Re: [RFC PATCH 0/5] ebpf: Added ebpf helper for libvirtd.

[Jason Wang]

在 2021/6/9 下午6:04, Andrew Melnychenko 写道:
> Libvirt usually launches qemu with strict permissions.
> To enable eBPF RSS steering, qemu-ebpf-rss-helper was added.


A silly question:

Kernel had the following permission checks in bpf syscall:

       if (sysctl_unprivileged_bpf_disabled && !bpf_capable())
                return -EPERM;
...

        err = security_bpf(cmd, &attr, size);
        if (err < 0)
                return err;

So if I understand the code correctly, bpf syscall can only be done if:

1) unprivileged_bpf is enabled or
2) has the capability  and pass the LSM checks

So I think the series is for unprivileged_bpf disabled. If I'm not 
wrong, I guess the policy is to grant CAP_BPF but do fine grain checks 
via LSM.

If this is correct, need to describe it in the commit log.


> Added property "ebpf_rss_fds" for "virtio-net" that allows to
> initialize eBPF RSS context with passed program & maps fds.
>
> Added qemu-ebpf-rss-helper - simple helper that loads eBPF
> context and passes fds through unix socket.
> Libvirt should call the helper and pass fds to qemu through
> "ebpf_rss_fds" property.
>
> Added explicit target OS check for libbpf dependency in meson.
> eBPF RSS works only with Linux TAP, so there is no reason to
> build eBPF loader/helper for non-Linux.
>
> Overall, libvirt process should not be aware of the "interface"
> of eBPF RSS, it will not be aware of eBPF maps/program "type" and
> their quantity.


I'm not sure this is the best. We have several examples that let libvirt 
to involve. Examples:

1) create TAP device (and the TUN_SETIFF)

2) open vhost devices


>   That's why qemu and the helper should be from
> the same build and be "synchronized". Technically each qemu may
> have its own helper. That's why "query-helper-paths" qmp command
> was added. Qemu should return the path to the helper that suits
> and libvirt should use "that" helper for "that" emulator.
>
> qmp sample:
> C: { "execute": "query-helper-paths" }
> S: { "return": [
>       {
>         "name": "qemu-ebpf-rss-helper",
>         "path": "/usr/local/libexec/qemu-ebpf-rss-helper"
>       }
>      ]
>     }


I think we need an example on the detail steps for how libvirt is 
expected to use this.

Thanks


> Andrew Melnychenko (5):
>    ebpf: Added eBPF initialization by fds.
>    virtio-net: Added property to load eBPF RSS with fds.
>    ebpf_rss_helper: Added helper for eBPF RSS.
>    qmp: Added qemu-ebpf-rss-path command.
>    meson: libbpf dependency now exclusively for Linux.
>
>   ebpf/ebpf_rss-stub.c           |   6 ++
>   ebpf/ebpf_rss.c                |  31 +++++++-
>   ebpf/ebpf_rss.h                |   5 ++
>   ebpf/qemu-ebpf-rss-helper.c    | 130 +++++++++++++++++++++++++++++++++
>   hw/net/virtio-net.c            |  77 ++++++++++++++++++-
>   include/hw/virtio/virtio-net.h |   1 +
>   meson.build                    |  37 ++++++----
>   monitor/qmp-cmds.c             |  78 ++++++++++++++++++++
>   qapi/misc.json                 |  29 ++++++++
>   9 files changed, 374 insertions(+), 20 deletions(-)
>   create mode 100644 ebpf/qemu-ebpf-rss-helper.c