use the switch_root function in shutdown

removes code duplication

also move switch-root to shared

src/shared/switch-root.c

@@ -0,0 +1,163 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2012 Harald Hoyer, Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <sys/stat.h>
+#include <stdbool.h>
+#include <errno.h>
+#include <string.h>
+#include <sys/mount.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+#include "util.h"
+#include "path-util.h"
+#include "switch-root.h"
+#include "mkdir.h"
+#include "base-filesystem.h"
+#include "missing.h"
+
+int switch_root(const char *new_root, const char *oldroot, bool detach_oldroot,  unsigned long mountflags) {
+
+        /*  Don't try to unmount/move the old "/", there's no way to do it. */
+        static const char move_mounts[] =
+                "/dev\0"
+                "/proc\0"
+                "/sys\0"
+                "/run\0";
+
+        _cleanup_close_ int old_root_fd = -1;
+        struct stat new_root_stat;
+        bool old_root_remove;
+        const char *i, *temporary_old_root;
+        int r;
+
+        if (path_equal(new_root, "/"))
+                return 0;
+
+        temporary_old_root = strappenda(new_root, oldroot);
+        mkdir_p_label(temporary_old_root, 0755);
+
+        old_root_remove = in_initrd();
+
+        if (stat(new_root, &new_root_stat) < 0) {
+                log_error("Failed to stat directory %s: %m", new_root);
+                return -errno;
+        }
+
+        /* Work-around for a kernel bug: for some reason the kernel
+         * refuses switching root if any file systems are mounted
+         * MS_SHARED. Hence remount them MS_PRIVATE here as a
+         * work-around.
+         *
+         * https://bugzilla.redhat.com/show_bug.cgi?id=847418 */
+        if (mount(NULL, "/", NULL, MS_REC|MS_PRIVATE, NULL) < 0)
+                log_warning("Failed to make \"/\" private mount: %m");
+
+        NULSTR_FOREACH(i, move_mounts) {
+                char new_mount[PATH_MAX];
+                struct stat sb;
+
+                snprintf(new_mount, sizeof(new_mount), "%s%s", new_root, i);
+                char_array_0(new_mount);
+
+                mkdir_p_label(new_mount, 0755);
+
+                if ((stat(new_mount, &sb) < 0) ||
+                    sb.st_dev != new_root_stat.st_dev) {
+
+                        /* Mount point seems to be mounted already or
+                         * stat failed. Unmount the old mount
+                         * point. */
+                        if (umount2(i, MNT_DETACH) < 0)
+                                log_warning("Failed to unmount %s: %m", i);
+                        continue;
+                }
+
+                if (mount(i, new_mount, NULL, mountflags, NULL) < 0) {
+                        if (mountflags & MS_MOVE) {
+                                log_error("Failed to move mount %s to %s, forcing unmount: %m", i, new_mount);
+
+                                if (umount2(i, MNT_FORCE) < 0)
+                                        log_warning("Failed to unmount %s: %m", i);
+                        }
+                        if (mountflags & MS_BIND)
+                                log_error("Failed to bind mount %s to %s: %m", i, new_mount);
+
+                }
+        }
+
+        r = base_filesystem_create(new_root);
+        if (r < 0) {
+                log_error("Failed to create the base filesystem: %s", strerror(-r));
+                return r;
+        }
+
+        if (chdir(new_root) < 0) {
+                log_error("Failed to change directory to %s: %m", new_root);
+                return -errno;
+        }
+
+        if (old_root_remove) {
+                old_root_fd = open("/", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_NOCTTY|O_DIRECTORY);
+                if (old_root_fd < 0)
+                        log_warning("Failed to open root directory: %m");
+        }
+
+        /* We first try a pivot_root() so that we can umount the old
+         * root dir. In many cases (i.e. where rootfs is /), that's
+         * not possible however, and hence we simply overmount root */
+        if (pivot_root(new_root, temporary_old_root) >= 0) {
+
+                /* Immediately get rid of the old root, if detach_oldroot is set.
+                 * Since we are running off it we need to do this lazily. */
+                if (detach_oldroot && umount2(oldroot, MNT_DETACH) < 0) {
+                        log_error("Failed to umount old root dir %s: %m", oldroot);
+                        return -errno;
+                }
+
+        } else if (mount(new_root, "/", NULL, MS_MOVE, NULL) < 0) {
+                log_error("Failed to mount moving %s to /: %m", new_root);
+                return -errno;
+        }
+
+        if (chroot(".") < 0) {
+                log_error("Failed to change root: %m");
+                return -errno;
+        }
+
+        if (chdir("/") < 0) {
+                log_error("Failed to change directory: %m");
+                return -errno;
+        }
+
+        if (old_root_fd >= 0) {
+                struct stat rb;
+
+                if (fstat(old_root_fd, &rb) < 0)
+                        log_warning("Failed to stat old root directory, leaving: %m");
+                else {
+                        rm_rf_children(old_root_fd, false, false, &rb);
+                        old_root_fd = -1;
+                }
+        }
+
+        return 0;
+}