This patch refactors the dhcp() function to accept `struct iov_tail *data`
directly as its packet input, replacing the previous `const struct pool *p`
parameter. Consequently, dhcp() no longer fetches packet data internally
using packet_data().

This change simplifies callers, such as tap4_handler(), which now pass
the iov_tail representing the L2 frame directly to dhcp(). This removes
the need for intermediate packet pool handling for DHCP processing.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

In practical terms, passt doesn't benefit from the additional
protection offered by the AGPL over the GPL, because it's not
suitable to be executed over a computer network.

Further, restricting the distribution under the version 3 of the GPL
wouldn't provide any practical advantage either, as long as the passt
codebase is concerned, and might cause unnecessary compatibility
dilemmas.

Change licensing terms to the GNU General Public License Version 2,
or any later version, with written permission from all current and
past contributors, namely: myself, David Gibson, Laine Stump, Andrea
Bolognani, Paul Holzinger, Richard W.M. Jones, Chris Kuhn, Florian
Weimer, Giuseppe Scrivano, Stefan Hajnoczi, and Vasiliy Ulyanov.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

...at the moment, just for consistency with packet.h, icmp.h,
tcp.h and udp.h.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Implement a packet abstraction providing boundary and size checks
based on packet descriptors: packets stored in a buffer can be queued
into a pool (without storage of its own), and data can be retrieved
referring to an index in the pool, specifying offset and length.

Checks ensure data is not read outside the boundaries of buffer and
descriptors, and that packets added to a pool are within the buffer
range with valid offset and indices.

This implies a wider rework: usage of the "queueing" part of the
abstraction mostly affects tap_handler_{passt,pasta}() functions and
their callees, while the "fetching" part affects all the guest or tap
facing implementations: TCP, UDP, ICMP, ARP, NDP, DHCP and DHCPv6
handlers.

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

SPDX tags don't replace license files. Some notices were missing and
some tags were not according to the SPDX specification, too.

Now reuse --lint from the REUSE tool (https://reuse.software/) passes.

Reported-by: Martin Hauke <mardnh@gmx.de>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

...from 11MiB to 155KiB for 'make avx2', 95KiB with -Os and stripped.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

A bunch of fixes not worth single commits at this stage, notably:

- make buffer, length parameter ordering consistent in ARP, DHCP,
  NDP handlers

- strict checking of buffer, message and option length in DHCP
  handler (a malicious client could have easily crashed it)

- set up forwarding for IPv4 and IPv6, and masquerading with nft for
  IPv4, from demo script

- get rid of separate slow and fast timers, we don't save any
  overhead that way

- stricter checking of buffer lengths as passed to tap handlers

- proper dequeuing from qemu socket back-end: I accidentally trashed
  messages that were bundled up together in a single tap read
  operation -- the length header tells us what's the size of the next
  frame, but there's no apparent limit to the number of messages we
  get with one single receive

- rework some bits of the TCP state machine, now passive and active
  connection closes appear to be robust -- introduce a new
  FIN_WAIT_1_SOCK_FIN state indicating a FIN_WAIT_1 with a FIN flag
  from socket

- streamline TCP option parsing routine

- track TCP state changes to stderr (this is temporary, proper
  debugging and syslogging support pending)

- observe that multiplying a number by four might very well change
  its value, and this happens to be the case for the data offset
  from the TCP header as we check if it's the same as the total
  length to find out if it's a duplicated ACK segment

- recent estimates suggest that the duration of a millisecond is
  closer to a million nanoseconds than a thousand of them, this
  trend is now reflected into the timespec_diff_ms() convenience
  routine

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

With this, merd provides a fully functional IPv4 environment to
guests, requiring a single capability, CAP_NET_RAW.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>