zdtm: Check fd from pidfd_getfd is C/Red correctly

We get the read end of a pipe using `pidfd_getfd` and check if we can
read from it after C/R.

signed-off-by: Bhavik Sachdev <[email protected]>

test/zdtm/static/Makefile

@@ -57,6 +57,7 @@ TST_NOFILE	:=				\
 		pidfd_dead			\
 		pidfd_child			\
 		pidfd_kill			\
+		fd_from_pidfd			\
 		pipe00				\
 		pipe01				\
 		pipe02				\

test/zdtm/static/fd_from_pidfd.c

@@ -0,0 +1,81 @@
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#include "zdtmtst.h"
+
+const char *test_doc = "Check if fd obtained from pidfd_get_fd is C/R correctly\n";
+const char *test_author = "Bhavik Sachdev <[email protected]>";
+
+static int pidfd_open(pid_t pid, unsigned int flags)
+{
+	return syscall(__NR_pidfd_open, pid, flags);
+} static int pidfd_getfd(int pidfd, int targetfd, unsigned int flags)
+{
+	return syscall(__NR_pidfd_getfd, pidfd, targetfd, flags);
+}
+
+int main(int argc, char* argv[])
+{
+	#define READ 0
+	#define WRITE 1
+
+	int pidfd, child, p[2], child_read, read_data;
+	int data = 42;
+
+	test_init(argc, argv);
+
+	if (pipe(p)) {
+		pr_perror("pipe");
+		return 1;
+	}
+
+	child = fork();
+	if (child < 0) {
+		pr_perror("fork");
+		return 1;
+	}
+
+	if (child == 0) {
+		close(p[WRITE]);
+		test_waitsig();
+		return 0;
+	}
+
+	pidfd = pidfd_open(child, 0);
+	if (pidfd < 0) {
+		pr_perror("pidfd_open failed");
+		return 1;
+	}
+
+	close(p[READ]);
+	if (write(p[WRITE], &data, sizeof(data)) != sizeof(data)) {
+		pr_perror("write");
+		return 1;
+	}
+	close(p[WRITE]);
+
+	child_read = pidfd_getfd(pidfd, p[READ], 0);
+	if (child_read < 0) {
+		pr_perror("pidfd_getfd");
+		return 1;
+	}
+
+	test_daemon();
+	test_waitsig();
+
+	if (read(child_read, &read_data, sizeof(read_data)) != sizeof(read_data)) {
+		pr_perror("read");
+		return 1;
+	}
+
+	if (read_data != data) {
+		fail("data from fd obtained using pidfd_getfd incorrect\n");
+		goto out;
+	}
+
+	pass();
+out:
+	close(child_read);
+	close(pidfd);
+	return 0;
+}