libc/system_properties/prop_area.h

/*
 * Copyright (C) 2008 The Android Open Source Project
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *  * Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *  * Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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 */

#include <stdatomic.h>
#include <stdint.h>
#include <string.h>
#include <sys/mman.h>

#include "private/bionic_macros.h"

#include "prop_info.h"

#ifndef SYSTEM_PROPERTIES_PROP_AREA_H
#define SYSTEM_PROPERTIES_PROP_AREA_H

// Properties are stored in a hybrid trie/binary tree structure.
// Each property's name is delimited at '.' characters, and the tokens are put
// into a trie structure.  Siblings at each level of the trie are stored in a
// binary tree.  For instance, "ro.secure"="1" could be stored as follows:
//
// +-----+   children    +----+   children    +--------+
// |     |-------------->| ro |-------------->| secure |
// +-----+               +----+               +--------+
//                       /    \                /   |
//                 left /      \ right   left /    |  prop   +===========+
//                     v        v            v     +-------->| ro.secure |
//                  +-----+   +-----+     +-----+            +-----------+
//                  | net |   | sys |     | com |            |     1     |
//                  +-----+   +-----+     +-----+            +===========+

// Represents a node in the trie.
struct prop_bt {
  uint32_t namelen;

  // The property trie is updated only by the init process (single threaded) which provides
  // property service. And it can be read by multiple threads at the same time.
  // As the property trie is not protected by locks, we use atomic_uint_least32_t types for the
  // left, right, children "pointers" in the trie node. To make sure readers who see the
  // change of "pointers" can also notice the change of prop_bt structure contents pointed by
  // the "pointers", we always use release-consume ordering pair when accessing these "pointers".

  // prop "points" to prop_info structure if there is a propery associated with the trie node.
  // Its situation is similar to the left, right, children "pointers". So we use
  // atomic_uint_least32_t and release-consume ordering to protect it as well.

  // We should also avoid rereading these fields redundantly, since not
  // all processor implementations ensure that multiple loads from the
  // same field are carried out in the right order.
  atomic_uint_least32_t prop;

  atomic_uint_least32_t left;
  atomic_uint_least32_t right;

  atomic_uint_least32_t children;

  char name[0];

  prop_bt(const char* name, const uint32_t name_length) {
    this->namelen = name_length;
    memcpy(this->name, name, name_length);
    this->name[name_length] = '\0';
  }

 private:
  DISALLOW_COPY_AND_ASSIGN(prop_bt);
};

class prop_area {
 public:
  static prop_area* map_prop_area_rw(const char* filename, const char* context,
                                     bool* fsetxattr_failed);
  static prop_area* map_prop_area(const char* filename);
  static void unmap_prop_area(prop_area** pa) {
    if (*pa) {
      munmap(*pa, pa_size_);
      *pa = nullptr;
    }
  }

  prop_area(const uint32_t magic, const uint32_t version) : magic_(magic), version_(version) {
    atomic_init(&serial_, 0);
    memset(reserved_, 0, sizeof(reserved_));
    // Allocate enough space for the root node.
    bytes_used_ = sizeof(prop_bt);
  }

  const prop_info* find(const char* name);
  bool add(const char* name, unsigned int namelen, const char* value, unsigned int valuelen);

  bool foreach (void (*propfn)(const prop_info* pi, void* cookie), void* cookie);

  atomic_uint_least32_t* serial() {
    return &serial_;
  }
  uint32_t magic() const {
    return magic_;
  }
  uint32_t version() const {
    return version_;
  }

 private:
  static prop_area* map_fd_ro(const int fd);

  void* allocate_obj(const size_t size, uint_least32_t* const off);
  prop_bt* new_prop_bt(const char* name, uint32_t namelen, uint_least32_t* const off);
  prop_info* new_prop_info(const char* name, uint32_t namelen, const char* value, uint32_t valuelen,
                           uint_least32_t* const off);
  void* to_prop_obj(uint_least32_t off);
  prop_bt* to_prop_bt(atomic_uint_least32_t* off_p);
  prop_info* to_prop_info(atomic_uint_least32_t* off_p);

  prop_bt* root_node();

  prop_bt* find_prop_bt(prop_bt* const bt, const char* name, uint32_t namelen, bool alloc_if_needed);

  const prop_info* find_property(prop_bt* const trie, const char* name, uint32_t namelen,
                                 const char* value, uint32_t valuelen, bool alloc_if_needed);

  bool foreach_property(prop_bt* const trie, void (*propfn)(const prop_info* pi, void* cookie),
                        void* cookie);

  // The original design doesn't include pa_size or pa_data_size in the prop_area struct itself.
  // Since we'll need to be backwards compatible with that design, we don't gain much by adding it
  // now, especially since we don't have any plans to make different property areas different sizes,
  // and thus we share these two variables among all instances.
  static size_t pa_size_;
  static size_t pa_data_size_;

  uint32_t bytes_used_;
  atomic_uint_least32_t serial_;
  uint32_t magic_;
  uint32_t version_;
  uint32_t reserved_[28];
  char data_[0];

  DISALLOW_COPY_AND_ASSIGN(prop_area);
};

#endif