Daniel P. Berrangé <berra...@redhat.com> writes: > On Wed, Mar 01, 2023 at 03:53:47PM +0100, Toke Høiland-Jørgensen wrote: >> Daniel P. Berrangé <berra...@redhat.com> writes: >> >> > On Tue, Feb 28, 2023 at 11:21:56PM +0100, Toke Høiland-Jørgensen wrote: >> >> Daniel P. Berrangé <berra...@redhat.com> writes: >> >> >> >> > On Tue, Feb 28, 2023 at 08:01:51PM +0100, Toke Høiland-Jørgensen wrote: >> >> >> Daniel P. Berrangé <berra...@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.
Right, I don't have a strong opinion about the exact mechanism, just wanted to convey a general "loading an untrusted BPF program is bad" kind of vibe ;) -Toke