SchOS/kernel/net/netlink.c

// SPDX-License-Identifier: GPL-2.0-or-later
//
// rtnetlink, enough of it that getifaddrs(3) works.
//
// Why this exists rather than a patch to whatever program wanted it: asking
// "what interfaces does this machine have, and what addresses do they carry" is
// something every networked program does, and on Linux there is exactly one
// answer -- a NETLINK_ROUTE socket. musl implements getifaddrs() over it, so a
// libc call that looks like it should be a syscall is really a small protocol
// conversation, and without this side of it every caller fails identically.
//
// It is a *responder*, not a socket family in the general sense. There is no
// kernel-to-userspace event stream, no multicast groups, nothing subscribes to
// link state. A request arrives, the whole answer is generated into the
// socket's buffer, and reads drain it. That shape is chosen deliberately:
// getifaddrs sends a dump request and reads until NLMSG_DONE, so generating the
// answer at send time means a read never has to block and MSG_DONTWAIT -- which
// musl passes -- is trivially honoured.
//
// The information is not this file's. Interfaces come from the netdev class
// (include/netdev.h) and addresses from lwIP's netifs, exactly as /proc/netif
// reads them. Nothing here knows what an e1000 is.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "mmu.h"
#include "proc.h"
#include "fs.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "file.h"
#include "errno.h"
#include "device.h"
#include "netdev.h"

#define _KERNEL_SOCKET_INTERNALS
#include "socket.h"

#include "lwip/netif.h"
#include "lwip/ip4_addr.h"

// Writing into a fixed buffer, with every append bounds-checked against the
// same `cap`. A netlink dump is the kind of code where a length field and a
// memmove disagree quietly, so there is one helper and everything goes through
// it.
struct nlbuf {
  char *p;
  uint  len;
  uint  cap;
  int   full;
};

static void *
nl_take(struct nlbuf *b, uint n)
{
  if (b->full || b->len + n > b->cap) {
    b->full = 1;
    return 0;
  }
  void *at = b->p + b->len;
  memset(at, 0, n);
  b->len += n;
  return at;
}

// One attribute: header, payload, padding to a 4-byte boundary. The header's
// length counts itself and the payload but *not* the padding -- getting that
// backwards is the classic netlink bug, and it desynchronises the reader's walk
// rather than failing outright.
static void
nl_attr(struct nlbuf *b, uint16 type, const void *val, uint vlen)
{
  uint total = RTA_ALIGN(sizeof(struct rtattr) + vlen);
  struct rtattr *a = nl_take(b, total);
  if (!a)
    return;
  a->rta_type = type;
  a->rta_len  = (uint16)(sizeof(struct rtattr) + vlen);
  memmove((char *)a + sizeof(struct rtattr), val, vlen);
}

// Start a message, returning where its length must be written once the
// attributes are known.
static struct nlmsghdr *
nl_msg(struct nlbuf *b, uint16 type, uint32 seq, uint body)
{
  struct nlmsghdr *h = nl_take(b, sizeof(*h) + NLMSG_ALIGN(body));
  if (!h)
    return 0;
  h->nlmsg_type  = type;
  h->nlmsg_flags = NLM_F_MULTI;
  h->nlmsg_seq   = seq;
  h->nlmsg_pid   = 0;
  return h;
}

static void
nl_done(struct nlbuf *b, uint32 seq)
{
  struct nlmsghdr *h = nl_take(b, sizeof(*h));
  if (!h)
    return;
  h->nlmsg_len  = sizeof(*h);
  h->nlmsg_type = NLMSG_DONE;
  h->nlmsg_seq  = seq;
}

// Interface index. Linux numbers from 1 and reserves nothing, but getifaddrs
// matches RTM_NEWADDR records to RTM_NEWLINK records *by index*, so the two
// dumps below have to agree. Loopback is 1 and the netdevs follow.
#define NL_LO_INDEX  1
#define NL_DEV_BASE  2

static void
dump_links(struct nlbuf *b, uint32 seq)
{
  // Loopback first, and it is not a netdev: it has no driver, no MAC and no
  // entry in the device model. It is a netif lwIP creates, and a program
  // enumerating interfaces still expects to see it.
  {
    struct nlmsghdr *h = nl_msg(b, RTM_NEWLINK, seq, sizeof(struct ifinfomsg));
    if (!h) return;
    struct ifinfomsg *ifi = (struct ifinfomsg *)((char *)h + sizeof(*h));
    ifi->ifi_family = 0;                       /* AF_UNSPEC */
    ifi->ifi_type   = ARPHRD_LOOPBACK;
    ifi->ifi_index  = NL_LO_INDEX;
    ifi->ifi_flags  = IFF_UP | IFF_LOOPBACK | IFF_RUNNING;
    nl_attr(b, IFLA_IFNAME, "lo", 3);
    h->nlmsg_len = b->len - (uint)((char *)h - b->p);
  }

  for (int i = 0; i < netdev_count(); i++) {
    struct device *d = netdev_at(i);
    if (!d) continue;

    struct nlmsghdr *h = nl_msg(b, RTM_NEWLINK, seq, sizeof(struct ifinfomsg));
    if (!h) return;
    struct ifinfomsg *ifi = (struct ifinfomsg *)((char *)h + sizeof(*h));
    ifi->ifi_family = 0;
    ifi->ifi_type   = ARPHRD_ETHER;
    ifi->ifi_index  = NL_DEV_BASE + i;
    ifi->ifi_flags  = IFF_BROADCAST | IFF_MULTICAST
                    | (netdev_link_up(d) ? (IFF_UP | IFF_RUNNING) : 0);

    nl_attr(b, IFLA_IFNAME, d->name, (uint)strlen(d->name) + 1);
    const unsigned char *mac = netdev_mac(d);
    if (mac)
      nl_attr(b, IFLA_ADDRESS, mac, 6);

    h->nlmsg_len = b->len - (uint)((char *)h - b->p);
  }
}

// How many leading ones are in a netmask. getifaddrs does not use the prefix
// length itself -- it reconstructs the netmask from it -- so an answer that
// disagrees with the netmask is a wrong answer nobody notices until something
// computes a broadcast address.
static uchar
prefix_of(uint32 mask_net_order)
{
  uint32 m = ntohl_u32(mask_net_order);
  uchar n = 0;
  while (m & 0x80000000u) { n++; m <<= 1; }
  return n;
}

static void
addr_msg(struct nlbuf *b, uint32 seq, int index, uint32 addr, uchar prefix,
         const char *label, int loopback)
{
  struct nlmsghdr *h = nl_msg(b, RTM_NEWADDR, seq, sizeof(struct ifaddrmsg));
  if (!h) return;
  struct ifaddrmsg *ifa = (struct ifaddrmsg *)((char *)h + sizeof(*h));
  ifa->ifa_family    = AF_INET;
  ifa->ifa_prefixlen = prefix;
  ifa->ifa_flags     = 0;
  ifa->ifa_scope     = loopback ? 254 : 0;      /* RT_SCOPE_HOST : UNIVERSE */
  ifa->ifa_index     = (uint32)index;

  // IFA_ADDRESS and IFA_LOCAL are the same on anything that is not a
  // point-to-point link. musl reads whichever arrives and treats a second one
  // as the peer address, so sending both in this order is what makes it record
  // the address rather than a destination.
  nl_attr(b, IFA_ADDRESS, &addr, 4);
  nl_attr(b, IFA_LOCAL,   &addr, 4);
  nl_attr(b, IFA_LABEL, label, (uint)strlen(label) + 1);

  h->nlmsg_len = b->len - (uint)((char *)h - b->p);
}

static void
dump_addrs(struct nlbuf *b, uint32 seq)
{
  addr_msg(b, seq, NL_LO_INDEX, htonl_u32(0x7F000001), 8, "lo", 1);

  for (int i = 0; i < netdev_count(); i++) {
    struct device *d = netdev_at(i);
    if (!d) continue;
    struct netif *nif = net_eth_netif(i);
    if (!nif) continue;

    uint32 a = netif_addr4(nif);
    if (a == 0)
      continue;                                 /* no lease yet: not an address */
    addr_msg(b, seq, NL_DEV_BASE + i, a, prefix_of(netif_mask4(nif)), d->name, 0);
  }
}

// A request has arrived. Generate the whole answer now; see the note at the top
// about why this is not done lazily on read.
//
// Returns the number of bytes of `req` consumed, which is all of them: a
// malformed request is answered with an empty dump rather than an error,
// because the caller's next move is a read either way and an error message it
// does not expect is worse than nothing to read.
int
netlink_request(const char *req, uint reqlen, char *out, uint outcap)
{
  struct nlbuf b = { .p = out, .len = 0, .cap = outcap, .full = 0 };

  if (reqlen < sizeof(struct nlmsghdr))
    return 0;

  const struct nlmsghdr *h = (const struct nlmsghdr *)req;
  uint32 seq = h->nlmsg_seq;

  switch (h->nlmsg_type) {
  case RTM_GETLINK: dump_links(&b, seq); break;
  case RTM_GETADDR: dump_addrs(&b, seq); break;
  default:                                      /* nothing to say about it */
    break;
  }
  nl_done(&b, seq);

  // Truncation would hand the reader a half-written message and it would walk
  // off the end of it. Better to answer with only the terminator: an empty
  // interface list is a legible answer, a corrupt one is not.
  if (b.full) {
    b.len = 0; b.full = 0;
    nl_done(&b, seq);
  }
  return (int)b.len;
}