diff options
Diffstat (limited to 'src/shared/switch-root.c')
| -rw-r--r-- | src/shared/switch-root.c | 212 |
1 files changed, 212 insertions, 0 deletions
diff --git a/src/shared/switch-root.c b/src/shared/switch-root.c new file mode 100644 index 0000000..b620156 --- /dev/null +++ b/src/shared/switch-root.c @@ -0,0 +1,212 @@ +/* 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 "chase.h" +#include "creds-util.h" +#include "fd-util.h" +#include "initrd-util.h" +#include "log.h" +#include "missing_syscall.h" +#include "mkdir-label.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" + +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; may be NULL to unmount it */ + SwitchRootFlags flags) { + + /* Stuff mounted below /run/ we don't save on soft reboot, as it might have lost its relevance, i.e. + * credentials, removable media and such, we rather want that the new boot mounts this fresh. But on + * the switch from initrd we do use MS_REC, as it is expected that mounts set up in /run/ are + * maintained. */ + static const struct { + const char *path; + unsigned long mount_flags; /* Flags to apply if SWITCH_ROOT_RECURSIVE_RUN is unset */ + unsigned long mount_flags_recursive_run; /* Flags to apply if SWITCH_ROOT_RECURSIVE_RUN is set (0 if shall be skipped) */ + } transfer_table[] = { + { "/dev", MS_BIND|MS_REC, MS_BIND|MS_REC }, /* Recursive, because we want to save the original /dev/shm/ + /dev/pts/ and similar */ + { "/sys", MS_BIND|MS_REC, MS_BIND|MS_REC }, /* Similar, we want to retain various API VFS, or the cgroupv1 /sys/fs/cgroup/ tree */ + { "/proc", MS_BIND|MS_REC, MS_BIND|MS_REC }, /* Similar */ + { "/run", MS_BIND, MS_BIND|MS_REC }, /* Recursive except on soft reboot, see above */ + { SYSTEM_CREDENTIALS_DIRECTORY, MS_BIND, 0 /* skip! */ }, /* Credentials passed into the system should survive */ + { ENCRYPTED_SYSTEM_CREDENTIALS_DIRECTORY, MS_BIND, 0 /* skip! */ }, /* Similar */ + { "/run/host", MS_BIND|MS_REC, 0 /* skip! */ }, /* Host supplied hierarchy should also survive */ + }; + + _cleanup_close_ int old_root_fd = -EBADF, new_root_fd = -EBADF; + _cleanup_free_ char *resolved_old_root_after = NULL; + int r, istmp; + + assert(new_root); + + /* Check if we shall remove the contents of the old root */ + old_root_fd = open("/", O_DIRECTORY|O_CLOEXEC); + if (old_root_fd < 0) + return log_error_errno(errno, "Failed to open root directory: %m"); + + new_root_fd = open(new_root, O_DIRECTORY|O_CLOEXEC); + if (new_root_fd < 0) + return log_error_errno(errno, "Failed to open target directory '%s': %m", new_root); + + r = inode_same_at(old_root_fd, "", new_root_fd, "", AT_EMPTY_PATH); + if (r < 0) + return log_error_errno(r, "Failed to determine if old and new root directory are the same: %m"); + if (r > 0) { + log_debug("Skipping switch root, as old and new root directory are the same."); + return 0; + } + + /* Make the new root directory a mount point if it isn't */ + r = fd_make_mount_point(new_root_fd); + if (r < 0) + return log_error_errno(r, "Failed to make new root directory a mount point: %m"); + if (r > 0) { + int fd; + + /* When the path was not a mount point, then we need to reopen the path, otherwise, it still + * points to the underlying directory. */ + + fd = open(new_root, O_DIRECTORY|O_CLOEXEC); + if (fd < 0) + return log_error_errno(errno, "Failed to reopen target directory '%s': %m", new_root); + + close_and_replace(new_root_fd, fd); + } + + if (FLAGS_SET(flags, SWITCH_ROOT_DESTROY_OLD_ROOT)) { + istmp = fd_is_temporary_fs(old_root_fd); + if (istmp < 0) + return log_error_errno(istmp, "Failed to stat root directory: %m"); + if (istmp > 0) + log_debug("Root directory is on tmpfs, will do cleanup later."); + } else + istmp = -1; /* don't know */ + + if (old_root_after) { + /* Determine where we shall place the old root after the transition */ + r = chase(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); + } + + /* We are about to unmount various file systems with MNT_DETACH (either explicitly via umount() or + * indirectly via pivot_root()), and thus do not synchronously wait for them to be fully sync'ed — + * all while making them invisible/inaccessible in the file system tree for later code. That makes + * sync'ing them then difficult. Let's hence issue a manual sync() here, so that we at least can + * guarantee all file systems are an a good state before entering this state. */ + if (!FLAGS_SET(flags, SWITCH_ROOT_DONT_SYNC)) + sync(); + + /* 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"); + + /* 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_fd(new_root_fd, new_root, UID_INVALID, GID_INVALID); + + FOREACH_ARRAY(transfer, transfer_table, ELEMENTSOF(transfer_table)) { + _cleanup_free_ char *chased = NULL; + unsigned long mount_flags; + + mount_flags = FLAGS_SET(flags, SWITCH_ROOT_RECURSIVE_RUN) ? transfer->mount_flags_recursive_run : transfer->mount_flags; + if (mount_flags == 0) /* skip if zero */ + continue; + + if (access(transfer->path, F_OK) < 0) { + log_debug_errno(errno, "Path '%s' to move to target root directory, not found, ignoring: %m", transfer->path); + continue; + } + + r = chase(transfer->path, new_root, CHASE_PREFIX_ROOT, &chased, NULL); + if (r < 0) + return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, transfer->path); + + /* 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; + + r = mount_nofollow_verbose(LOG_ERR, transfer->path, chased, NULL, mount_flags, NULL); + if (r < 0) + return r; + } + + if (fchdir(new_root_fd) < 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 (resolved_old_root_after) + r = RET_NERRNO(pivot_root(".", resolved_old_root_after)); + else { + r = RET_NERRNO(pivot_root(".", ".")); + if (r >= 0) { + /* Now unmount the upper of the two stacked file systems */ + if (umount2(".", MNT_DETACH) < 0) + return log_error_errno(errno, "Failed to unmount the old root: %m"); + } + } + if (r < 0) { + log_debug_errno(r, "Pivoting root file system failed, moving mounts instead: %m"); + + if (resolved_old_root_after) { + r = mount_nofollow_verbose(LOG_ERR, "/", resolved_old_root_after, NULL, MS_BIND|MS_REC, NULL); + if (r < 0) + return r; + } + + /* If we have to use MS_MOVE let's first try to get rid of *all* mounts we can, with the + * exception of the path we want to switch to, plus everything leading to it and within + * it. This is necessary because unlike pivot_root() just moving the mount to the root via + * MS_MOVE won't magically unmount anything below it. Once the chroot() succeeds the mounts + * below would still be around but invisible to us, because not accessible via + * /proc/self/mountinfo. Hence, let's clean everything up first, as long as we still can. */ + (void) umount_recursive_full(NULL, MNT_DETACH, STRV_MAKE(new_root)); + + if (mount(".", "/", 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 (istmp > 0) { + struct stat rb; + + if (fstat(old_root_fd, &rb) < 0) + return log_error_errno(errno, "Failed to stat old root directory: %m"); + + /* Note: the below won't operate on non-memory file systems (i.e. only on tmpfs, ramfs), and + * it will stop at mount boundaries */ + (void) rm_rf_children(TAKE_FD(old_root_fd), 0, &rb); /* takes possession of the dir fd, even on failure */ + } + + return 0; +} |