This information is based upon v21.02, last updated for commit 6d266ef158 on 2022-02-10.

Sysupgrade – Technical Reference

In contrast to opkg, mtd and others, sysupgrade is merely a shell script: /sbin/sysupgrade intended to facilitate easy updates.

This page lists all sysupgrade command-line options. For the overall upgrade procedure and typical usage, please read OpenWrt OS upgrade procedure (sysupgrade or LuCI) instead.

sysupgrade supports the following options

Usage: /sbin/sysupgrade [<upgrade-option>...] <image file or URL>
       /sbin/sysupgrade [-q] [-i] [-c] [-u] [-o] [-k] <backup-command> <file>

        -f <config>  restore configuration from .tar.gz (file or url)
        -i           interactive mode
        -c           attempt to preserve all changed files in /etc/
        -o           attempt to preserve all changed files in /, except those
                     from packages but including changed confs.
        -u           skip from backup files that are equal to those in /rom
        -n           do not save configuration over reflash
        -p           do not attempt to restore the partition table after flash.
        -k           include in backup a list of current installed packages at
        -T | --test
                     Verify image and config .tar.gz but do not actually flash.
        -F | --force
                     Flash image even if image checks fail, this is dangerous!
        -q           less verbose
        -v           more verbose
        -h | --help  display this help

        -b | --create-backup <file>
                     create .tar.gz of files specified in sysupgrade.conf
                     then exit. Does not flash an image. If file is '-',
                     i.e. stdout, verbosity is set to 0 (i.e. quiet).
        -r | --restore-backup <file>
                     restore a .tar.gz created with sysupgrade -b
                     then exit. Does not flash an image. If file is '-',
                     the archive is read from stdin.
        -l | --list-backup
                     list the files that would be backed up when calling
                     sysupgrade -b. Does not create a backup file.

WARNING: Preserving files across sysupgrades can be fatal (see 'NOTE: …') on systems with weak cpu and exceptionally large rootfs_data partitions.

Files to be preserved depend on the following:

  • /etc/sysupgrade.conf - customizable backup configuration.
  • /lib/upgrade/keep.d/* - system configurations provided by specific packages preserved by default.
  • opkg list-changed-conffiles - list of files derived by package manager.
  • -o will cause the entire /overlay directory to be saved (with the -u caveat below).
  • -n will cause NO files will be saved and all configuration settings will be initialized from default values.
  • -u will prevent preservation of any file that has not been changed since the last sysupgrade. This prevents the need for programs to migrate an old configuration and reduces time needed for sysupgrade.
  • -f will COMPLETELY OVERRIDE all behaviour described above. Instead, the exact files provided in the .tar.gz file will be extracted into /overlay/upper after the sysupgrade.

Q: Does this mean, I make an archive.tar.gz of /etc and /root for example and sysupgrade -f archive.tar.gz will flash the router and afterwards restores the configs from this archive?

A: That's what is says: 'restore configuration from .tar.gz (file or url)'. Anything archived in the tgz will be written to /overlay after the flash. This way you can hand-pick the files that will be the system after new firmware boot.

The sysupgrade process starts with the execution of /sbin/sysupgrade. The below list describes its behavior.

  1. Parse command line and validate no mutually exclusive options passed.
  2. At this point, sysupgrade calls include /lib/upgrade – a function in /lib/ that will source all *.sh files in the given directory.
    1. NOTE: An optional, platform-specific /lib/upgrade/ can override behavior, so for a full understanding, you should examine this file. (See target/linux/<arch>/<sub-arch>/base-files/lib/upgrade/ in your source tree.)
    2. The optional functions platform_copy_config and platform_do_upgrade are at the end of this list.
  3. Create list of files to preserve and store them in /tmp/sysupgrade.tgz (unless -f was supplied).
  4. If the image supplied is an http or https URL, wget is run to retrieve it.
  5. The firmware image is saved (if a URL) or copied to /tmp/sysupgrade.img.
  6. Validates the firmware image, and that the root compatible node in the device tree file matches the value in /sys/firmware/devicetree/base/compatible from the existing firmware's device tree. (May be overridden with -F).
  7. Copies /sbin/upgraded into /tmp
  8. Builds a json message and sends a message, via ubus, to procd, to initiate the upgrade. Among other things, the message specifies:
    1. path: /tmp/sysupgrade.img,
    2. backup: /tmp/sysupgrade.tgz if any files are being preserved, unset otherwise,
    3. force: if -F was supplied,
    4. and command: /lib/upgrade/do_stage2.
  9. The sysupgrade function in procd will unpack and validate the message, then validate the firmware image and such.
  10. Notably, procd does not terminate any services here.
  11. This is where things get funky! If all is well, we call sysupgrade_exec_upgraded, which further parses the original json and then passes control to /sbin/upgraded via execvp.
    1. Note that at boot time, the kernel passes control to /sbin/init as PID=1, which in turn execs /sbin/procd. Thus, this results in /tmp/upgraded becoming the new PID=1 (“init”) process.
    2. At this point, service management is no longer possible.
  12. /sbin/upgraded executes the command passed (/lib/upgrade/stage2) with parameters. The remaining sequence runs from this shell script.
    1. /bin/sh /lib/upgrade/stage2 is run via fork/exec, so is not PID1.
  13. Terminate (SIGKILL) all telnet, ash, and dropbear processes.
  14. Loop over all remaining processes, sending them the TERM signal.
  15. Loop over all remaining processes, sending them the KILL signal.
  16. Create a new RAM filesystem, mount it, and copy over a very small set of binaries into it.
  17. Change root into the new RAM filesystem.
  18. Remount /overlay read-only, and lazy-unmount it.
  19. Write the upgraded firmware to disk. If platform_do_upgrade is defined then it is run. Otherwise, this is done via default_do_upgrade, using mtd to flash the firmware.
  20. If any files are being preserved, the tarball is passed via mtd's -j option. This causes mtd to write a raw jffs2 inode (with id = 1) and file data, resulting in the tarball to appearing as /sysupgrade.tgz once the jffs2 file system is mounted.
    1. A consequence is that the mtd sources are intimately tied to the jffs2 implementation and can break if changes are made to jffs2 in the kernel (it should probably be using the uapi header from the kernel make headers_install instead of copying it into its source tree).
    2. After reboot, this file will be extracted by /lib/preinit/80_mount_root (/sbin/init during preinit –> /bin/sh /etc/preinit –> /lib/preinit/80_mount_root)
    3. and finally deleted by /etc/rc.d/S95done (/sbin/procd –> at STATE_INIT –> procd_inittab_run("sysinit"); –> /etc/inittab)
  21. If a platform_copy_config is implemented, it is run at this point.
  22. Unmount any remaining filesystems.
  23. Reboot.

There are plenty of potential deficiencies in this process, among them:

  • Hardcodes a list of “potentially-interfering” / interactive processes (ash, telnet, dropbear) to kill first; this is not exhaustive or up-to-date (e.g., telnet is no longer in the base install; openssh is not handled).
  • Does not give processes much time in between TERM and KILL signals.
  • Does not utilise procd to tear down services.
  • Susceptible to fork bombs.
  • Easily broken by open files on storage devices (e.g., for swap, or explicitly opened), as these can prevent unmounting /overlay.
  • Does not handle errors (e.g., I/O) in writing the disk image, potentially leaving a partially-upgraded system upon reboot.
  • Various error conditions are ignored in mtd, which can also result in preserved files not being written to the new jffs2 file system or and leaving the file system corrupted, but without reporting an error.

Many of these deficiencies are historical artifacts, remaining simply because no one has fixed them.

Thanks to Michael Jones for writing most of this down on the mailing list [OpenWrt-Devel] Sysupgrade and Failed to kill all processes.

TODO: Explain how this process works during failsafe mode.

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  • Last modified: 2022/02/10 18:41
  • by dansan