[CRIU] Dealing with other mount types

[Tycho Andersen]

Hi all,

[As a preface, I don't understand all the issues at play here, so any
input or corrections are very much welcome.]

Recent changes in Ubuntu and LXC mean that c/r of LXC containers no longer
works out of the box, so I'd like to fix that. The first step is to fix some of
the mount handling. When I start a container on Vivid with LXC 1.1, I get a
mountinfo that looks like:

44 45 253:1 /usr/local/var/lib/lxc/u1/rootfs / rw,relatime master:1 - ext4 /dev/disk/by-uuid/6c5a78e0-95fa-49a8-aa91-a8093d295e58 rw,data=ordered
78 44 0:36 / /dev rw,relatime - tmpfs none rw,size=100k,mode=755
79 44 0:38 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
80 81 0:38 /sys/net /proc/sys/net rw,nosuid,nodev,noexec,relatime - proc proc rw
81 79 0:38 /sys /proc/sys ro,nosuid,nodev,noexec,relatime - proc proc rw
82 79 0:38 /sysrq-trigger /proc/sysrq-trigger ro,nosuid,nodev,noexec,relatime - proc proc rw
83 44 0:39 / /sys rw,nosuid,nodev,noexec,relatime - sysfs sysfs rw
84 83 0:39 / /sys ro,nosuid,nodev,noexec,relatime - sysfs sysfs rw
85 84 0:39 / /sys/devices/virtual/net rw,relatime - sysfs sysfs rw
86 85 0:39 /devices/virtual/net /sys/devices/virtual/net rw,nosuid,nodev,noexec,relatime - sysfs sysfs rw
87 84 0:34 / /sys/fs/fuse/connections rw,relatime master:23 - fusectl fusectl rw
88 84 0:7 / /sys/kernel/debug rw,relatime master:25 - debugfs debugfs rw
89 84 0:11 / /sys/kernel/security rw,nosuid,nodev,noexec,relatime master:8 - securityfs securityfs rw
90 84 0:23 / /sys/fs/pstore rw,nosuid,nodev,noexec,relatime master:11 - pstore pstore rw
91 84 0:40 / /sys/fs/cgroup rw,relatime - tmpfs cgroup rw,size=12k,mode=755
92 91 0:21 /cgmanager /sys/fs/cgroup/cgmanager rw - tmpfs tmpfs rw,mode=755
46 78 0:41 / /dev/pts rw,relatime - devpts devpts rw,gid=5,mode=620,ptmxmode=666
47 44 0:42 / /run rw,nosuid,noexec,relatime - tmpfs none rw,size=199952k,mode=755
48 47 0:43 / /run/lock rw,nosuid,nodev,noexec,relatime - tmpfs none rw,size=5120k
49 47 0:44 / /run/shm rw,nosuid,nodev,relatime - tmpfs none rw
50 47 0:45 / /run/user rw,nosuid,nodev,noexec,relatime - tmpfs none rw,size=102400k,mode=755

First, several things (the rootfs, fuse, pstore, etc.) are mounted as slaves.
My understanding is that this happens because systemd remounts / as MS_SHARED
instead of MS_PRIVATE, but it means that we need some way of handling slave
mounts. One thought is to have an argument similar to --ext-mount-map which
tells criu which peer group a particular mount is a slave to. For e.g. pstore
above, this would look like:

1. criu ... --slave-mount-map=/sys/fs/pstore:/sys/fs/pstore # source:target
2. criu walks the mount tree as usual, and when it sees something in
   --slave-mount-map:
    1. criu bind mounts /sys/fs/pstore into $root_yard/sys/fs/pstore
    2. criu sets MS_SLAVE (by calling restore_shared_options())

Second, for e.g. /proc/sys, the root of the mount is a path that's relative to
it's parent's mountpoint. I think (?) this just means that mount.c's
find_fsroot_mount_for() needs to be a little smarter when it resolves things,
so it should return /'s mountinfo when called for /proc/sys, instead of
complaining about a proper root mount. Is there something else here that I'm
missing?

Tycho