Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature

[Akihiko Odaki]

On 2023/10/09 17:13, Willem de Bruijn wrote:
> On Sun, Oct 8, 2023 at 12:22 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> > virtio-net have two usage of hashes: one is RSS and another is hash
> > reporting. Conventionally the hash calculation was done by the VMM.
> > However, computing the hash after the queue was chosen defeats the
> > purpose of RSS.
> >
> > Another approach is to use eBPF steering program. This approach has
> > another downside: it cannot report the calculated hash due to the
> > restrictive nature of eBPF.
> >
> > Introduce the code to compute hashes to the kernel in order to overcome
> > thse challenges. An alternative solution is to extend the eBPF steering
> > program so that it will be able to report to the userspace, but it makes
> > little sense to allow to implement different hashing algorithms with
> > eBPF since the hash value reported by virtio-net is strictly defined by
> > the specification.
> >
> > The hash value already stored in sk_buff is not used and computed
> > independently since it may have been computed in a way not conformant
> > with the specification.
> >
> > Signed-off-by: Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>
>
> > @@ -2116,31 +2172,49 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> >          }
> >
> >          if (vnet_hdr_sz) {
> > -               struct virtio_net_hdr gso;
> > +               union {
> > +                       struct virtio_net_hdr hdr;
> > +                       struct virtio_net_hdr_v1_hash v1_hash_hdr;
> > +               } hdr;
> > +               int ret;
> >
> >                  if (iov_iter_count(iter) < vnet_hdr_sz)
> >                          return -EINVAL;
> >
> > -               if (virtio_net_hdr_from_skb(skb, &gso,
> > -                                           tun_is_little_endian(tun), true,
> > -                                           vlan_hlen)) {
> > +               if ((READ_ONCE(tun->vnet_hash.flags) & TUN_VNET_HASH_REPORT) &&
> > +                   vnet_hdr_sz >= sizeof(hdr.v1_hash_hdr) &&
> > +                   skb->tun_vnet_hash) {
>
> Isn't vnet_hdr_sz guaranteed to be >= hdr.v1_hash_hdr, by virtue of
> the set hash ioctl failing otherwise?
>
> Such checks should be limited to control path where possible

There is a potential race since tun->vnet_hash.flags and vnet_hdr_sz are 
not read at once.