Init: then and now

A story of starting systems up

Transcript

1. Init: then and now
   A story of starting systems up
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2. Grüß Gott
   Pierre-Yves Ritschard
   CTO & Co-founder at
   Unix since 96
   Open-Source developer at OpenBSD, Riemann, Collectd, and
   more
   @pyr
   Exoscale
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3. Exoscale
   Infrastructure as a service
   Part of A1 Digital
   Zones in Frankfurt, Vienna, Zürich,
   Geneva
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4. How do systems start?
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5. The rmware
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6. The rmware
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7. The rmware
   Exposes BUS access
   Rudimentary way to communicate list devices
   Looks for a bootloader in on devices
   Usually very basic environment (sometimes 16-
   bit)
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8. The bootloader
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9. The bootloader
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10. The bootloader
    An intermediate mini-OS
    Rudimentary le-system support
    Prepares environment to start
    kernel
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11. The kernel
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12. The kernel
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13. The kernel
    Discovers devices on Buses
    Prepares environment to run process(es) using a common
    format
    Exposes standard facilities for programs:
    Abstracted I/O: open(2)
    Memory management: mmap(2), malloc(3)
    Process handling: fork(2), signal(2)
    Network abstractions: socket(2), bind(2), connect(2)
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14. Init
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15. Init
    First process started by the kernel
    Runs boot sequence
    Mounts lesystems
    Starts the network
    Starts essential subsystems: syslog, ntpd,
    cron
    Starts background daemons: ssh, smtpd
    Root of process tree
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16. Multi-User mode
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17. Multi-User mode
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18. Multi-User mode
    All facilities up and
    running
    Ready to run user
    programs
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19. Our focus today
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20. A small detour: the process tree
    There is a process hierarchy in Unix
    The kernel runs only one user
    process
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21. A small detour: the process tree
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22. A small detour: the process tree
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23. Init duties
    Mount lesystems, start
    everything
    Stay around to watch over
    hierarchy
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24. Init resiliency
    What happens when init crashes?
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25. Init resiliency
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26. Init simpli ed
    How would you?
    Mount lesystems
    Start the network
    Start essential subsystems: syslog, ntpd,
    cron
    Start background daemons: ssh, smtpd
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27. Standard Init
    swapon -a
    umount -a >/dev/null 2>&1
    mount -a -t nonfs
    . /etc/rc.conf
    sh /etc/netstart
    if [ X${rwhod} = X"YES" ]; then
    echo -n ' rwhod'; rwhod
    fi
    if [ X${lpd} = X"YES" ]; then
    echo -n ' printer'; lpd
    fi
    . /etc/rc.local
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28. Standard Init
    #define _PATH_BSHELL "/bin/sh"
    #define _PATH_RUNCOM "/etc/rc"
    /* ... */
    execv(_PATH_BSHELL, argv);
    /* ... */
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29. Init ow
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30. Are we there?
    How does the system stop?
    How do I restart or stop
    services?
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31. Stopping the system
    /etc/rc.shutdown
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32. Stopping or restarting services
    Stop: pkill
    Restart: pkill -HUP
    
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33. This isn't ideal
    It's hard to keep track of startup order
    Plenty of services didn't react well to standard
    signals
    No way to easily gather service status
    No way to ensure a critical service stays up
    This puts a lot on application packagers
    Especially on non-standardized systems
    Hello !
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34. Case in point: daemons
    int
    daemon(void)
    {
    switch (fork()) {
    case -1:
    return (-1);
    case 0:
    break;
    default:
    _exit(0);
    }
    if (setsid() == -1)
    return (-1);
    (void)chdir("/");
    (void)close(STDIN_FILENO);
    (void)close(STDOUT_FILENO);
    (void)close(STDERR_FILENO);
    return (0);
    }
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35. Case in point: daemons
    How do I know the child's PID?
    How do I keep track of service
    availability
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36. Common things applications do
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37. More things applications must do
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38. A better world?
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39. The init landscape
    System
    V
    Upstart
    SystemD
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40. System V
    You all know it
    /etc/init.d/rc5.d/S99blargh
    Introduces runlevels
    Enforces the concept of service startup, status, and
    shutdown.
    inittab(5) to de ne runlevels as state transitions
    Still leaves much to be desired
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41. Ubuntu Upstart
    Simple DSL
    Event based start on (net-device-up and local-
    filesystems)
    Support for environment, logging, and PID tracking
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42. Ubuntu Upstart
    description "Warp agent"
    start on runlevel [2345]
    stop on runlevel [!2345]
    respawn
    respawn limit 5 60
    limit nofile 8192 8192
    pre-start script
    [ -x "/usr/sbin/warp-agent" ] || exit 0
    [ -r "/etc/warp/agent.json" ] || exit 0
    end script
    exec /usr/sbin/warp-agent /etc/warp/agent.json
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43. Ubuntu Upstart
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44. Ubuntu Upstart Drawbacks
    Very brittle job supervision
    Left the process in un xable states on a regular
    basis
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45. SystemD
    DSL based
    Builds a dependency graph
    Support for environment, logging, PID tracking
    Opt-in support for resource constraints,
    rewalling
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46. SystemD
    [Unit]
    Description="Warp agent"
    ConditionPathExists=/usr/sbin/warp-agent
    ConditionPathExists=/etc/warp/agent.json
    [Service]
    ExecStart=/usr/sbin/warp-agent /etc/warp/agent.json
    LimitNOFILE=8192
    [Install]
    WantedBy=multi-user.target
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47. SystemD
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48. SystemD drawbacks
    Hard to avoid heated discussions
    Wide scope means more room for error
    Mixed focus on desktop & servers can be off-puting for
    admins
    Heavily coupled to DBUS
    That giant, little-known attack vector on your system
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49. Shepperd
    (define nginx
    (make
    #:provides '(nginx web-server)
    #:start (make-forkexec-constructor
    (list "nginx"))
    #:stop (make-kill-destructor)))
    (register-services nginx)
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50. Plenty more
    Daemontools
    SMF
    OpenRC
    BSD rc.d
    runit
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51. Plenty more
    http://blog.darknedgy.net/technology/2015/09/05/0/
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52. Thanks!
    Questions?
    We're
    hiring!
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