On Fri, Nov 28, 2025 at 04:28:20PM -0500, Demi Marie Obenour wrote:
On 11/28/25 16:08, Alyssa Ross wrote:
Demi Marie Obenour <demiobenour@gmail.com> writes:
On 11/28/25 15:41, Alyssa Ross wrote:
Demi Marie Obenour <demiobenour@gmail.com> writes:
On 11/28/25 08:47, Alyssa Ross wrote:
Demi Marie Obenour <demiobenour@gmail.com> writes:
> diff --git a/host/rootfs/image/usr/bin/spectrum-update b/host/rootfs/image/usr/bin/spectrum-update > new file mode 100755 > index 0000000000000000000000000000000000000000..613b43570d0538fce20296ccb1de2a6364e0df55 > --- /dev/null > +++ b/host/rootfs/image/usr/bin/spectrum-update > @@ -0,0 +1,92 @@ > +#!/bin/execlineb -WS1 > +# SPDX-License-Identifier: EUPL-1.2+ > +# SPDX-FileCopyrightText: 2025 Demi Marie Obenour <demiobenour@gmail.com> > + > +if { mkdir -p -m 0700 /run/updater } > + > +# Take a global lock to avoid races. > +s6-setlock /run/update-lock > + > +foreground { redirfd -w 2 /dev/null rmdir -- $1 } > +if { umask 0077 mkdir -p -- $1 } > +cd $1 > +foreground { > + # If this exists already that is okay. > + foreground { redirfd -w 2 /dev/null btrfs subvolume create -- shared } > + > + # Delete any stale temporary files. Delete any existing signature > + # files. If the VM is still running (it should not be), the VM might > + # have write access to the directory. However, updates-dir-check is > + # safe against that. > + if { updates-dir-check cleanup shared } > + > + if { > + foreground { > + # TODO: suppress only "subvolume does not exist" errors. > + redirfd -w 2 /dev/null > + btrfs subvolume delete snapshot > + } > + rm -f snapshot > + } > + > + backtick -E update_vm_id { > + backtick -E id_path { readlink /run/vm/by-name/sys.appvm-systemd-sysupdate } > + basename -- $id_path > + } > + > + # $fsdir is read-only to the guest, but read-write to the host. > + # Directories bind-mounted into it are read-write to the guest. > + # See etc/s6-linux-init/run-image/service/vhost-user-fs/template/run > + # for details. > +
This still refers to a non-existent variable.
Whoops!
> + # Set up /etc with what the VM needs. The VM will overlay this > + # on its own /etc. > + # > + # In the future, this should use a bind mount instead of copying > + # into a tmpfs. However, this would significantly complicate the > + # cleanup code. Deleting fs/etc would require undoing the bind > + # mounts instead of rm -rf. Once this code is in a separate mount > + # namespace, the copies should be replaced by bind mounts. > + if { > + if { rm -rf -- /run/vm/by-id/${update_vm_id}/fs/etc } > + umask 022 > + if { mkdir -p -- /run/vm/by-id/${update_vm_id}/fs/updates /run/vm/by-id/${update_vm_id}/fs/etc/systemd } > + if { cp -R -- /etc/vm-sysupdate.d /etc/update-url /run/vm/by-id/${update_vm_id}/fs/etc } > + cp -- /etc/systemd/import-pubring.gpg /run/vm/by-id/${update_vm_id}/fs/etc/systemd > + } > + > + # If the directory is already mounted, unmount it. This prevents a > + # confusing error from mount. > + foreground { redirfd -w 2 /dev/null umount -- /run/vm/by-id/${update_vm_id}/fs/updates } > + > + # Share the update directory with the VM. > + if { mount --bind -- shared /run/vm/by-id/${update_vm_id}/fs/updates } > + > + # Start the update VM. > + if { vm-start $update_vm_id } > + > + # Wait for the VM to exit. > + # TODO: This is racy. If the update finishes before this code runs, > + # the s6-svwait call will fail. > + if { s6-svwait -D /run/service/vmm/instance/${update_vm_id} } > + > + # Remove the bind mount. > + if { umount -- /run/vm/by-id/${update_vm_id}/fs/updates } > + > + # Ensure that the VM cannot change the directory > + # while systemd-sysupdate is using it. > + if { btrfs subvolume snapshot -- shared snapshot } > + > + # Validate the update directory. Delete any stale temporary files. > + # Check that a signature file was downloaded. > + if { updates-dir-check check snapshot } > + > + unshare --mount > + if { mount --bind -o ro -- snapshot /run/updater } > + > + /usr/lib/systemd/systemd-sysupdate update
Why not just make a readonly snapshot? (btrfs subvolume snapshot -r)
The checker will delete any temporary files it comes across, so it needs write access. A snapshot is much heavier than a bind mount and isn't automatically cleaned up.
Okay, but why do we need to block systemd-sysupdate from writing to this directory, but not to anywhere else on the system?
We don't. I'm fine with a writable mount. Either should work.
Then let's keep things simple and drop the unshare and mount. We can deprivilege it properly in the future.
The mount is absolutely necessary: it ensures that the update is available where the .transfer file says it will be. It just doesn't need to be read-only.
The unshare is there to allow trivial cleanup by the kernel once systemd-sysupdate exits.
Ah, then I was misreading. If the mount has to be there, it might as well be read-only. With the outdated comment removed: Reviewed-by: Alyssa Ross <hi@alyssa.is>