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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Mon, 9 Oct 2023 04:54:56 -0500
Message-ID: <CAF=yD-K4bCBpUVtDR_cv=bagRL+vM4Rusez+uHFTb4_kR8XkpA@mail.gmail.com>
Subject: Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature
To:     Akihiko Odaki <akihiko.odaki@daynix.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kselftest@vger.kernel.org, bpf@vger.kernel.org,
        davem@davemloft.net, kuba@kernel.org, ast@kernel.org,
        daniel@iogearbox.net, andrii@kernel.org, kafai@fb.com,
        songliubraving@fb.com, yhs@fb.com, john.fastabend@gmail.com,
        kpsingh@kernel.org, rdunlap@infradead.org, willemb@google.com,
        gustavoars@kernel.org, herbert@gondor.apana.org.au,
        steffen.klassert@secunet.com, nogikh@google.com,
        pablo@netfilter.org, decui@microsoft.com, cai@lca.pw,
        jakub@cloudflare.com, elver@google.com, pabeni@redhat.com,
        Yuri Benditovich <yuri.benditovich@daynix.com>
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On Mon, Oct 9, 2023 at 3:44=E2=80=AFAM Akihiko Odaki <akihiko.odaki@daynix.=
com> wrote:
>
> On 2023/10/09 17:13, Willem de Bruijn wrote:
> > On Sun, Oct 8, 2023 at 12:22=E2=80=AFAM Akihiko Odaki <akihiko.odaki@da=
ynix.com> 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 overcom=
e
> >> thse challenges. An alternative solution is to extend the eBPF steerin=
g
> >> program so that it will be able to report to the userspace, but it mak=
es
> >> little sense to allow to implement different hashing algorithms with
> >> eBPF since the hash value reported by virtio-net is strictly defined b=
y
> >> 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@daynix.com>
> >
> >> @@ -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_R=
EPORT) &&
> >> +                   vnet_hdr_sz >=3D sizeof(hdr.v1_hash_hdr) &&
> >> +                   skb->tun_vnet_hash) {
> >
> > Isn't vnet_hdr_sz guaranteed to be >=3D 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.

It should not be possible to downgrade the hdr_sz once v1 is selected.