Add kselftest support for open, linkat, unshare, mount tests

- Replace direct error handling with
 `ksft_test_result_*` macros for better reporting
  of test outcomes.

- Add `ksft_print_header()` and `ksft_set_plan()`
 to structure test outputs more effectively.

- Introduce the helper function `is_unshare()` to
  handle unshare() related checks.

- Improve the test flow by adding more detailed pass/fail
  reporting for unshare, mounting, file opening, and linking
  operations.

- Skip the test if it's not run as root, providing an
  appropriate Warning.

Test logs:

Before change:

- Without root
 error: unshare, errno 1

- With root
 No, output

After change:

- Without root
 TAP version 13
 1..1
 ok 1 # SKIP This test needs root to run

- With root
 TAP version 13
 1..1
 ok 1 unshare(): we have a new mount namespace.
 1..2
 ok 2 mount(): Root filesystem private mount: Success
 1..3
 ok 3 mount(): Mounting tmpfs on /tmp: Success
 1..4
 ok 4 openat(): Open first temporary file: Success
 1..5
 ok 5 linkat(): Linking the temporary file: Success
 1..6
 ok 6 openat(): Opening the second temporary file: Success
 # Totals: pass:6 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Shivam Chaudhary <cvam0000@gmail.com>
---
Changes in v2:
  - Make the commit message more clear.

 .../selftests/tmpfs/bug-link-o-tmpfile.c      | 72 +++++++++++++++----
 1 file changed, 58 insertions(+), 14 deletions(-)

diff --git a/tools/testing/selftests/tmpfs/bug-link-o-tmpfile.c b/tools/testing/selftests/tmpfs/bug-link-o-tmpfile.c
index b5c3ddb90942..26dea19c1614 100644
--- a/tools/testing/selftests/tmpfs/bug-link-o-tmpfile.c
+++ b/tools/testing/selftests/tmpfs/bug-link-o-tmpfile.c
@@ -23,45 +23,89 @@
 #include <sys/mount.h>
 #include <unistd.h>
 
-int main(void)
-{
-	int fd;
+#include "../kselftest.h"
 
-	if (unshare(CLONE_NEWNS) == -1) {
+static int is_unshare(int flag)
+{
+	if (unshare(flag) == -1) {
 		if (errno == ENOSYS || errno == EPERM) {
-			fprintf(stderr, "error: unshare, errno %d\n", errno);
-			return 4;
+			ksft_test_result_fail("error: unshare, errno %d\n", errno);
+			return -1; // Return -1 for failure
 		}
 		fprintf(stderr, "error: unshare, errno %d\n", errno);
+		return -1;
+	}
+
+	return 0; // Return 0 for success
+}
+
+int main(void)
+{
+	int fd;
+
+	// Setting up kselftest framework
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	// Check if test is run as root
+	if (geteuid()) {
+		ksft_test_result_skip("This test needs root to run!\n");
 		return 1;
 	}
-	if (mount(NULL, "/", NULL, MS_PRIVATE|MS_REC, NULL) == -1) {
-		fprintf(stderr, "error: mount '/', errno %d\n", errno);
+
+	if (is_unshare(CLONE_NEWNS) == 0) {
+		ksft_test_result_pass("unshare(): we have a new mount namespace.\n");
+	} else {
+		ksft_test_result_fail("unshare(): failed\n");
 		return 1;
 	}
 
+	ksft_set_plan(2);
+
+	if (mount(NULL, "/", NULL, MS_PRIVATE | MS_REC, NULL) == -1) {
+		ksft_test_result_fail("mount(): Root filesystem private mount: Fail %d\n", errno);
+		return 1;
+	} else {
+		ksft_test_result_pass("mount(): Root filesystem private mount: Success\n");
+	}
+
+	ksft_set_plan(3);
 	/* Our heroes: 1 root inode, 1 O_TMPFILE inode, 1 permanent inode. */
 	if (mount(NULL, "/tmp", "tmpfs", 0, "nr_inodes=3") == -1) {
-		fprintf(stderr, "error: mount tmpfs, errno %d\n", errno);
+		ksft_test_result_fail("mount(): Mounting tmpfs on /tmp: Fail %d\n", errno);
 		return 1;
+	} else {
+		ksft_test_result_pass("mount(): Mounting tmpfs on /tmp: Success\n");
 	}
 
-	fd = openat(AT_FDCWD, "/tmp", O_WRONLY|O_TMPFILE, 0600);
+	ksft_set_plan(4);
+	fd = openat(AT_FDCWD, "/tmp", O_WRONLY | O_TMPFILE, 0600);
 	if (fd == -1) {
-		fprintf(stderr, "error: open 1, errno %d\n", errno);
+		ksft_test_result_fail("openat(): Open first temporary file: Fail %d\n", errno);
 		return 1;
+	} else {
+		ksft_test_result_pass("openat(): Open first temporary file: Success\n");
 	}
+
+	ksft_set_plan(5);
 	if (linkat(fd, "", AT_FDCWD, "/tmp/1", AT_EMPTY_PATH) == -1) {
-		fprintf(stderr, "error: linkat, errno %d\n", errno);
+		ksft_test_result_fail("linkat(): Linking the temporary file: Fail %d\n", errno);
+		close(fd); // Ensure fd is closed on failure
 		return 1;
+	} else {
+		ksft_test_result_pass("linkat(): Linking the temporary file: Success\n");
 	}
 	close(fd);
 
-	fd = openat(AT_FDCWD, "/tmp", O_WRONLY|O_TMPFILE, 0600);
+	ksft_set_plan(6);
+	fd = openat(AT_FDCWD, "/tmp", O_WRONLY | O_TMPFILE, 0600);
 	if (fd == -1) {
-		fprintf(stderr, "error: open 2, errno %d\n", errno);
+		ksft_test_result_fail("openat(): Opening the second temporary file: Fail %d\n", errno);
 		return 1;
+	} else {
+		ksft_test_result_pass("openat(): Opening the second temporary file: Success\n");
 	}
 
+	ksft_exit_pass();
 	return 0;
 }
-- 
2.34.1