Adds the client half of TCP -- tcp_connect/tcp_send/tcp_recv/tcp_close --
and wires SOCK_STREAM sockets to it, so the kernel can originate an
outbound stream rather than only answer inbound ones. Inbound segments
are now matched against the connection table before being offered to the
listeners; previously anything not addressed to a listening port was
answered with RST, which would have rejected the SYN-ACK completing our
own handshake.
Transmission is stop-and-wait: one segment outstanding, retransmitted
until acknowledged. Every blocking call is bounded against the timer tick
and halts the CPU between checks, so a silent peer costs a timeout rather
than a hang. Connections linger in TIME_WAIT so a late duplicate cannot
be mistaken for traffic on a new incarnation.
Verifying it against a real peer uncovered two pre-existing bugs that had
made TCP unusable in both directions. Neither was visible from inside the
guest, which is why the servers had looked fine:
1. The TCP and UDP checksums were computed by reading the header a
uint16_t at a time, immediately after the caller stored zero into that
same header's checksum field. Nothing stops the compiler reordering
those word reads around that store, and at -O2 it did: the stale
checksum field was folded into its own result. Every TCP segment the
kernel had ever sent carried a wrong checksum and was silently dropped
by the peer -- no client could complete a handshake with sshd or
httpd, and it is the likely reason DHCP and DNS replies never arrived.
The checksum is now summed a byte at a time through unsigned char
pointers, which may alias any object and so cannot be reordered, and
which also drops the alignment assumption that reading a packed header
as uint16_t made -- a fault rather than a slowdown on ARM. The helper
is shared by IP, TCP and UDP instead of being open-coded four times.
2. net_receive derived the datagram length from the Ethernet frame rather
than from the IP header's total_length, so the padding on any frame
shorter than 60 bytes was delivered as protocol payload. A bare ACK or
FIN appeared to carry six bytes, which pushed the acknowledgement
number out of step with the peer and turned every close into a FIN
retransmission storm answered with RST.
Also adds `tcpconnect` and `httpget` applets, and gives netstat the real
per-connection state and direction from the engine's table.
Verified on x86_64 against a real TCP stack, checked packet by packet in
a capture: `httpget` fetched a 349-byte page from a server on the host --
handshake, 81-byte request, response, orderly four-way close, every
sequence number exact, zero bad checksums, no retransmissions. In the
other direction curl on the host now fetches a live page from the guest's
httpd, which had never worked. Builds clean on x86_64, aarch64 and armv8i.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Fh5z6V7nGPbHvqVG5p4CXX
