Messages in this thread | | | Date | Mon, 9 Oct 2023 19:05:17 +0900 | Subject | Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature | From | Akihiko Odaki <> |
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On 2023/10/09 18:54, Willem de Bruijn wrote: > On Mon, Oct 9, 2023 at 3:44 AM 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 AM Akihiko Odaki <akihiko.odaki@daynix.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 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@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_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.
I see nothing that prevents shrinking the header size.
tun->vnet_hash.flags is read after vnet_hdr_sz so the race can happen even for the case the header size grows though this can be fixed by reordering the two reads.
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