Re: [PATCH v5 11/13] Support updates via systemd-sysupdate

[Demi Marie Obenour <demiobenour@gmail.com>]

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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.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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