[PATCH] selftests/seccomp: fix check of fds being assigned

[Andrea Righi <andrea.righi@canonical.com>]

There might be an arbitrary free open fd slot when we run the addfd
sub-test, so checking for progressive numbers of file descriptors
starting from memfd is not always a reliable check and we could get the
following failure:

  #  RUN           global.user_notification_addfd ...
  # seccomp_bpf.c:3989:user_notification_addfd:Expected listener (18) == nextfd++ (9)
  # user_notification_addfd: Test terminated by assertion

Simply check if memfd and listener are valid file descriptors and start
counting for progressive file checking with the listener fd.

Fixes: 93e720d710df ("selftests/seccomp: More closely track fds being assigned")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
---
 tools/testing/selftests/seccomp/seccomp_bpf.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index d425688cf59c..4f37153378a1 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -3975,18 +3975,17 @@ TEST(user_notification_addfd)
 	/* There may be arbitrary already-open fds at test start. */
 	memfd = memfd_create("test", 0);
 	ASSERT_GE(memfd, 0);
-	nextfd = memfd + 1;
 
 	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
 	ASSERT_EQ(0, ret) {
 		TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
 	}
 
-	/* fd: 4 */
 	/* Check that the basic notification machinery works */
 	listener = user_notif_syscall(__NR_getppid,
 				      SECCOMP_FILTER_FLAG_NEW_LISTENER);
-	ASSERT_EQ(listener, nextfd++);
+	ASSERT_GE(listener, 0);
+	nextfd = listener + 1;
 
 	pid = fork();
 	ASSERT_GE(pid, 0);
-- 
2.32.0