path: root/resolv/resolv_conf.c

/* Extended resolver state separate from struct __res_state.
   Copyright (C) 2017-2025 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 <resolv_conf.h>

#include <alloc_buffer.h>
#include <assert.h>
#include <libc-lock.h>
#include <resolv-internal.h>
#include <sys/stat.h>
#include <libc-symbols.h>
#include <file_change_detection.h>

/* _res._u._ext.__glibc_extension_index is used as an index into a
   struct resolv_conf_array object.  The intent of this construction
   is to make reasonably sure that even if struct __res_state objects
   are copied around and patched by applications, we can still detect
   accesses to stale extended resolver state.  The array elements are
   either struct resolv_conf * pointers (if the LSB is cleared) or
   free list entries (if the LSB is set).  The free list is used to
   speed up finding available entries in the array.  */
#define DYNARRAY_STRUCT resolv_conf_array
#define DYNARRAY_ELEMENT uintptr_t
#define DYNARRAY_PREFIX resolv_conf_array_
#define DYNARRAY_INITIAL_SIZE 0
#include <malloc/dynarray-skeleton.c>

/* A magic constant for XORing the extension index
   (_res._u._ext.__glibc_extension_index).  This makes it less likely
   that a valid index is created by accident.  In particular, a zero
   value leads to an invalid index.  */
#define INDEX_MAGIC 0x26a8fa5e48af8061ULL

/* Global resolv.conf-related state.  */
struct resolv_conf_global
{
  /* struct __res_state objects contain the extension index
     (_res._u._ext.__glibc_extension_index ^ INDEX_MAGIC), which
     refers to an element of this array.  When a struct resolv_conf
     object (extended resolver state) is associated with a struct
     __res_state object (legacy resolver state), its reference count
     is increased and added to this array.  Conversely, if the
     extended state is detached from the basic state (during
     reinitialization or deallocation), the index is decremented, and
     the array element is overwritten with NULL.  */
  struct resolv_conf_array array;

  /* Start of the free list in the array.  Zero if the free list is
     empty.  Otherwise, free_list_start >> 1 is the first element of
     the free list (and the free list entries all have their LSB set
     and are shifted one to the left).  */
  uintptr_t free_list_start;

  /* Cached current configuration object for /etc/resolv.conf.  */
  struct resolv_conf *conf_current;

  /* File system identification for /etc/resolv.conf.  */
  struct file_change_detection file_resolve_conf;
};

/* Lazily allocated storage for struct resolv_conf_global.  */
static struct resolv_conf_global *global;

/* The lock synchronizes access to global and *global.  It also
   protects the __refcount member of struct resolv_conf.  */
__libc_lock_define_initialized (static, lock);

/* Ensure that GLOBAL is allocated and lock it.  Return NULL if
   memory allocation fails.  */
static struct resolv_conf_global *
get_locked_global (void)
{
  __libc_lock_lock (lock);
  /* Use relaxed MO through because of load outside the lock in
     __resolv_conf_detach.  */
  struct resolv_conf_global *global_copy = atomic_load_relaxed (&global);
  if (global_copy == NULL)
    {
      global_copy = calloc (1, sizeof (*global));
      if (global_copy == NULL)
	{
	  __libc_lock_unlock (lock);
	  return NULL;
	}
      atomic_store_relaxed (&global, global_copy);
      resolv_conf_array_init (&global_copy->array);
    }
  return global_copy;
}

/* Relinquish the lock acquired by get_locked_global.  */
static void
put_locked_global (struct resolv_conf_global *global_copy)
{
  __libc_lock_unlock (lock);
}

/* Decrement the reference counter.  The caller must acquire the lock
   around the function call.  */
static void
conf_decrement (struct resolv_conf *conf)
{
  assert (conf->__refcount > 0);
  if (--conf->__refcount == 0)
    free (conf);
}

struct resolv_conf *
__resolv_conf_get_current (void)
{
  struct file_change_detection initial;
  if (!__file_change_detection_for_path (&initial, _PATH_RESCONF))
    return NULL;

  struct resolv_conf_global *global_copy = get_locked_global ();
  if (global_copy == NULL)
    return NULL;
  struct resolv_conf *conf;
  if (global_copy->conf_current != NULL
      && __file_is_unchanged (&