[pve-devel] [PATCH container v2] lxc_config: mount /sys as mixed for unprivileged by default

[Thomas Lamprecht]

CONTAINER_INTERFACE[0] is something systemd people call their API and
we need to adapt to it a bit, even if it means doing stupid
unnecessary things, as else systemd decides to regress and suddenly
break network stack in CT after an upgrade[1].

This mounts the parent /sys as mixed, which is:
> mount /sys as read-only but with /sys/devices/virtual/net writable.
-- man 5 lxc.container.conf

Allow users to overwrite that with a features knob, as surely some
run into other issues else and manually adding a "lxc.mount.auto"
entry in the container .conf is not an nice user experience for most.

Fixes the system regression in up to date Arch installations
introduced by[2].

[0]: https://systemd.io/CONTAINER_INTERFACE/
[1]: https://github.com/systemd/systemd/issues/15101#issuecomment-598607582
[2]: https://github.com/systemd/systemd/commit/bf331d87171b7750d1c72ab0b140a240c0cf32c3

Signed-off-by: Thomas Lamprecht <t.lamprecht at proxmox.com>
---

changes v1 -> v2:
* use sys:mixed and only do this for upriv. CTs
* add knob to allow easier opting out of this

 src/PVE/LXC.pm        | 6 ++++++
 src/PVE/LXC/Config.pm | 7 +++++++
 2 files changed, 13 insertions(+)

diff --git a/src/PVE/LXC.pm b/src/PVE/LXC.pm
index 0742a53..df52afa 100644
--- a/src/PVE/LXC.pm
+++ b/src/PVE/LXC.pm
@@ -662,6 +662,12 @@ sub update_lxc_config {
 	$raw .= "lxc.mount.entry = /dev/fuse dev/fuse none bind,create=file 0 0\n";
     }
 
+    if ($unprivileged && !$features->{force_rw_sys}) {
+	# unpriv. CT default to sys:rw, but that doesn't always plays well with
+	# systemd, e.g., systemd-networkd https://systemd.io/CONTAINER_INTERFACE/
+	$raw .= "lxc.mount.auto = sys:mixed\n";
+    }
+
     # WARNING: DO NOT REMOVE this without making sure that loop device nodes
     # cannot be exposed to the container with r/w access (cgroup perms).
     # When this is enabled mounts will still remain in the monitor's namespace
diff --git a/src/PVE/LXC/Config.pm b/src/PVE/LXC/Config.pm
index e88ba0b..0909773 100644
--- a/src/PVE/LXC/Config.pm
+++ b/src/PVE/LXC/Config.pm
@@ -331,6 +331,13 @@ my $features_desc = {
 	    ." This requires a kernel with seccomp trap to user space support (5.3 or newer)."
 	    ." This is experimental.",
     },
+    force_rw_sys => {
+	optional => 1,
+	type => 'boolean',
+	default => 0,
+	description => "Mount /sys in unprivileged containers as `rw` instead of `mixed`."
+	    ." This can break networking under newer (>= v245) systemd-network use."
+    },
 };
 
 my $confdesc = {
-- 
2.20.1