Re: [libvirt PATCH v3 09/18] qemu: add functions to start and stop nbdkit

On Thu, Oct 20, 2022 at 16:59:00 -0500, Jonathon Jongsma wrote: > Add some helper functions to build a virCommand object and run the > nbdkit process for a given virStorageSource. > > Signed-off-by: Jonathon Jongsma <jjongsma(a)redhat.com&gt; > --- > src/qemu/qemu_nbdkit.c | 251 +++++++++++++++++++++++++++++++++++++++++ > src/qemu/qemu_nbdkit.h | 10 ++ > 2 files changed, 261 insertions(+) >

> +static int > +qemuNbdkitProcessBuildCommandSSH(qemuNbdkitProcess *proc, > + virCommand *cmd) > +{ > + const char *user = NULL; > + virStorageNetHostDef *host = &proc->source->hosts[0]; > + g_autofree char *portstr = g_strdup_printf("%u", host->port); > + > + /* nbdkit plugin name */ > + virCommandAddArg(cmd, "ssh"); > + > + virCommandAddArgPair(cmd, "host", host->name); > + virCommandAddArgPair(cmd, "port", portstr); > + virCommandAddArgPair(cmd, "path", proc->source->path); > + > + if (proc->source->auth) > + user = proc->source->auth->username; > + else if (proc->source->ssh_user) > + user = proc->source->ssh_user; So there's a bit problem with this, which also explains my reluctance to simply supporting the SSH protocol in the current state without properly designing other required parameters for SSH authentication. And even then it will most likely break "historical compatibility". So historically we didn't implement ssh protocol at all. Users (notably libguestfs) worked this around by adding a wrapper QCOW2 image and setting up the backing store string such that qemu would access the ssh image. When blockdev support was added I needed a way to port the existing functionality specifically for libguestfs. This made me add the ssh protocol and forwart the minimum set of properties to the image (thus the 'ssh_user' field. But there's another thing that was always configured out-of-band (via environment variables) and that is the ssh agent socket path or ssh key path. These are not present in the virStorageSource because they are not even configured for the blockdev 'ssh' protocol driver directly. Now with switching to nbdkit this means agent/sshkey authentication will necessarily break for such usage as we simply don't have the information. So what to do about it? We can either break the old way completely, which I guess would be mostly okay since libguestfs most likely now uses nbdkit anyways, and then design it properly. Obviously since the qemu ssh blockdev backend driver still configures stuff using environment variables thus the new configuration will be available only when nbdkit is in use. Other possibility, if we want to keep old functionality working as it was, is to avoid the use of nbdkit. Either way the implementation of SSH as done in this series simply just breaks SSH usage completely, by not being able to support the old hacky way of using agent/sshkey, not implementing any new support and not even implementing password authentication. What now? I don't really care too deeply about the hacky impl we have now. If libguestfs is no longer using I reckon we can simply break it and not deal with the fallout. I'm not sure how others care about that though. Obviously another option (besides doing a proper desing on authentication config) is to not touch anything ssh-related for now.