[PATCH v2 0/2] vhost-user, dhcp: Fix iPXE network boot over vhost-user
iPXE network boot over vhost-user was broken because passt
unconditionally skipped TCP/UDP checksum computation, relying on
VIRTIO_NET_HDR_F_DATA_VALID to tell the guest the checksums were
valid. Linux guests happened to work because their virtio-net driver
honours DATA_VALID regardless of feature negotiation, but iPXE
verifies checksums strictly and never negotiates VIRTIO_NET_F_GUEST_CSUM.
This series handles correctly VIRTIO_NET_F_GUEST_CSUM feature to fix that
and adds a minimal --dhcp-boot option that populates the BOOTP/DHCP 'file'
field, providing just enough for testing iPXE UDP and TCP support.
This can be tested as following:
- Create an ipxe file configuration
cat > boot-alpine.ipxe <
According to the virtio-net specification, when the VIRTIO_NET_F_GUEST_CSUM
is negotiated, the device can set VIRTIO_NET_HDR_F_DATA_VALID in the
virtio-net header to indicate that packet checksums have been validated,
allowing the guest to skip verification. Without this feature, the device
must provide fully checksummed packets.
The vhost-user TCP and UDP paths were unconditionally skipping checksum
computation, regardless of whether GUEST_CSUM was negotiated. This
went undetected with Linux guests because Linux's virtio-net driver
honours VIRTIO_NET_HDR_F_DATA_VALID regardless of whether
VIRTIO_NET_F_GUEST_CSUM was negotiated, marking such packets as
CHECKSUM_UNNECESSARY and skipping verification.
iPXE, however, does not negotiate GUEST_CSUM, ignores the DATA_VALID
flag entirely, and always verifies checksums. This caused TCP
connections to fail: the SYN-ACK had a zero TCP checksum, iPXE rejected
it, and the connection timed out in SYN_RCVD.
Adding --pcap happened to mask the bug, because the pcap code path
forces checksum computation to ensure correct captures.
Offer VIRTIO_NET_F_GUEST_CSUM in the device features, and only skip
checksum computation when the guest has actually negotiated it. When
GUEST_CSUM is not negotiated, always compute valid checksums as required
by the specification.
We keep setting VIRTIO_NET_HDR_F_DATA_VALID unconditionally in
VU_HEADER: when GUEST_CSUM is negotiated, the flag lets the guest skip
checksum verification; when it is not, the spec says the guest should
ignore the flags field, so setting it is harmless.
Signed-off-by: Laurent Vivier
Add a --dhcp-boot option that populates the 'file' field in DHCP reply
messages with the given filename.
Using --dhcp-boot together with --no-dhcp is rejected at startup.
Signed-off-by: Laurent Vivier
participants (1)
-
Laurent Vivier