On Sat, Apr 05, 2025 at 05:04:28PM +0900, Akihiko Odaki wrote:
> The goal of commit 7987d2be5a8b ("virtio-net: Copy received header to
> buffer") was to remove the need to patch the (const) input buffer with a
> recomputed UDP checksum by copying headers to a RW region and inject the
> checksum there. The patch computed the checksum only from the header
> fields (missing the rest of the payload) producing an invalid one
> and making guests fail to acquire a DHCP lease.
>
> Fix the issue by copying the entire packet instead of only copying the
> headers.
>
> Fixes: 7987d2be5a8b ("virtio-net: Copy received header to buffer")
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2727
> Cc: qemu-sta...@nongnu.org
> Signed-off-by: Akihiko Odaki <akihiko.od...@daynix.com>Tested-By: Antoine Damhet <adam...@scaleway.com> > --- > This patch aims to resolves the issue the following one also does: > https://lore.kernel.org/qemu-devel/20250404151835.328368-1-adam...@scaleway.com > > The difference from the mentioned patch is that this patch also > preserves that the original intent of regressing change, which is to > remove the need to patch the (const) input buffer with a recomputed UDP > checksum. > > To Antoine Damhet: > I confirmed that DHCP is currently not working and this patch fixes the > issue, but I would appreciate if you also confirm the fix as I already > have done testing badly for the regressing patch. Thanks for the swift response, ideally I'd like a non-regression test in the testsuite but a quick test showed me that I couldn't easily reproduce with user networking so unless someone has a great idea it would be a pain. > --- > hw/net/virtio-net.c | 35 ++++++++++++++++------------------- > 1 file changed, 16 insertions(+), 19 deletions(-) > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c > index de87cfadffe1..a920358a89c5 100644 > --- a/hw/net/virtio-net.c > +++ b/hw/net/virtio-net.c > @@ -1687,6 +1687,11 @@ static void virtio_net_hdr_swap(VirtIODevice *vdev, > struct virtio_net_hdr *hdr) > virtio_tswap16s(vdev, &hdr->csum_offset); > } > > +typedef struct Header { > + struct virtio_net_hdr_v1_hash virtio_net; > + uint8_t payload[1500]; > +} Header; > + > /* dhclient uses AF_PACKET but doesn't pass auxdata to the kernel so > * it never finds out that the packets don't have valid checksums. This > * causes dhclient to get upset. Fedora's carried a patch for ages to > @@ -1701,7 +1706,7 @@ static void virtio_net_hdr_swap(VirtIODevice *vdev, > struct virtio_net_hdr *hdr) > * we should provide a mechanism to disable it to avoid polluting the host > * cache. > */ > -static void work_around_broken_dhclient(struct virtio_net_hdr *hdr, > +static void work_around_broken_dhclient(struct Header *hdr, > size_t *hdr_len, const uint8_t *buf, > size_t buf_size, size_t *buf_offset) > { > @@ -1711,20 +1716,20 @@ static void work_around_broken_dhclient(struct > virtio_net_hdr *hdr, > buf += *buf_offset; > buf_size -= *buf_offset; > > - if ((hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && /* missing csum */ > - (buf_size >= csum_size && buf_size < 1500) && /* normal sized MTU */ > + if ((hdr->virtio_net.hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && /* > missing csum */ > + (buf_size >= csum_size && buf_size < sizeof(hdr->payload)) && /* > normal sized MTU */ > (buf[12] == 0x08 && buf[13] == 0x00) && /* ethertype == IPv4 */ > (buf[23] == 17) && /* ip.protocol == UDP */ > (buf[34] == 0 && buf[35] == 67)) { /* udp.srcport == bootps */ > - memcpy((uint8_t *)hdr + *hdr_len, buf, csum_size); > - net_checksum_calculate((uint8_t *)hdr + *hdr_len, csum_size, > CSUM_UDP); > - hdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM; > - *hdr_len += csum_size; > - *buf_offset += csum_size; > + memcpy((uint8_t *)hdr + *hdr_len, buf, buf_size); > + net_checksum_calculate((uint8_t *)hdr + *hdr_len, buf_size, > CSUM_UDP); > + hdr->virtio_net.hdr.flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM; > + *hdr_len += buf_size; > + *buf_offset += buf_size; > } > } > > -static size_t receive_header(VirtIONet *n, struct virtio_net_hdr *hdr, > +static size_t receive_header(VirtIONet *n, Header *hdr, > const void *buf, size_t buf_size, > size_t *buf_offset) `receive_header` can now "receive" the whole packet that's kinda misleading. I though another approach would be to only do the detection/flag patching from receive_header and recompute the checksum directly in the final `iov`, this would also eliminate the extra payload copy. Cheers, -- Antoine 'xdbob' Damhet Engineer @scaleway > { > @@ -1736,7 +1741,7 @@ static size_t receive_header(VirtIONet *n, struct > virtio_net_hdr *hdr, > work_around_broken_dhclient(hdr, &hdr_len, buf, buf_size, buf_offset); > > if (n->needs_vnet_hdr_swap) { > - virtio_net_hdr_swap(VIRTIO_DEVICE(n), hdr); > + virtio_net_hdr_swap(VIRTIO_DEVICE(n), (struct virtio_net_hdr *)hdr); > } > > return hdr_len; > @@ -1907,13 +1912,6 @@ static int virtio_net_process_rss(NetClientState *nc, > const uint8_t *buf, > return (index == new_index) ? -1 : new_index; > } > > -typedef struct Header { > - struct virtio_net_hdr_v1_hash virtio_net; > - struct eth_header eth; > - struct ip_header ip; > - struct udp_header udp; > -} Header; > - > static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf, > size_t size) > { > @@ -2002,8 +2000,7 @@ static ssize_t virtio_net_receive_rcu(NetClientState > *nc, const uint8_t *buf, > } > > guest_offset = n->has_vnet_hdr ? > - receive_header(n, (struct virtio_net_hdr *)&hdr, > - buf, size, &offset) : > + receive_header(n, &hdr, buf, size, &offset) : > n->guest_hdr_len; > > iov_from_buf(sg, elem->in_num, 0, &hdr, guest_offset); > > --- > base-commit: 825b96dbcee23d134b691fc75618b59c5f53da32 > change-id: 20250405-mtu-834d4c62c93c > > Best regards, > -- > Akihiko Odaki <akihiko.od...@daynix.com>
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