core: when recursively bind-remounting nested mounts, use options from top one

core: when recursively bind-remounting nested mounts, use options from top one

When mount points are stacked, bind_remount_recursive_with_mountinfo()
uses the existing mount options of the "lower" level mount (ie: the
first one that was mounted on a mount point). But the actual mount
point in use is the "top" one (ie: the last one that was mounted on a
mount point), so in practice if the mount options are different between
the layers, the bottom options are used by mistake on the top mount,
which is not what we want. This is because libmount returns the "bottom"
one first.

If the hashmap returns EEXIST, which means the same key (path) with different
value (options) is already present, update the hashmap instead of discarding
the result. This way, the last/top mount options are always used when
mounts are stacked on a mount point.

This was found to cause problems as LXC version 4.x stacks two /sys mounts,
the bottom one read-write and the top one read-only. systemd accidentally
remounts the top-one read-write, breaking various expectations since a
read-only /sys is the way we decide whether we are running in a container
or not (in this particular case, networkd tests are broken as networkd
expects to be able to modify network settings with a writable /sys).

Future versions of LXC will no longer do this double-stacking, but we
need to support running inside older versions too.

This was triggered by https://github.com/systemd/systemd/commit/6720e356c137
as that causes a recursive remount of '/', which processes '/sys' as one
of the submounts, from make_nosuid(). But it's likely that other combinations
of options could trigger this as well.

Before:

root@systemd-debug:/# systemd-run -t --wait --property ProtectSystem=yes findmnt
Running as unit: run-u9.service
Press ^] three times within 1s to disconnect TTY.
TARGET                         SOURCE                           FSTYPE    OPTIONS
/                              /dev/sda2[/var/lib/lxc/systemd-debug/rootfs]
│                                                               ext4      ro,nosuid,relatime,errors=remount-ro,stripe=
├─/dev                         none                             tmpfs     rw,nosuid,relatime,size=492k,mode=755
│ ├─/dev/.lxc/proc             proc                             proc      rw,nosuid,relatime
│ ├─/dev/.lxc/sys              sys                              sysfs     rw,nosuid,relatime
│ ├─/dev/console               devpts[/2]                       devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptm
│ ├─/dev/pts                   devpts                           devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptm
│ ├─/dev/ptmx                  devpts[/ptmx]                    devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptm
│ ├─/dev/tty1                  devpts[/0]                       devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptm
│ ├─/dev/tty2                  devpts[/1]                       devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptm
│ ├─/dev/tty3                  devpts[/2]                       devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptm
│ ├─/dev/tty4                  devpts[/3]                       devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptm
│ ├─/dev/shm                   tmpfs                            tmpfs     rw,nosuid,nodev
│ ├─/dev/hugepages             hugetlbfs                        hugetlbfs rw,nosuid,relatime,pagesize=2M
│ └─/dev/mqueue                mqueue                           mqueue    rw,nosuid,nodev,noexec,relatime
├─/proc                        proc                             proc      rw,nosuid,nodev,noexec,relatime
│ ├─/proc/sys                  proc[/sys]                       proc      ro,nosuid,nodev,noexec,relatime
│ │ ├─/proc/sys/net            proc[/sys/net]                   proc      rw,nosuid,nodev,noexec,relatime
│ │ └─/proc/sys/kernel/random/boot_id
│ │                            none[/.lxc-boot-id]              tmpfs     ro,nosuid,nodev,noexec,relatime,size=492k,mo
│ └─/proc/sysrq-trigger        proc[/sysrq-trigger]             proc      ro,nosuid,nodev,noexec,relatime
├─/sys                         sysfs                            sysfs     rw,nosuid,nodev,noexec,relatime
│ └─/sys                       sysfs                            sysfs     rw,nosuid,nodev,noexec,relatime
│   ├─/sys/devices/virtual/net sysfs                            sysfs     rw,relatime
│   │ └─/sys/devices/virtual/net
│   │                          sysfs[/devices/virtual/net]      sysfs     rw,nosuid,relatime
│   ├─/sys/fs/fuse/connections fusectl                          fusectl   rw,nosuid,nodev,noexec,relatime
│   └─/sys/fs/cgroup           cgroup                           cgroup2   rw,nosuid,nodev,noexec,relatime,nsdelegate,m
├─/run                         tmpfs                            tmpfs     ro,nosuid,nodev,size=4912348k,nr_inodes=8192
│ ├─/run/credentials           tmpfs[/systemd/inaccessible/dir] tmpfs     ro,nosuid,nodev,noexec,size=4912348k,nr_inod
│ └─/run/systemd/incoming      tmpfs[/systemd/propagate/run-u9.service]
│                                                               tmpfs     ro,nosuid,nodev,size=4912348k,nr_inodes=8192
├─/tmp                         tmpfs                            tmpfs     rw,nosuid,nodev,size=12280872k,nr_inodes=409
│ └─/tmp                       tmpfs[/systemd-private-b730df90da424397a3f246cb15dcdbb1-run-u9.service-K6EUwf/tmp]
│                                                               tmpfs     rw,nosuid,nodev,size=12280872k,nr_inodes=409
└─/var/tmp                     /dev/sda2[/var/lib/lxc/systemd-debug/rootfs/var/tmp/systemd-private-b730df90da424397a3f246cb15dcdbb1-run-u9.service-vEHyRi/tmp]
                                                                ext4      rw,nosuid,relatime,errors=remount-ro,stripe=
Finished with result: success
Main processes terminated with: code=exited/status=0
Service runtime: 14.249s
CPU time consumed: 37ms

After:

root@systemd-debug:/# systemd-run -t --wait --property ProtectSystem=yes findmnt
Running as unit: run-u3.service
Press ^] three times within 1s to disconnect TTY.
TARGET                         SOURCE                      FSTYPE    OPTIONS
/                              /dev/sda2[/var/lib/lxc/systemd-debug/rootfs]
│                                                          ext4      rw,relatime,errors=remount-ro,stripe=32699
├─/dev                         none                        tmpfs     rw,relatime,size=492k,mode=755
│ ├─/dev/.lxc/proc             proc                        proc      rw,relatime
│ ├─/dev/.lxc/sys              sys                         sysfs     rw,relatime
│ ├─/dev/console               devpts[/2]                  devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode
│ ├─/dev/pts                   devpts                      devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode
│ ├─/dev/ptmx                  devpts[/ptmx]               devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode
│ ├─/dev/tty1                  devpts[/0]                  devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode
│ ├─/dev/tty2                  devpts[/1]                  devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode
│ ├─/dev/tty3                  devpts[/2]                  devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode
│ ├─/dev/tty4                  devpts[/3]                  devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode
│ ├─/dev/shm                   tmpfs                       tmpfs     rw,nosuid,nodev
│ ├─/dev/hugepages             hugetlbfs                   hugetlbfs rw,relatime,pagesize=2M
│ └─/dev/mqueue                mqueue                      mqueue    rw,nosuid,nodev,noexec,relatime
├─/proc                        proc                        proc      rw,nosuid,nodev,noexec,relatime
│ ├─/proc/sys                  proc[/sys]                  proc      ro,nosuid,nodev,noexec,relatime
│ │ ├─/proc/sys/net            proc[/sys/net]              proc      rw,nosuid,nodev,noexec,relatime
│ │ └─/proc/sys/kernel/random/boot_id
│ │                            none[/.lxc-boot-id]         tmpfs     ro,nosuid,nodev,noexec,relatime,size=492k,mode=75
│ └─/proc/sysrq-trigger        proc[/sysrq-trigger]        proc      ro,nosuid,nodev,noexec,relatime
├─/sys                         sysfs                       sysfs     rw,nosuid,nodev,noexec,relatime
│ └─/sys                       sysfs                       sysfs     ro,nosuid,nodev,noexec,relatime
│   ├─/sys/devices/virtual/net sysfs                       sysfs     rw,relatime
│   │ └─/sys/devices/virtual/net
│   │                          sysfs[/devices/virtual/net] sysfs     rw,nosuid,nodev,noexec,relatime
│   ├─/sys/fs/fuse/connections fusectl                     fusectl   rw,nosuid,nodev,noexec,relatime
│   └─/sys/fs/cgroup           cgroup                      cgroup2   rw,nosuid,nodev,noexec,relatime,nsdelegate,memory
├─/run                         tmpfs                       tmpfs     rw,nosuid,nodev,size=4912348k,nr_inodes=819200,mo
│ ├─/run/credentials           tmpfs[/systemd/inaccessible/dir]
│ │                                                        tmpfs     ro,nosuid,nodev,noexec,size=4912348k,nr_inodes=81
│ └─/run/systemd/incoming      tmpfs[/systemd/propagate/run-u3.service]
│                                                          tmpfs     ro,nosuid,nodev,size=4912348k,nr_inodes=819200,mo
├─/tmp                         tmpfs                       tmpfs     rw,nosuid,nodev,size=12280872k,nr_inodes=409600
├─/boot                        /dev/sda2[/var/lib/lxc/systemd-debug/rootfs/boot]
│                                                          ext4      ro,relatime,errors=remount-ro,stripe=32699
└─/usr                         /dev/sda2[/var/lib/lxc/systemd-debug/rootfs/usr]
                                                           ext4      ro,relatime,errors=remount-ro,stripe=32699
Finished with result: success
Main processes terminated with: code=exited/status=0
Service runtime: 14ms
CPU time consumed: 5ms

Host (LXC):

root@systemd-debug:/# findmnt
TARGET                         SOURCE               FSTYPE    OPTIONS
/                              /dev/sda2[/var/lib/lxc/systemd-debug/rootfs]
│                                                   ext4      rw,relatime,errors=remount-ro,stripe=32699
├─/run                         tmpfs                tmpfs     rw,nosuid,nodev,size=4912348k,nr_inodes=819200,mode=755
├─/tmp                         tmpfs                tmpfs     rw,nosuid,nodev,size=12280872k,nr_inodes=409600
├─/dev                         none                 tmpfs     rw,relatime,size=492k,mode=755
│ ├─/dev/pts                   devpts               devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666,ma
│ ├─/dev/ptmx                  devpts[/ptmx]        devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666,ma
│ ├─/dev/tty1                  devpts[/0]           devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666,ma
│ ├─/dev/tty2                  devpts[/1]           devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666,ma
│ ├─/dev/tty3                  devpts[/2]           devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666,ma
│ ├─/dev/tty4                  devpts[/3]           devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666,ma
│ ├─/dev/shm                   tmpfs                tmpfs     rw,nosuid,nodev
│ ├─/dev/hugepages             hugetlbfs            hugetlbfs rw,relatime,pagesize=2M
│ ├─/dev/mqueue                mqueue               mqueue    rw,nosuid,nodev,noexec,relatime
│ ├─/dev/console               devpts[/2]           devpts    rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000
│ ├─/dev/.lxc/proc             proc                 proc      rw,relatime
│ └─/dev/.lxc/sys              sys                  sysfs     rw,relatime
├─/proc                        proc                 proc      rw,nosuid,nodev,noexec,relatime
│ ├─/proc/sys                  proc[/sys]           proc      ro,nosuid,nodev,noexec,relatime
│ │ ├─/proc/sys/kernel/random/boot_id
│ │ │                          none[/.lxc-boot-id]  tmpfs     ro,nosuid,nodev,noexec,relatime,size=492k,mode=755
│ │ └─/proc/sys/net            proc[/sys/net]       proc      rw,nosuid,nodev,noexec,relatime
│ └─/proc/sysrq-trigger        proc[/sysrq-trigger] proc      ro,nosuid,nodev,noexec,relatime
└─/sys                         sysfs                sysfs     rw,nosuid,nodev,noexec,relatime
  └─/sys                       sysfs                sysfs     ro,nosuid,nodev,noexec,relatime
    ├─/sys/devices/virtual/net sysfs                sysfs     rw,relatime
    │ └─/sys/devices/virtual/net
    │                          sysfs[/devices/virtual/net]
    │                                               sysfs     rw,nosuid,nodev,noexec,relatime
    ├─/sys/fs/fuse/connections fusectl              fusectl   rw,nosuid,nodev,noexec,relatime
    └─/sys/fs/cgroup           cgroup               cgroup2   rw,nosuid,nodev,noexec,relatime,nsdelegate,memory_recurs

Fixes https://github.com/systemd/systemd/issues/20032