t think it is necessary here.

There is also bpf_tcp_ca_check_member(), which also and only tests for
is_unsupported(). And check_member() will be called even earlier than
init_member() when loading a struct_ops. This deleted is_unsupported()
call should have been there only to test for missing but required &&
supported functions.

I added a test in v3 to make sure the check for the unsupported
get_info() (still) works.

Please correct me, if I misread/misfollowed the code paths!

and remove 'int prog_fd' as reported by the test bot.

Of course, sorry about that … Oversight on my side.

Test is still needed.  You can copy the simpler "bpf_dctcp"
to another tcp_congestion_ops.  Write+read the sk_packing
and also use .cong_control instead of .cong_avoid.  I think rs-

quoted

acked_sacked

is the 'delivered' and the 'ack' is not used.

Added tests in v3 of the patch series. I’m open for further feedback.