Martin KaFai Lau [off-list ref] writes:

On Thu, Mar 25, 2021 at 04:40:33PM +0100, Toke Høiland-Jørgensen wrote:

quoted

With the introduction of the struct_ops program type, it became possible to
implement kernel functionality in BPF, making it viable to use BPF in place
of a regular kernel module for these particular operations.

Thus far, the only user of this mechanism is for implementing TCP
congestion control algorithms. These are clearly marked as GPL-only when
implemented as modules (as seen by the use of EXPORT_SYMBOL_GPL for
tcp_register_congestion_control()), so it seems like an oversight that this
was not carried over to BPF implementations. And sine this is the only user
of the struct_ops mechanism, just enforcing GPL-only for the struct_ops
program type seems like the simplest way to fix this.

Fixes: 0baf26b0fcd7 ("bpf: tcp: Support tcp_congestion_ops in bpf")
Signed-off-by: Toke Høiland-Jørgensen <redacted>
---
 kernel/bpf/verifier.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 44e4ec1640f1..48dd0c0f087c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -12166,6 +12166,11 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
 		return -ENOTSUPP;
 	}
 
+	if (!prog->gpl_compatible) {
+		verbose(env, "struct ops programs must have a GPL compatible license\n");
+		return -EINVAL;
+	}
+

Thanks for the patch.

A nit.  Instead of sitting in between of the attach_btf_id check
and expected_attach_type check, how about moving it to the beginning
of this function.  Checking attach_btf_id and expected_attach_type
would make more sense to be done next to each other as in the current
code.

Yeah, good point. Not sure what I was thinking stuffing it in the middle
there; will fix and send a v2!

Acked-by: Martin KaFai Lau <redacted>

Thanks! :)

-Toke