Re: [PATCH 3/5] qmp: Added the helper stamp check.

[Daniel P . Berrangé]

On Wed, Mar 01, 2023 at 03:53:47PM +0100, Toke Høiland-Jørgensen wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > On Tue, Feb 28, 2023 at 11:21:56PM +0100, Toke Høiland-Jørgensen wrote:
> >> Daniel P. Berrangé <berrange@redhat.com> writes:
> >> 
> >> > On Tue, Feb 28, 2023 at 08:01:51PM +0100, Toke Høiland-Jørgensen wrote:
> >> >> Daniel P. Berrangé <berrange@redhat.com> writes:
> >> >> 
> >> >> Just to interject a note on this here: the skeleton code is mostly a
> >> >> convenience feature used to embed BPF programs into the calling binary.
> >> >> It is perfectly possible to just have the BPF object file itself reside
> >> >> directly in the file system and just use the regular libbpf APIs to load
> >> >> it. Some things get a bit more cumbersome (mostly setting values of
> >> >> global variables, if the BPF program uses those).
> >> >> 
> >> >> So the JSON example above could just be a regular compiled-from-clang
> >> >> BPF object file, and the management program can load that, inspect its
> >> >> contents using the libbpf APIs and pass the file descriptors on to Qemu.
> >> >> It's even possible to embed version information into this so that Qemu
> >> >> can check if it understands the format and bail out if it doesn't - just
> >> >> stick a version field in the configuration map as the first entry :)
> >> >
> >> > If all you have is the BPF object file is it possible to interrogate
> >> > it to get a list of all the maps, and get FDs associated for them ?
> >> > I had a look at the libbpf API and wasn't sure about that, it seemed
> >> > like you had to know the required maps upfront ?  If it is possible
> >> > to auto-discover everything you need, soley from the BPF object file
> >> > as input, then just dealing with that in isolation would feel simpler.
> >> 
> >> It is. You load the object file, and bpf_object__for_each_map() lets you
> >> discover which maps it contains, with the different bpf_map__*() APIs
> >> telling you the properties of that map (and you can modify them too
> >> before loading the object if needed).
> >> 
> >> The only thing that's not in the object file is any initial data you
> >> want to put into the map(s). But except for read-only maps that can be
> >> added by userspace after loading the maps, so you could just let Qemu do
> >> that...
> >> 
> >> > It occurrs to me that exposing the BPF program as data rather than
> >> > via binary will make more practical to integrate this into KubeVirt's
> >> > architecture. In their deployment setup both QEMU and libvirt are
> >> > running unprivileged inside a container. For any advanced nmetworking
> >> > a completely separate component creates the TAP device and passes it
> >> > into the container running QEMU. I don't think that the separate
> >> > precisely matched helper binary would be something they can use, but
> >> > it might be possible to expose a data file providing the BPF program
> >> > blob and describing its maps.
> >> 
> >> Well, "a data file providing the BPF program blob and describing its
> >> maps" is basically what a BPF .o file is. It just happens to be encoded
> >> in ELF format :)
> >> 
> >> You can embed it into some other data structure and have libbpf load it
> >> from a blob in memory as well as from the filesystem, though; that is
> >> basically what the skeleton file does (notice the big character string
> >> at the end, that's just the original .o file contents).
> >
> > Ok, in that case I'm really wondering why any of this helper program
> > stuff was proposed. I recall the rationale was that it was impossible
> > for an external program to load the BPF object on behalf of QEMU,
> > because it would not know how todo that without QEMU specific
> > knowledge.
> 
> I'm not sure either. Was there some bits that initially needed to be set
> before the program was loaded (read-only maps or something)? Also,
> upstream does encourage the use of skeletons for embedding into
> applications, so it's not an unreasonable thing to start with if you
> don't have the kind of deployment constraints that Qemu does in this
> case.
> 
> > It looks like we can simply expose the BPF object blob to mgmt apps
> > directly and get rid of this helper program entirely.
> 
> I believe so, yes. You'd still need to be sure that the BPF object file
> itself comes from a trusted place, but hopefully it should be enough to
> load it from a known filesystem path? (Sorry if this is a stupid
> question, I only have a fuzzy idea of how all the pieces fit together
> here).

It could be from a well known location on the filesystem, but might
be better to make it possible to query it from QMP, which is mostly
safe *provided* you've not yet started guest CPUs running. It could
be queried at startup and then cached for future use.

With regards,
Daniel
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