[PATCH net-next] selftests: tls: avoid flakiness in data_steal

[Jakub Kicinski <kuba@kernel.org>]

We see the following failure a few times a week:

  #  RUN           global.data_steal ...
  # tls.c:3280:data_steal:Expected recv(cfd, buf2, sizeof(buf2), MSG_DONTWAIT) (10000) == -1 (-1)
  # data_steal: Test failed
  #          FAIL  global.data_steal
  not ok 8 global.data_steal

The 10000 bytes read suggests that the child process did a recv()
of half of the data using the TLS ULP and we're now getting the
remaining half. The intent of the test is to get the child to
enter _TCP_ recvmsg handler, so it needs to enter the syscall before
parent installed the TLS recvmsg with setsockopt(SOL_TLS).

Instead of the 10msec sleep send 1 byte of data and wait for the
child to consume it.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: sd@queasysnail.net
CC: shuah@kernel.org
CC: linux-kselftest@vger.kernel.org
---
 tools/testing/selftests/net/tls.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/net/tls.c b/tools/testing/selftests/net/tls.c
index a4d16a460fbe..9e2ccea13d70 100644
--- a/tools/testing/selftests/net/tls.c
+++ b/tools/testing/selftests/net/tls.c
@@ -3260,17 +3260,25 @@ TEST(data_steal) {
 	ASSERT_EQ(setsockopt(cfd, IPPROTO_TCP, TCP_ULP, "tls", sizeof("tls")), 0);
 
 	/* Spawn a child and get it into the read wait path of the underlying
-	 * TCP socket.
+	 * TCP socket (before kernel .recvmsg is replaced with the TLS one).
 	 */
 	pid = fork();
 	ASSERT_GE(pid, 0);
 	if (!pid) {
-		EXPECT_EQ(recv(cfd, buf, sizeof(buf) / 2, MSG_WAITALL),
-			  sizeof(buf) / 2);
+		EXPECT_EQ(recv(cfd, buf, sizeof(buf) / 2 + 1, MSG_WAITALL),
+			  sizeof(buf) / 2 + 1);
 		exit(!__test_passed(_metadata));
 	}
 
-	usleep(10000);
+	/* Send a sync byte and poll until it's consumed to ensure
+	 * the child is in recv() before we proceed to install TLS.
+	 */
+	ASSERT_EQ(send(fd, buf, 1, 0), 1);
+	do {
+		usleep(500);
+	} while (recv(cfd, buf, 1, MSG_PEEK | MSG_DONTWAIT) == 1);
+	EXPECT_EQ(errno, EAGAIN);
+
 	ASSERT_EQ(setsockopt(fd, SOL_TLS, TLS_TX, &tls, tls.len), 0);
 	ASSERT_EQ(setsockopt(cfd, SOL_TLS, TLS_RX, &tls, tls.len), 0);
 
-- 
2.52.0