Merge branch 's390-bpf-fix-arch_prepare_bpf_trampoline'

Song Liu says:

====================
s390/bpf: Fix arch_prepare_bpf_trampoline

While working on trampoline, I found s390's arch_prepare_bpf_trampoline
returns 0 on success, which breaks struct_ops. However, the CI doesn't
catch this issue. Turns out test_progs:bpf_tcp_ca doesn't really test
members of a struct_ops are actually called via the trampolines.

1/2 fixes arch_prepare_bpf_trampoline for s390.
2/2 adds a check to test_progs:bpf_tcp_ca to verify bpf_cubic_acked() is
indeed called by the trampoline. Without 1/2, this check would fail on
s390.
====================

Link: https://lore.kernel.org/r/20230919060258.3237176-1-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

arch/s390/net/bpf_jit_comp.c

@@ -2513,7 +2513,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image,
 			return -E2BIG;
 	}
 
-	return ret;
+	return tjit.common.prg;
 }
 
 bool bpf_jit_supports_subprog_tailcalls(void)

tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c

@@ -185,6 +185,8 @@ static void test_cubic(void)
 
 	do_test("bpf_cubic", NULL);
 
+	ASSERT_EQ(cubic_skel->bss->bpf_cubic_acked_called, 1, "pkts_acked called");
+
 	bpf_link__destroy(link);
 	bpf_cubic__destroy(cubic_skel);
 }

tools/testing/selftests/bpf/progs/bpf_cubic.c

@@ -490,6 +490,8 @@ static __always_inline void hystart_update(struct sock *sk, __u32 delay)
 	}
 }
 
+int bpf_cubic_acked_called = 0;
+
 void BPF_STRUCT_OPS(bpf_cubic_acked, struct sock *sk,
 		    const struct ack_sample *sample)
 {
@@ -497,6 +499,7 @@ void BPF_STRUCT_OPS(bpf_cubic_acked, struct sock *sk,
 	struct bictcp *ca = inet_csk_ca(sk);
 	__u32 delay;
 
+	bpf_cubic_acked_called = 1;
 	/* Some calls are for duplicates without timetamps */
 	if (sample->rtt_us < 0)
 		return;