selftests: cachestat: test for cachestat availability

JIRA: https://issues.redhat.com/browse/RHEL-50302

commit 5e56982
Author: Andre Przywara <andre.przywara@arm.com>
Date:   Mon Aug 21 17:05:33 2023 +0100

    selftests: cachestat: test for cachestat availability

    Patch series "selftests: cachestat: fix run on older kernels", v2.

    I ran all kernel selftests on some test machine, and stumbled upon
    cachestat failing (among others).  These patches fix the run on older
    kernels and when the current directory is on a tmpfs instance.

    This patch (of 2):

    As cachestat is a new syscall, it won't be available on older kernels, for
    instance those running on a development machine.  At the moment the test
    reports all tests as "not ok" in this case.

    Test for the cachestat syscall availability first, before doing further
    tests, and bail out early with a TAP SKIP comment.

    This also uses the opportunity to add the proper TAP headers, and add one
    check for proper error handling (illegal file descriptor).

    Link: https://lkml.kernel.org/r/20230821160534.3414911-1-andre.przywara@arm.com
    Link: https://lkml.kernel.org/r/20230821160534.3414911-2-andre.przywara@arm.com
    Signed-off-by: Andre Przywara <andre.przywara@arm.com>
    Acked-by: Nhat Pham <nphamcs@gmail.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Shuah Khan <shuah@kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Eric Chanudet <echanude@redhat.com>

Author: eric-ch

tools/testing/selftests/cachestat/test_cachestat.c

@@ -15,6 +15,8 @@
 
 #include "../kselftest.h"
 
+#define NR_TESTS	8
+
 static const char * const dev_files[] = {
 	"/dev/zero", "/dev/null", "/dev/urandom",
 	"/proc/version", "/proc"
@@ -236,7 +238,23 @@ bool test_cachestat_shmem(void)
 
 int main(void)
 {
-	int ret = 0;
+	int ret;
+
+	ksft_print_header();
+
+	ret = syscall(__NR_cachestat, -1, NULL, NULL, 0);
+	if (ret == -1 && errno == ENOSYS)
+		ksft_exit_skip("cachestat syscall not available\n");
+
+	ksft_set_plan(NR_TESTS);
+
+	if (ret == -1 && errno == EBADF) {
+		ksft_test_result_pass("bad file descriptor recognized\n");
+		ret = 0;
+	} else {
+		ksft_test_result_fail("bad file descriptor ignored\n");
+		ret = 1;
+	}
 
 	for (int i = 0; i < 5; i++) {
 		const char *dev_filename = dev_files[i];