zdtm: Check pidfd for thread is valid after C/R

We open a pidfd to a thread using `PIDFD_THREAD` flag and after C/R
ensure that we can send signals using it with `PIDFD_SIGNAL_THREAD`.

signed-off-by: Bhavik Sachdev <b.sachdev1904@gmail.com>

test/zdtm/static/Makefile

@@ -54,6 +54,7 @@ TST_NOFILE	:=				\
 		shm-mp				\
 		ptrace_sig			\
 		pidfd_self			\
+		pidfd_of_thread			\
 		pidfd_dead			\
 		pidfd_child			\
 		pidfd_kill			\

test/zdtm/static/pidfd_of_thread.c

@@ -0,0 +1,114 @@
+#include <sched.h>
+#include <sys/stat.h>
+#include <sys/statfs.h>
+#include <fcntl.h>
+
+#include "zdtmtst.h"
+#include "lock.h"
+
+const char *test_doc = "Check C/R of pidfds that point to threads\n";
+const char *test_author = "Bhavik Sachdev <b.sachdev1904@gmail.com>";
+
+/* see also: https://codebrowser.dev/glibc/glibc/sysdeps/unix/sysv/linux/tst-clone3.c.html */
+
+#ifndef PIDFD_THREAD
+#define PIDFD_THREAD	O_EXCL
+#endif
+
+#ifndef PIDFD_SIGNAL_THREAD
+#define PIDFD_SIGNAL_THREAD		(1UL << 0)
+#endif
+
+#ifndef PID_FS_MAGIC
+#define PID_FS_MAGIC 0x50494446
+#endif
+
+static long get_fs_type(int lfd)
+{
+	struct statfs fst;
+
+	if (fstatfs(lfd, &fst)) {
+		return -1;
+	}
+	return fst.f_type;
+}
+
+static int pidfd_open(pid_t pid, unsigned int flags)
+{
+	return syscall(__NR_pidfd_open, pid, flags);
+}
+
+static int pidfd_send_signal(int pidfd, int sig, siginfo_t* info, unsigned int flags)
+{
+	return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
+}
+
+static int thread_func(void *a)
+{
+	test_waitsig();
+	return 0;
+}
+
+#define CTID_INIT_VAL 1
+
+int main(int argc, char* argv[])
+{
+	char st[64 * 1024] __attribute__ ((aligned));
+	pid_t tid;
+	int pidfd, test_pidfd;
+	futex_t exited;
+
+	int clone_flags = CLONE_THREAD;
+	clone_flags |= CLONE_VM | CLONE_SIGHAND;
+	clone_flags |= CLONE_CHILD_CLEARTID;
+
+	test_init(argc, argv);
+
+	test_pidfd = pidfd_open(getpid(), 0);
+	if (test_pidfd < 0) {
+		pr_perror("pidfd_open() failed");
+		return 1;
+	}
+
+	/* PIDFD_THREAD, PIDFD_SIGNAL_THREAD are supported only with pidfs */
+	if (get_fs_type(test_pidfd) != PID_FS_MAGIC) {
+		test_daemon();
+		test_waitsig();
+		skip("pidfs not supported.");
+		close(test_pidfd);
+		return 0;
+	}
+	close(test_pidfd);
+
+	futex_set(&exited, CTID_INIT_VAL);
+
+	tid = clone(thread_func, st + sizeof(st), clone_flags, NULL, NULL, NULL, &(exited.raw));
+	if (tid == -1) {
+	    pr_perror("clone() failed");
+	    return 1;
+	}
+
+	test_msg("Successfully created a thread with tid: %d\n", tid);
+	pidfd = pidfd_open(tid, PIDFD_THREAD);
+	if (pidfd < 0) {
+	    pr_perror("pidfd_open() failed");
+	    return 1;
+	}
+
+	test_daemon();
+	test_waitsig();
+
+	if (pidfd_send_signal(pidfd, SIGTERM, NULL, PIDFD_SIGNAL_THREAD)) {
+	    pr_perror("pidfd_send_signal() failed");
+	    fail();
+	    close(pidfd);
+	    return 1;
+	}
+
+	test_msg("Waiting for thread to exit\n");
+	futex_wait_until(&exited, 0);
+
+	pass();
+	close(pidfd);
+	return 0;
+}

test/zdtm/static/pidfd_of_thread.desc

@@ -0,0 +1 @@
+{'flags': 'noauto crfail'}