Martin KaFai Lau [off-list ref] writes:

On Thu, Mar 25, 2021 at 04:40:34PM +0100, Toke Høiland-Jørgensen wrote:

quoted

This adds a selftest to check that the verifier rejects a TCP CC struct_ops
with a non-GPL license. To save having to add a whole new BPF object just
for this, reuse the dctcp CC, but rewrite the license field before loading.

Signed-off-by: Toke Høiland-Jørgensen <redacted>
---
 .../selftests/bpf/prog_tests/bpf_tcp_ca.c     | 31 +++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c b/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c
index 37c5494a0381..613cf8a00b22 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c

@@ -227,10 +227,41 @@ static void test_dctcp(void)
 	bpf_dctcp__destroy(dctcp_skel);
 }
 
+static void test_invalid_license(void)
+{
+	/* We want to check that the verifier refuses to load a non-GPL TCP CC.
+	 * Rather than create a whole new file+skeleton, just reuse an existing
+	 * object and rewrite the license in memory after loading. Sine libbpf
+	 * doesn't expose this, we define a struct that includes the first couple
+	 * of internal fields for struct bpf_object so we can overwrite the right
+	 * bits. Yes, this is a bit of a hack, but it makes the test a lot simpler.
+	 */
+	struct bpf_object_fragment {
+		char name[BPF_OBJ_NAME_LEN];
+		char license[64];
+	} *obj;

It is fragile.  A new bpf_nogpltcp.c should be created and it does
not have to be a full tcp-cc.  A very minimal implementation with
only .init. Something like this (uncompiled code):

char _license[] SEC("license") = "X";

void BPF_STRUCT_OPS(nogpltcp_init, struct sock *sk)
{
}

SEC(".struct_ops")
struct tcp_congestion_ops bpf_nogpltcp = {
	.init           = (void *)nogpltcp_init,
	.name           = "bpf_nogpltcp",
};

libbpf_set_print() can also be used to look for the
the verifier log "struct ops programs must have a GPL compatible license".

Ah, thanks for the pointers! Will fix this up as well...

-Toke