1 files changed, 127 insertions, 0 deletions
diff --git a/src/shared/switch-root.c b/src/shared/switch-root.c
new file mode 100644
index 0000000..e0dd17a
--- /dev/null
+++ b/src/shared/switch-root.c
@@ -0,0 +1,127 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <stdbool.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include "base-filesystem.h"
+#include "fd-util.h"
+#include "fs-util.h"
+#include "log.h"
+#include "missing_syscall.h"
+#include "mkdir.h"
+#include "mount-util.h"
+#include "mountpoint-util.h"
+#include "path-util.h"
+#include "rm-rf.h"
+#include "stdio-util.h"
+#include "string-util.h"
+#include "strv.h"
+#include "switch-root.h"
+#include "user-util.h"
+#include "util.h"
+
+int switch_root(const char *new_root,
+                const char *old_root_after, /* path below the new root, where to place the old root after the transition */
+                bool unmount_old_root,
+                unsigned long mount_flags) {  /* MS_MOVE or MS_BIND */
+
+        _cleanup_free_ char *resolved_old_root_after = NULL;
+        _cleanup_close_ int old_root_fd = -1;
+        bool old_root_remove;
+        const char *i;
+        int r;
+
+        assert(new_root);
+        assert(old_root_after);
+
+        if (path_equal(new_root, "/"))
+                return 0;
+
+        /* Check if we shall remove the contents of the old root */
+        old_root_remove = in_initrd();
+        if (old_root_remove) {
+                old_root_fd = open("/", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_NOCTTY|O_DIRECTORY);
+                if (old_root_fd < 0)
+                        return log_error_errno(errno, "Failed to open root directory: %m");
+        }
+
+        /* Determine where we shall place the old root after the transition */
+        r = chase_symlinks(old_root_after, new_root, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &resolved_old_root_after, NULL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, old_root_after);
+        if (r == 0) /* Doesn't exist yet. Let's create it */
+                (void) mkdir_p_label(resolved_old_root_after, 0755);
+
+        /* Work-around for kernel design: the kernel refuses MS_MOVE 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)
+                return log_error_errno(errno, "Failed to set \"/\" mount propagation to private: %m");
+
+        FOREACH_STRING(i, "/sys", "/dev", "/run", "/proc") {
+                _cleanup_free_ char *chased = NULL;
+
+                r = chase_symlinks(i, new_root, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &chased, NULL);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, i);
+                if (r > 0) {
+                        /* Already exists. Let's see if it is a mount point already. */
+                        r = path_is_mount_point(chased, NULL, 0);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to determine whether %s is a mount point: %m", chased);
+                        if (r > 0) /* If it is already mounted, then do nothing */
+                                continue;
+                } else
+                         /* Doesn't exist yet? */
+                        (void) mkdir_p_label(chased, 0755);
+
+                if (mount(i, chased, NULL, mount_flags, NULL) < 0)
+                        return log_error_errno(errno, "Failed to mount %s to %s: %m", i, chased);
+        }
+
+        /* Do not fail if base_filesystem_create() fails. Not all switch roots are like base_filesystem_create() wants
+         * them to look like. They might even boot, if they are RO and don't have the FS layout. Just ignore the error
+         * and switch_root() nevertheless. */
+        (void) base_filesystem_create(new_root, UID_INVALID, GID_INVALID);
+
+        if (chdir(new_root) < 0)
+                return log_error_errno(errno, "Failed to change directory to %s: %m", new_root);
+
+        /* 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, resolved_old_root_after) >= 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 (unmount_old_root) {
+                        r = umount_recursive(old_root_after, MNT_DETACH);
+                        if (r < 0)
+                                log_warning_errno(r, "Failed to unmount old root directory tree, ignoring: %m");
+                }
+
+        } else if (mount(new_root, "/", NULL, MS_MOVE, NULL) < 0)
+                return log_error_errno(errno, "Failed to move %s to /: %m", new_root);
+
+        if (chroot(".") < 0)
+                return log_error_errno(errno, "Failed to change root: %m");
+
+        if (chdir("/") < 0)
+                return log_error_errno(errno, "Failed to change directory: %m");
+
+        if (old_root_fd >= 0) {
+                struct stat rb;
+
+                if (fstat(old_root_fd, &rb) < 0)
+                        log_warning_errno(errno, "Failed to stat old root directory, leaving: %m");
+                else
+                        (void) rm_rf_children(TAKE_FD(old_root_fd), 0, &rb); /* takes possession of the dir fd, even on failure */
+        }
+
+        return 0;
+}