libbpf: Fix ring_buffer__poll() to return number of consumed samples

Fix ring_buffer__poll() to return the number of non-discarded records
consumed, just like its documentation states. It's also consistent with
ring_buffer__consume() return. Fix up selftests with wrong expected results.

Fixes: bf99c936 ("libbpf: Add BPF ring buffer support")
Fixes: cb1c9ddd ("selftests/bpf: Add BPF ringbuf selftests")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201130223336.904192-1-andrii@kernel.org

tools/lib/bpf/ringbuf.c

@@ -278,7 +278,7 @@ int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms)
 		err = ringbuf_process_ring(ring);
 		if (err < 0)
 			return err;
-		res += cnt;
+		res += err;
 	}
 	return cnt < 0 ? -errno : res;
 }

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

@@ -217,7 +217,7 @@ void test_ringbuf(void)
 	if (CHECK(err, "join_bg", "err %d\n", err))
 		goto cleanup;
 
-	if (CHECK(bg_ret != 1, "bg_ret", "epoll_wait result: %ld", bg_ret))
+	if (CHECK(bg_ret <= 0, "bg_ret", "epoll_wait result: %ld", bg_ret))
 		goto cleanup;
 
 	/* 3 rounds, 2 samples each */

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

@@ -81,7 +81,7 @@ void test_ringbuf_multi(void)
 
 	/* poll for samples, should get 2 ringbufs back */
 	err = ring_buffer__poll(ringbuf, -1);
-	if (CHECK(err != 4, "poll_res", "expected 4 records, got %d\n", err))
+	if (CHECK(err != 2, "poll_res", "expected 2 records, got %d\n", err))
 		goto cleanup;
 
 	/* expect extra polling to return nothing */