vfs: Add a sample program for the new mount API

Add a sample program to demonstrate fsopen/fsmount/move_mount to mount
something.

To make it compile on all arches, irrespective of whether or not syscall
numbers are assigned, define the syscall number to -1 if it isn't to cause
the kernel to return -ENOSYS.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

samples/vfs/test-fsmount.c

@@ -0,0 +1,133 @@
+/* fd-based mount test.
+ *
+ * Copyright (C) 2017 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/prctl.h>
+#include <sys/wait.h>
+#include <linux/mount.h>
+#include <linux/unistd.h>
+
+#define E(x) do { if ((x) == -1) { perror(#x); exit(1); } } while(0)
+
+static void check_messages(int fd)
+{
+	char buf[4096];
+	int err, n;
+
+	err = errno;
+
+	for (;;) {
+		n = read(fd, buf, sizeof(buf));
+		if (n < 0)
+			break;
+		n -= 2;
+
+		switch (buf[0]) {
+		case 'e':
+			fprintf(stderr, "Error: %*.*s\n", n, n, buf + 2);
+			break;
+		case 'w':
+			fprintf(stderr, "Warning: %*.*s\n", n, n, buf + 2);
+			break;
+		case 'i':
+			fprintf(stderr, "Info: %*.*s\n", n, n, buf + 2);
+			break;
+		}
+	}
+
+	errno = err;
+}
+
+static __attribute__((noreturn))
+void mount_error(int fd, const char *s)
+{
+	check_messages(fd);
+	fprintf(stderr, "%s: %m\n", s);
+	exit(1);
+}
+
+/* Hope -1 isn't a syscall */
+#ifndef __NR_fsopen
+#define __NR_fsopen -1
+#endif
+#ifndef __NR_fsmount
+#define __NR_fsmount -1
+#endif
+#ifndef __NR_fsconfig
+#define __NR_fsconfig -1
+#endif
+#ifndef __NR_move_mount
+#define __NR_move_mount -1
+#endif
+
+
+static inline int fsopen(const char *fs_name, unsigned int flags)
+{
+	return syscall(__NR_fsopen, fs_name, flags);
+}
+
+static inline int fsmount(int fsfd, unsigned int flags, unsigned int ms_flags)
+{
+	return syscall(__NR_fsmount, fsfd, flags, ms_flags);
+}
+
+static inline int fsconfig(int fsfd, unsigned int cmd,
+			   const char *key, const void *val, int aux)
+{
+	return syscall(__NR_fsconfig, fsfd, cmd, key, val, aux);
+}
+
+static inline int move_mount(int from_dfd, const char *from_pathname,
+			     int to_dfd, const char *to_pathname,
+			     unsigned int flags)
+{
+	return syscall(__NR_move_mount,
+		       from_dfd, from_pathname,
+		       to_dfd, to_pathname, flags);
+}
+
+#define E_fsconfig(fd, cmd, key, val, aux)				\
+	do {								\
+		if (fsconfig(fd, cmd, key, val, aux) == -1)		\
+			mount_error(fd, key ?: "create");		\
+	} while (0)
+
+int main(int argc, char *argv[])
+{
+	int fsfd, mfd;
+
+	/* Mount a publically available AFS filesystem */
+	fsfd = fsopen("afs", 0);
+	if (fsfd == -1) {
+		perror("fsopen");
+		exit(1);
+	}
+
+	E_fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "#grand.central.org:root.cell.", 0);
+	E_fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+
+	mfd = fsmount(fsfd, 0, MOUNT_ATTR_RDONLY);
+	if (mfd < 0)
+		mount_error(fsfd, "fsmount");
+	E(close(fsfd));
+
+	if (move_mount(mfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH) < 0) {
+		perror("move_mount");
+		exit(1);
+	}
+
+	E(close(mfd));
+	exit(0);
+}