On Mon, Oct 9, 2023 at 3:44 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
>
> 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.
It should not be possible to downgrade the hdr_sz once v1 is selected.
