Re: [RFC PATCH 3/7] tun: allow use of BPF_PROG_TYPE_SCHED_CLS program type

[Yuri Benditovich]

On Tue, Jan 12, 2021 at 10:40 PM Yuri Benditovich
<yuri.benditovich@xxxxxxxxxx> wrote:
>
> On Tue, Jan 12, 2021 at 9:42 PM Yuri Benditovich
> <yuri.benditovich@xxxxxxxxxx> wrote:
> >
> > This program type can set skb hash value. It will be useful
> > when the tun will support hash reporting feature if virtio-net.
> >
> > Signed-off-by: Yuri Benditovich <yuri.benditovich@xxxxxxxxxx>
> > ---
> >  drivers/net/tun.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index 7959b5c2d11f..455f7afc1f36 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -2981,6 +2981,8 @@ static int tun_set_ebpf(struct tun_struct *tun, struct tun_prog __rcu **prog_p,
> >                 prog = NULL;
> >         } else {
> >                 prog = bpf_prog_get_type(fd, BPF_PROG_TYPE_SOCKET_FILTER);
> > +               if (IS_ERR(prog))
> > +                       prog = bpf_prog_get_type(fd, BPF_PROG_TYPE_SCHED_CLS);
> >                 if (IS_ERR(prog))
> >                         return PTR_ERR(prog);
> >         }
>
> Comment from Alexei Starovoitov:
> Patches 1 and 2 are missing for me, so I couldn't review properly,
> but this diff looks odd.
> It allows sched_cls prog type to attach to tun.
> That means everything that sched_cls progs can do will be done from tun hook?

We do not have an intention to modify the packet in this steering eBPF.
There is just one function that unavailable for BPF_PROG_TYPE_SOCKET_FILTER
that the eBPF needs to make possible to deliver the hash to the guest
VM - it is 'bpf_set_hash'

Does it mean that we need to define a new eBPF type for socket filter
operations + set_hash?

Our problem is that the eBPF calculates 32-bit hash, 16-bit queue
index and 8-bit of hash type.
But it is able to return only 32-bit integer, so in this set of
patches the eBPF returns
queue index and hash type and saves the hash in skb->hash using bpf_set_hash().

If this is unacceptable, can you please recommend a better solution?

> sched_cls assumes l2 and can modify the packet.

The steering eBPF in TUN module also assumes l2.

> I think crashes are inevitable.
>
> > --
> > 2.17.1
> >