zdtm: Check pidfd can send signal after C/R

Ensure `pidfd_send_signal()` syscall works as expected after C/R.

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_child			\
 		pipe00				\
 		pipe01				\
 		pipe02				\

test/zdtm/static/pidfd_child.c

@@ -0,0 +1,66 @@
+#include <sys/syscall.h>
+#include <unistd.h>
+#include <sys/wait.h>
+
+#include "zdtmtst.h"
+
+const char *test_doc = "Checks pidfd sends signal to child process after restore\n";
+const char *test_author = "Bhavik Sachdev <b.sachdev1904@gmail.com>";
+
+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);
+}
+
+int main(int argc, char* argv[])
+{
+	int pidfd, status;
+	pid_t child;
+
+	test_init(argc, argv);
+
+	child = fork();
+	if (child < 0) {
+		pr_perror("Unable to fork a new process");
+		return 1;
+	} else if (child == 0) {
+		test_waitsig();
+		return 0;
+	}
+
+	pidfd = pidfd_open(child, 0);
+	if (pidfd < 0) {
+		pr_perror("pidfd_open failed");
+		return 1;
+	}
+
+	test_daemon();
+	test_waitsig();
+
+	if (pidfd_send_signal(pidfd, SIGTERM, NULL, 0)) {
+		fail("Could not send signal");
+		goto err_close;
+	}
+
+	if (waitpid(child, &status, 0) != child) {
+		pr_perror("waitpid()");
+		goto err_close;
+	}
+
+	if (status != 0) {
+		fail("%d:%d:%d:%d", WIFEXITED(status), WEXITSTATUS(status), WIFSIGNALED(status), WTERMSIG(status));
+		goto err_close;
+	}
+
+	pass();
+	close(pidfd);
+	return 0;
+err_close:
+	close(pidfd);
+	return 1;
+}