glibc/sysdeps/unix/sysv/linux/check_pf.c

/* Determine protocol families for which interfaces exist.  Linux version.
   Copyright (C) 2003-2024 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

#include <assert.h>
#include <errno.h>
#include <ifaddrs.h>
#include <netdb.h>
#include <stddef.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <stdint.h>
#include <sys/socket.h>

#include <asm/types.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>

#include <not-cancel.h>
#include <libc-lock.h>
#include <atomic.h>
#include <nscd/nscd-client.h>

#include "netlinkaccess.h"

#ifndef IFA_F_HOMEADDRESS
# define IFA_F_HOMEADDRESS 0
#endif
#ifndef IFA_F_OPTIMISTIC
# define IFA_F_OPTIMISTIC 0
#endif


struct cached_data
{
  uint32_t timestamp;
  uint32_t usecnt;
  bool seen_ipv4;
  bool seen_ipv6;
  size_t in6ailen;
  struct in6addrinfo in6ai[0];
};

static struct cached_data noai6ai_cached =
  {
    .usecnt = 1,	/* Make sure we never try to delete this entry.  */
    .in6ailen = 0
  };

static struct cached_data *cache;
__libc_lock_define_initialized (static, lock);


static inline uint32_t
get_nl_timestamp (void)
{
#if defined USE_NSCD
  return __nscd_get_nl_timestamp ();
#else
  return 0;
#endif
}

static inline bool
cache_valid_p (void)
{
  if (cache != NULL)
    {
      uint32_t timestamp = get_nl_timestamp ();
      return timestamp != 0 && cache->timestamp == timestamp;
    }
  return false;
}


static struct cached_data *
make_request (int fd, pid_t pid)
{
  struct cached_data *result = NULL;

  size_t result_len = 0;
  size_t result_cap = 32;

  struct req
  {
    struct nlmsghdr nlh;
    struct rtgenmsg g;
    /* struct rtgenmsg consists of a single byte.  This means there
       are three bytes of padding included in the REQ definition.
       We make them explicit here.  */
    char pad[3];
  } req;
  struct sockaddr_nl nladdr;

  req.nlh.nlmsg_len = sizeof (req);
  req.nlh.nlmsg_type = RTM_GETADDR;
  req.nlh.nlmsg_flags = NLM_F_ROOT | NLM_F_MATCH | NLM_F_REQUEST;
  req.nlh.nlmsg_pid = 0;
  req.nlh.nlmsg_seq = time_now ();
  req.g.rtgen_family = AF_UNSPEC;

  assert (sizeof (req) - offsetof (struct req, pad) == 3);
  memset (req.pad, '\0', sizeof (req.pad));

  memset (&nladdr, '\0', sizeof (nladdr));
  nladdr.nl_family = AF_NETLINK;

#ifdef PAGE_SIZE
  const size_t buf_size = PAGE_SIZE;
#else
  const size_t buf_size = 4096;
#endif
  char buf[buf_size];

  struct iovec iov = { buf, buf_size };

  if (TEMP_FAILURE_RETRY (__sendto (fd, (void *) &req, sizeof (req), 0,
				    (struct sockaddr *) &nladdr,
				    sizeof (nladdr))) < 0)
    goto out_fail;

  bool done = false;

  bool seen_ipv4 = false;
  bool seen_ipv6 = false;

  do
    {
      struct msghdr msg =
	{
	  .msg_name = (void *) &nladdr,
	  .msg_namelen =  sizeof (nladdr),
	  .msg_iov = &iov,
	  .msg_iovlen = 1,
	  .msg_control = NULL,
	  .msg_controllen = 0,
	  .msg_flags = 0
	};

      ssize_t read_len = TEMP_FAILURE_RETRY (__recvmsg (fd, &msg, 0));
      __netlink_assert_response (fd, read_len);
      if (read_len < 0)
	goto out_fail;

      if (msg.msg_flags & MSG_TRUNC)
	goto out_fail;

      struct nlmsghdr *nlmh;
      for (nlmh = (struct nlmsghdr *) buf;
	   NLMSG_OK (nlmh, (size_t) read_len);
	   nlmh = (struct nlmsghdr *) NLMSG_NEXT (nlmh, read_len))
	{
	  if (nladdr.nl_pid != 0 || (pid_t) nlmh->nlmsg_pid != pid
	      || nlmh->nlmsg_seq != req.nlh.nlmsg_seq)
	    continue;

	  if (nlmh->nlmsg_type == RTM_NEWADDR)
	    {
	      struct ifaddrmsg *ifam = (struct ifaddrmsg *) NLMSG_DATA (nlmh);
	      struct rtattr *rta = IFA_RTA (ifam);
	      size_t len = nlmh->nlmsg_len - NLMSG_LENGTH (sizeof (*ifam));

	      if (ifam->ifa_family != AF_INET
		  && ifam->ifa_family != AF_INET6)
		continue;

	      const void *local = NULL;
	      const void *address = NULL;
	      while (RTA_OK (rta, len))
		{
		  switch (rta->rta_type)
		    {
		    case IFA_LOCAL:
		      local = RTA_DATA (rta);
		      break;

		    case IFA_ADDRESS:
		      address = RTA_DATA (rta);
		      goto out;
		    }

		  rta = RTA_NEXT (rta, len);
		}

	      if (local != NULL)
		{
		  address = local;
		out:
		  if (ifam->ifa_family == AF_INET)
		    {
		      if (*(const in_addr_t *) address
			  != htonl (INADDR_LOOPBACK))
			seen_ipv4 = true;
		    }
		  else
		    {
		      if (!IN6_IS_ADDR_LOOPBACK (address))
			seen_ipv6 = true;
		    }
		}

	      if (result_len == 0 || result_len == result_cap)
		{
		  result_cap = 2 * result_cap;
		  result = realloc (result, sizeof (*result)
				    + result_cap
				      * sizeof (struct in6addrinfo));
		}

	      if (!result)
		goto out_fail;

	      struct in6addrinfo *info = &result->in6ai[result_len++];

	      info->flags = (((ifam->ifa_flags
			       & (IFA_F_DEPRECATED | IFA_F_OPTIMISTIC))
			      ? in6ai_deprecated : 0)
			     | ((ifam->ifa_flags & IFA_F_HOMEADDRESS)
			         ? in6ai_homeaddress : 0));
	      info->prefixlen = ifam->ifa_prefixlen;
	      info->index = ifam->ifa_index;
	      if (ifam->ifa_family == AF_INET)
		{
		  info->addr[0] = 0;
		  info->addr[1] = 0;
		  info->addr[2] = htonl (0xffff);
		  info->addr[3] = *(const in_addr_t *) address;
		}
	      else
		memcpy (info->addr, address, sizeof (info->addr));
	    }
	  else if (nlmh->nlmsg_type == NLMSG_DONE)
	    /* We found the end, leave the loop.  */
	    done = true;
	}
    }
  while (! done);

  if (seen_ipv6 && result != NULL)
    {
      result->timestamp = get_nl_timestamp ();
      result->usecnt = 2;
      result->seen_ipv4 = seen_ipv4;
      result->seen_ipv6 = true;
      result->in6ailen = result_len;
    }
  else
    {
      free (result);

      atomic_fetch_add_relaxed (&noai6ai_cached.usecnt, 2);
      noai6ai_cached.seen_ipv4 = seen_ipv4;
      noai6ai_cached.seen_ipv6 = seen_ipv6;
      result = &noai6ai_cached;
    }

  return result;

 out_fail:

  free (result);
  return NULL;
}

#ifdef __EXCEPTIONS
static void
cancel_handler (void *arg __attribute__((unused)))
{
  /* Release the lock.  */
  __libc_lock_unlock (lock);
}
#endif

void
attribute_hidden
__check_pf (bool *seen_ipv4, bool *seen_ipv6,
	    struct in6addrinfo **in6ai, size_t *in6ailen)
{
  *in6ai = NULL;
  *in6ailen = 0;

  struct cached_data *olddata = NULL;
  struct cached_data *data = NULL;

#ifdef __EXCEPTIONS
  /* Make sure that lock is released when the thread is cancelled.  */
  __libc_cleanup_push (cancel_handler, NULL);
#endif
  __libc_lock_lock (lock);

  if (cache_valid_p ())
    {
      data = cache;
      atomic_fetch_add_relaxed (&cache->usecnt, 1);
    }
  else
    {
      int fd = __socket (PF_NETLINK, SOCK_RAW | SOCK_CLOEXEC, NETLINK_ROUTE);

      if (__glibc_likely (fd >= 0))
	{
	  struct sockaddr_nl nladdr;
	  memset (&nladdr, '\0', sizeof (nladdr));
	  nladdr.nl_family = AF_NETLINK;

	  socklen_t addr_len = sizeof (nladdr);

	  if (__bind (fd, (struct sockaddr *) &nladdr, sizeof (nladdr)) == 0
	      && __getsockname (fd, (struct sockaddr *) &nladdr,
				&addr_len) == 0)
	    data = make_request (fd, nladdr.nl_pid);

	  __close_nocancel_nostatus (fd);
	}

      if (data != NULL)
	{
	  olddata = cache;
	  cache = data;
	}
    }

#ifdef __EXCEPTIONS
  __libc_cleanup_pop (0);
#endif
  __libc_lock_unlock (lock);

  if (data != NULL)
    {
      /* It worked.  */
      *seen_ipv4 = data->seen_ipv4;
      *seen_ipv6 = data->seen_ipv6;
      *in6ailen = data->in6ailen;
      *in6ai = data->in6ai;

      if (olddata != NULL && olddata->usecnt > 0
	  && atomic_fetch_add_relaxed (&olddata->usecnt, -1) == 1)
	free (olddata);

      return;
    }

  /* We cannot determine what interfaces are available.  Be
     pessimistic.  */
  *seen_ipv4 = true;
  *seen_ipv6 = true;
}

/* Free the cache if it has been allocated.  */
void
__check_pf_freemem (void)
{
  if (cache)
    __free_in6ai (cache->in6ai);
}

void
__free_in6ai (struct in6addrinfo *ai)
{
  if (ai != NULL)
    {
      struct cached_data *data =
	(struct cached_data *) ((char *) ai
				- offsetof (struct cached_data, in6ai));

      if (atomic_fetch_add_relaxed (&data->usecnt, -1) == 1)
	{
	  __libc_lock_lock (lock);

	  if (data->usecnt == 0)
	    /* Still unused.  */
	    free (data);

	  __libc_lock_unlock (lock);
	}
    }
}