RFC: Do we still need COPY_AS_IS+=( /dev ... ) ?

[jsmeix]

• ReaR version ("/usr/sbin/rear -V"):
  current master code

• Description of the issue (ideally so that others can reproduce it):

See
#3027 (comment)

We copy by default all in /dev/ into the recovery system
because in current master code there is

COPY_AS_IS+=( /dev ...

in usr/share/rear/conf/GNU/Linux.conf
https://github.com/rear/rear/blob/master/usr/share/rear/conf/GNU/Linux.conf#L231
without a comment why we copy all in /dev/
so I guess it was obvious why that is needed
at the time when it was implemented.

It is there since the beginning (at least since Mar 15 2009)
according to

# git log --follow -p usr/share/rear/conf/GNU/Linux.conf

so I guess copying all in /dev/ into the recovery system
is from older times where things in /dev/ had been static.

Meanwhile /dev/ is a mountpoint in the running recovery system:

RESCUE localhost:~ # mountpoint /dev
/dev is a mountpoint

RESCUE localhost:~ # findmnt  
TARGET                                SOURCE             FSTYPE     OPTIONS
/                                     none               rootfs     rw
|-/proc                               proc               proc       rw,nosuid,nodev,noexec,relatime
| `-/proc                             none               proc       rw,relatime
|-/sys                                sysfs              sysfs      rw,nosuid,nodev,noexec,relatime
| |-/sys/kernel/security              securityfs         securityfs rw,nosuid,nodev,noexec,relatime
| |-/sys/fs/cgroup                    tmpfs              tmpfs      ro,nosuid,nodev,noexec,size=4096k,nr_inodes=1024,mode=755,inode64
| | |-/sys/fs/cgroup/unified          cgroup2            cgroup2    rw,nosuid,nodev,noexec,relatime,nsdelegate
| | |-/sys/fs/cgroup/systemd          cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,xattr,name=systemd
| | |-/sys/fs/cgroup/pids             cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,pids
| | |-/sys/fs/cgroup/cpu,cpuacct      cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,cpu,cpuacct
| | |-/sys/fs/cgroup/hugetlb          cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,hugetlb
| | |-/sys/fs/cgroup/rdma             cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,rdma
| | |-/sys/fs/cgroup/blkio            cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,blkio
| | |-/sys/fs/cgroup/cpuset           cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,cpuset
| | |-/sys/fs/cgroup/devices          cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,devices
| | |-/sys/fs/cgroup/freezer          cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,freezer
| | |-/sys/fs/cgroup/memory           cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,memory
| | |-/sys/fs/cgroup/net_cls,net_prio cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,net_cls,net_prio
| | |-/sys/fs/cgroup/misc             cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,misc
| | `-/sys/fs/cgroup/perf_event       cgroup             cgroup     rw,nosuid,nodev,noexec,relatime,perf_event
| |-/sys/fs/pstore                    pstore             pstore     rw,nosuid,nodev,noexec,relatime
| |-/sys/firmware/efi/efivars         efivarfs           efivarfs   rw,nosuid,nodev,noexec,relatime
| `-/sys/fs/bpf                       none               bpf        rw,nosuid,nodev,noexec,relatime,mode=700
|-/dev                                devtmpfs           devtmpfs   rw,nosuid,size=4096k,nr_inodes=1048576,mode=755,inode64
| |-/dev/shm                          tmpfs              tmpfs      rw,nosuid,nodev,inode64
| `-/dev/pts                          devpts             devpts     rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000
|   `-/dev/pts                        none               devpts     rw,relatime,gid=5,mode=620,ptmxmode=000
|-/run                                tmpfs              tmpfs      rw,nosuid,nodev,size=402384k,nr_inodes=819200,mode=755,inode64
`-/etc/machine-id                     tmpfs[/machine-id] tmpfs      ro,size=402384k,nr_inodes=819200,mode=755,inode64

So what there was copied from the original system in /dev/
into the recovery system is hidden behind what is now
mounted at /dev/ in the running recovery system.

[pcahyna]

Is it related to #2808 / #2798 ?

[jsmeix]

For comparison:

On the original system:

# find /dev | wc -l
464

# find /var/tmp/rear.NCVRvE31DCUFZIa/rootfs/dev | wc -l
445

# find /dev/mqueue -ls
 10194  0 drwxrwxrwt  2 root  root       40 Jul 19 16:50 /dev/mqueue

# find /var/tmp/rear.NCVRvE31DCUFZIa/rootfs/dev/mqueue -ls
271020  4 drwxrwxrwt  2 root  root     4096 Jul 19 16:50 /var/tmp/rear.NCVRvE31DCUFZIa/rootfs/dev/mqueue

# find /dev/watchdog -ls
   385  0 crw-------  1 root  root  10, 130 Jul 19 16:50 /dev/watchdog

# find /var/tmp/rear.NCVRvE31DCUFZIa/rootfs/dev/watchdog -ls
find: ‘/var/tmp/rear.NCVRvE31DCUFZIa/rootfs/dev/watchdog’: No such file or directory

In the running recovery system:

RESCUE localhost:~ # find /dev | wc -l
398

RESCUE localhost:~ # find /dev/mqueue -ls
find: '/dev/mqueue': No such file or directory

RESCUE localhost:~ # find /dev/watchdog -ls
289  0 crw-------  1 root  root  10, 130 Jul 19 16:40 /dev/watchdog

Regarding '/dev/mqueue' see
#3027

[jsmeix]

@pcahyna
see my above
#3028 (comment)
I have /dev/watchdog in the running recovery system
regardless that it was not copied into the recovery system.

Nevertheless #2808
is right to not have /dev/watchdog in ReaR's build area.

With this issue here a generic solution could be
to not have anything from /dev in ReaR's build area.

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[pcahyna]

FYI: in RHEL 6 /dev is already a mountpoint (devtmpfs). Even in RHEL 5 (which we do not care about anymore), it is a mountpoint (tmpfs). @jsmeix I suggest that you check the distributions that you care about.

[pcahyna]

In skel/default/etc/scripts/boot we have this:

    udev_version=$(udevd --version)
    if [[ "$udev_version" -gt 175 ]]; then
        ### udev > 175 needs devtmpfs
        mountpoint /dev || mount -t devtmpfs none /dev
    fi

so, for older udev versions than 176 we won't mount /dev in the recovery ssytem, despite being a mountpoint in the original system. I suppose it is not a problem though, if older udev can populate an empty /dev in the rescue ramdisk without it being a mountpoint.
The bigger issue is, is udev even started on systems that don't use systemd yet? (RHEL 6 is not.) I don't see udev being executed anywhere in the recovery system startup scripts.

[pcahyna]

On RHEL 7 there is no udevd anymore:

# udevd --version
bash: udevd: command not found

so, how does /dev get mounted in the recovery system, if at all, is a mystery for me.

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