最初的问题
升级到 Ubuntu 20.04 后，snapd 似乎出现了问题。如果我安装并执行任何 snap 包，都会出现以下错误：
$ snap --version
snap 2.44.3+20.04
snapd 2.44.3+20.04
series 16
ubuntu 20.04
kernel 5.4.0-29-generic
$ mailspring
fopen /sys/fs/cgroup/devices/snap.mailspring.mailspring/cgroup.procs failed: Permission denied
$ htop
cannot perform operation: mount --rbind /var/lib/dhcp /tmp/snap.rootfs_mVUnzd//var/lib/dhcp: Permission denied
有任何想法吗？
更新
$ snap debug sandbox-features
apparmor: kernel:caps kernel:dbus kernel:domain kernel:file kernel:mount kernel:namespaces kernel:network kernel:network_v8 kernel:policy kernel:ptrace kernel:query kernel:rlimit kernel:signal parser:unsafe policy:default support-level:full
confinement-options: classic devmode strict
dbus: mediated-bus-access
kmod: mediated-modprobe
mount: freezer-cgroup-v1 layouts mount-namespace per-snap-persistency per-snap-profiles per-snap-updates per-snap-user-profiles stale-base-invalidation
seccomp: bpf-actlog bpf-argument-filtering kernel:allow kernel:errno kernel:kill_process kernel:kill_thread kernel:log kernel:trace kernel:trap kernel:user_notif
udev: device-cgroup-v1 device-filtering tagging
$ SNAPD_DEBUG=1 SNAP_CONFINE_DEBUG=1 snap run htop
2020/05/18 12:39:32.533861 cmd_linux.go:94: DEBUG: snap (at "/snap/core/current") is older ("2.44.3") than distribution package ("2.44.3+20.04")
2020/05/18 12:39:32.547500 cmd_run.go:395: DEBUG: SELinux not enabled
DEBUG: umask reset, old umask was 02
DEBUG: security tag: snap.htop.htop
DEBUG: executable: /usr/lib/snapd/snap-exec
DEBUG: confinement: non-classic
DEBUG: base snap: core18
DEBUG: ruid: 1000, euid: 0, suid: 0
DEBUG: rgid: 1000, egid: 1000, sgid: 1000
DEBUG: apparmor label on snap-confine is: /usr/lib/snapd/snap-confine
DEBUG: apparmor mode is: enforce
DEBUG: creating lock directory /run/snapd/lock (if missing)
DEBUG: set_effective_identity uid:0 (change: no), gid:0 (change: yes)
DEBUG: opening lock directory /run/snapd/lock
DEBUG: set_effective_identity uid:0 (change: no), gid:1000 (change: yes)
DEBUG: opening lock file: /run/snapd/lock/.lock
DEBUG: set_effective_identity uid:0 (change: no), gid:0 (change: yes)
DEBUG: set_effective_identity uid:0 (change: no), gid:1000 (change: yes)
DEBUG: sanity timeout initialized and set for 30 seconds
DEBUG: acquiring exclusive lock (scope (global), uid 0)
DEBUG: sanity timeout reset and disabled
DEBUG: ensuring that snap mount directory is shared
DEBUG: unsharing snap namespace directory
DEBUG: set_effective_identity uid:0 (change: no), gid:0 (change: yes)
DEBUG: set_effective_identity uid:0 (change: no), gid:1000 (change: yes)
DEBUG: releasing lock 5
DEBUG: opened snap-update-ns executable as file descriptor 5
DEBUG: opened snap-discard-ns executable as file descriptor 6
DEBUG: creating lock directory /run/snapd/lock (if missing)
DEBUG: set_effective_identity uid:0 (change: no), gid:0 (change: yes)
DEBUG: opening lock directory /run/snapd/lock
DEBUG: set_effective_identity uid:0 (change: no), gid:1000 (change: yes)
DEBUG: opening lock file: /run/snapd/lock/htop.lock
DEBUG: set_effective_identity uid:0 (change: no), gid:0 (change: yes)
DEBUG: set_effective_identity uid:0 (change: no), gid:1000 (change: yes)
DEBUG: sanity timeout initialized and set for 30 seconds
DEBUG: acquiring exclusive lock (scope htop, uid 0)
DEBUG: sanity timeout reset and disabled
DEBUG: initializing mount namespace: htop
DEBUG: snappy_udev_init
DEBUG: forked support process 50478
DEBUG: unsharing the mount namespace (per-snap)
DEBUG: changing apparmor hat to mount-namespace-capture-helper
DEBUG: helper process waiting for command
DEBUG: sanity timeout initialized and set for 30 seconds
DEBUG: scratch directory for constructing namespace: /tmp/snap.rootfs_79D7R4
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
DEBUG: performing operation: (disabled) use debug build to see details
cannot perform operation: mount --rbind /var/lib/dhcp /tmp/snap.rootfs_79D7R4//var/lib/dhcp: Permission denied
DEBUG: sanity timeout reset and disabled
DEBUG: helper process received command -1
谢谢！- 杰克