Document mount order

[flx42]

Mostly a nice to have, but I got confused for a while on why our mount hook ("a hook to be run in the container's namespace after mounting has been done, but before the pivot_root") was called before the lxc.mount.entry but after the lxc.mount.auto.

From the code:

lxc/src/lxc/conf.c

Lines 3537 to 3614 in 9810d19

	ret = lxc_mount_auto_mounts(lxc_conf, lxc_conf->auto_mounts & ~LXC_AUTO_CGROUP_MASK, handler);
	if (ret < 0) {
	ERROR("Failed to setup first automatic mounts");
	return -1;
	}
	ret = setup_mount(lxc_conf, &lxc_conf->rootfs, lxc_conf->fstab, name, lxcpath);
	if (ret < 0) {
	ERROR("Failed to setup mounts");
	return -1;
	}
	if (lxc_conf->is_execute) {
	if (execveat_supported()) {
	int fd;
	char path[PATH_MAX];
	ret = snprintf(path, PATH_MAX, SBINDIR "/init.lxc.static");
	if (ret < 0 || ret >= PATH_MAX) {
	ERROR("Path to init.lxc.static too long");
	return -1;
	}
	fd = open(path, O_PATH | O_CLOEXEC);
	if (fd < 0) {
	SYSERROR("Unable to open lxc.init.static");
	return -1;
	}
	((struct execute_args *)handler->data)->init_fd = fd;
	((struct execute_args *)handler->data)->init_path = NULL;
	} else {
	ret = lxc_execute_bind_init(handler);
	if (ret < 0) {
	ERROR("Failed to bind-mount the lxc init system");
	return -1;
	}
	}
	}
	/* Now mount only cgroups, if wanted. Before, /sys could not have been
	* mounted. It is guaranteed to be mounted now either through
	* automatically or via fstab entries.
	*/
	ret = lxc_mount_auto_mounts(lxc_conf, lxc_conf->auto_mounts & LXC_AUTO_CGROUP_MASK, handler);
	if (ret < 0) {
	ERROR("Failed to setup remaining automatic mounts");
	return -1;
	}
	ret = run_lxc_hooks(name, "mount", lxc_conf, NULL);
	if (ret < 0) {
	ERROR("Failed to run mount hooks");
	return -1;
	}
	if (lxc_conf->autodev > 0) {
	ret = run_lxc_hooks(name, "autodev", lxc_conf, NULL);
	if (ret < 0) {
	ERROR("Failed to run autodev hooks");
	return -1;
	}
	ret = lxc_fill_autodev(&lxc_conf->rootfs);
	if (ret < 0) {
	ERROR("Failed to populate \"/dev\"");
	return -1;
	}
	}
	if (!lxc_list_empty(&lxc_conf->mount_list)) {
	ret = setup_mount_entries(lxc_conf, &lxc_conf->rootfs,
	&lxc_conf->mount_list, name, lxcpath);
	if (ret < 0) {
	ERROR("Failed to setup mount entries");
	return -1;
	}
	}

The order is:

• lxc.mount.auto = proc and lxc.mount.auto = sys
• lxc.mount.fstab
• lxc.mount.auto = cgroup
• lxc.hook.mount
• lxc.autodev
• lxc.mount.entry

By the way, was there a reason to not have lxc.mount.entry immediately after lxc.mount.fstab?

[brauner]

By the way, was there a reason to not have lxc.mount.entry immediately after lxc.mount.fstab?

Not that I know of.

You want the mount order documented in lxc.container.conf or in the code?

[flx42]

By the way, was there a reason to not have lxc.mount.entry immediately after lxc.mount.fstab?

Should we try to fix it or is too late and part of the implicit API?

You want the mount order documented in lxc.container.conf or in the code?

In lxc.container.conf. The code is easy to follow once you find the right place.

[brauner]

Yeah, I think we can send a patch, clean this up and then document it. :)

[brauner]

Care to send one, @flx42?

[flx42]

Was out of the office for a few days, I'll try to do it this week.

[flx42]

Let's see if changing the mount order breaks anything before tackling documentation.

[tardyp]

By the way, was there a reason to not have lxc.mount.entry immediately after lxc.mount.fstab?

A use case is to bind mount additional stuff like /dev/bus/usb

This is documented to be working here:
https://medium.com/@konpat/usb-passthrough-to-an-lxc-proxmox-15482674f11d

[oxr463]

Any updates on this one?