libc/system_properties/system_properties.cpp - platform/bionic.git - Git at Google

 /*
  * 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,
  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */

 #include "system_properties/system_properties.h"

 #include <errno.h>
 #include <private/android_filesystem_config.h>
 #include <stdatomic.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>

 #include <new>

 #include <async_safe/CHECK.h>
 #include <async_safe/log.h>

 #include "private/ErrnoRestorer.h"
 #include "private/bionic_futex.h"

 #include "system_properties/context_node.h"
 #include "system_properties/prop_area.h"
 #include "system_properties/prop_info.h"

 #define SERIAL_DIRTY(serial) ((serial)&1)
 #define SERIAL_VALUE_LEN(serial) ((serial) >> 24)
 #define APPCOMPAT_PREFIX "ro.appcompat_override."

 static bool is_dir(const char* pathname) {
   struct stat info;
   if (stat(pathname, &info) == -1) {
     return false;
   }
   return S_ISDIR(info.st_mode);
 }

 bool SystemProperties::Init(const char* filename) {
   // This is called from __libc_init_common, and should leave errno at 0 (http://b/37248982).
   ErrnoRestorer errno_restorer;

   if (initialized_) {
     contexts_->ResetAccess();
     return true;
   }

   properties_filename_ = filename;

   if (!InitContexts(false)) {
     return false;
   }

   initialized_ = true;
   return true;
 }

 bool SystemProperties::InitContexts(bool load_default_path) {
   if (is_dir(properties_filename_.c_str())) {
     if (access(PROP_TREE_FILE, R_OK) == 0) {
       auto serial_contexts = new (contexts_data_) ContextsSerialized();
       contexts_ = serial_contexts;
       if (!serial_contexts->Initialize(false, properties_filename_.c_str(), nullptr,
                                        load_default_path)) {
         return false;
       }
     } else {
       contexts_ = new (contexts_data_) ContextsSplit();
       if (!contexts_->Initialize(false, properties_filename_.c_str(), nullptr)) {
         return false;
       }
     }
   } else {
     contexts_ = new (contexts_data_) ContextsPreSplit();
     if (!contexts_->Initialize(false, properties_filename_.c_str(), nullptr)) {
       return false;
     }
   }
   return true;
 }

 bool SystemProperties::AreaInit(const char* filename, bool* fsetxattr_failed) {
   return AreaInit(filename, fsetxattr_failed, false);
 }

 // Note: load_default_path is only used for testing, as it will cause properties to be loaded from
 // one file (specified by PropertyInfoAreaFile.LoadDefaultPath), but be written to "filename".
 bool SystemProperties::AreaInit(const char* filename, bool* fsetxattr_failed,
                                 bool load_default_path) {
   properties_filename_ = filename;
   auto serial_contexts = new (contexts_data_) ContextsSerialized();
   contexts_ = serial_contexts;
   if (!serial_contexts->Initialize(true, properties_filename_.c_str(), fsetxattr_failed,
                                    load_default_path)) {
     return false;
   }

   appcompat_filename_ = PropertiesFilename(properties_filename_.c_str(), "appcompat_override");
   appcompat_override_contexts_ = nullptr;
   if (access(appcompat_filename_.c_str(), F_OK) != -1) {
     auto* appcompat_contexts = new (appcompat_override_contexts_data_) ContextsSerialized();
     if (!appcompat_contexts->Initialize(true, appcompat_filename_.c_str(), fsetxattr_failed,
                                         load_default_path)) {
       // The appcompat folder exists, but initializing it failed
       return false;
     } else {
       appcompat_override_contexts_ = appcompat_contexts;
     }
   }

   initialized_ = true;
   return true;
 }

 bool SystemProperties::Reload(bool load_default_path) {
   if (!initialized_) {
     return true;
   }

   return InitContexts(load_default_path);
 }

 uint32_t SystemProperties::AreaSerial() {
   if (!initialized_) {
     return -1;
   }

   prop_area* pa = contexts_->GetSerialPropArea();
   if (!pa) {
     return -1;
   }

   // Make sure this read fulfilled before __system_property_serial
   return atomic_load_explicit(pa->serial(), memory_order_acquire);
 }

 const prop_info* SystemProperties::Find(const char* name) {
   if (!initialized_) {
     return nullptr;
   }

   prop_area* pa = contexts_->GetPropAreaForName(name);
   if (!pa) {
     async_safe_format_log(ANDROID_LOG_WARN, "libc", "Access denied finding property \"%s\"", name);
     return nullptr;
   }

   return pa->find(name);
 }

 static bool is_appcompat_override(const char* name) {
   return strncmp(name, APPCOMPAT_PREFIX, strlen(APPCOMPAT_PREFIX)) == 0;
 }

 static bool is_read_only(const char* name) {
   return strncmp(name, "ro.", 3) == 0;
 }

 uint32_t SystemProperties::ReadMutablePropertyValue(const prop_info* pi, char* value) {
   // We assume the memcpy below gets serialized by the acquire fence.
   uint32_t new_serial = load_const_atomic(&pi->serial, memory_order_acquire);
   uint32_t serial;
   unsigned int len;
   for (;;) {
     serial = new_serial;
     len = SERIAL_VALUE_LEN(serial);
     if (__predict_false(SERIAL_DIRTY(serial))) {
       // See the comment in the prop_area constructor.
       prop_area* pa = contexts_->GetPropAreaForName(pi->name);
       memcpy(value, pa->dirty_backup_area(), len + 1);
     } else {
       memcpy(value, pi->value, len + 1);
     }
     atomic_thread_fence(memory_order_acquire);
     new_serial = load_const_atomic(&pi->serial, memory_order_relaxed);
     if (__predict_true(serial == new_serial)) {
       break;
     }
     // We need another fence here because we want to ensure that the memcpy in the
     // next iteration of the loop occurs after the load of new_serial above. We could
     // get this guarantee by making the load_const_atomic of new_serial
     // memory_order_acquire instead of memory_order_relaxed, but then we'd pay the
     // penalty of the memory_order_acquire even in the overwhelmingly common case
     // that the serial number didn't change.
     atomic_thread_fence(memory_order_acquire);
   }
   return serial;
 }

 int SystemProperties::Read(const prop_info* pi, char* name, char* value) {
   uint32_t serial = ReadMutablePropertyValue(pi, value);
   if (name != nullptr) {
     size_t namelen = strlcpy(name, pi->name, PROP_NAME_MAX);
     if (namelen >= PROP_NAME_MAX) {
       async_safe_format_log(ANDROID_LOG_ERROR, "libc",
                             "The property name length for \"%s\" is >= %d;"
                             " please use __system_property_read_callback"
                             " to read this property. (the name is truncated to \"%s\")",
                             pi->name, PROP_NAME_MAX - 1, name);
     }
   }
   if (is_read_only(pi->name) && pi->is_long()) {
     async_safe_format_log(
         ANDROID_LOG_ERROR, "libc",
         "The property \"%s\" has a value with length %zu that is too large for"
         " __system_property_get()/__system_property_read(); use"
         " __system_property_read_callback() instead.",
         pi->name, strlen(pi->long_value()));
   }
   return SERIAL_VALUE_LEN(serial);
 }

 void SystemProperties::ReadCallback(const prop_info* pi,
                                     void (*callback)(void* cookie, const char* name,
                                                      const char* value, uint32_t serial),
                                     void* cookie) {
   // Read only properties don't need to copy the value to a temporary buffer, since it can never
   // change.  We use relaxed memory order on the serial load for the same reason.
   if (is_read_only(pi->name)) {
     uint32_t serial = load_const_atomic(&pi->serial, memory_order_relaxed);
     if (pi->is_long()) {
       callback(cookie, pi->name, pi->long_value(), serial);
     } else {
       callback(cookie, pi->name, pi->value, serial);
     }
     return;
   }

   char value_buf[PROP_VALUE_MAX];
   uint32_t serial = ReadMutablePropertyValue(pi, value_buf);
   callback(cookie, pi->name, value_buf, serial);
 }

 int SystemProperties::Get(const char* name, char* value) {
   const prop_info* pi = Find(name);

   if (pi != nullptr) {
     return Read(pi, nullptr, value);
   } else {
     value[0] = 0;
     return 0;
   }
 }

 int SystemProperties::Update(prop_info* pi, const char* value, unsigned int len) {
   if (len >= PROP_VALUE_MAX) {
     return -1;
   }

   if (!initialized_) {
     return -1;
   }
   bool have_override = appcompat_override_contexts_ != nullptr;

   prop_area* serial_pa = contexts_->GetSerialPropArea();
   prop_area* override_serial_pa =
       have_override ? appcompat_override_contexts_->GetSerialPropArea() : nullptr;
   if (!serial_pa) {
     return -1;
   }
   prop_area* pa = contexts_->GetPropAreaForName(pi->name);
   prop_area* override_pa =
       have_override ? appcompat_override_contexts_->GetPropAreaForName(pi->name) : nullptr;
   if (__predict_false(!pa)) {
     async_safe_format_log(ANDROID_LOG_ERROR, "libc", "Could not find area for \"%s\"", pi->name);
     return -1;
   }
   CHECK(!have_override || (override_pa && override_serial_pa));

   auto* override_pi = const_cast<prop_info*>(have_override ? override_pa->find(pi->name) : nullptr);

   uint32_t serial = atomic_load_explicit(&pi->serial, memory_order_relaxed);
   unsigned int old_len = SERIAL_VALUE_LEN(serial);

   // The contract with readers is that whenever the dirty bit is set, an undamaged copy
   // of the pre-dirty value is available in the dirty backup area. The fence ensures
   // that we publish our dirty area update before allowing readers to see a
   // dirty serial.
   memcpy(pa->dirty_backup_area(), pi->value, old_len + 1);
   if (have_override) {
     memcpy(override_pa->dirty_backup_area(), override_pi->value, old_len + 1);
   }
   atomic_thread_fence(memory_order_release);
   serial |= 1;
   atomic_store_explicit(&pi->serial, serial, memory_order_relaxed);
   strlcpy(pi->value, value, len + 1);
   if (have_override) {
     atomic_store_explicit(&override_pi->serial, serial, memory_order_relaxed);
     strlcpy(override_pi->value, value, len + 1);
   }
   // Now the primary value property area is up-to-date. Let readers know that they should
   // look at the property value instead of the backup area.
   atomic_thread_fence(memory_order_release);
   int new_serial = (len << 24) | ((serial + 1) & 0xffffff);
   atomic_store_explicit(&pi->serial, new_serial, memory_order_relaxed);
   if (have_override) {
     atomic_store_explicit(&override_pi->serial, new_serial, memory_order_relaxed);
   }
   __futex_wake(&pi->serial, INT32_MAX);  // Fence by side effect
   atomic_store_explicit(serial_pa->serial(),
                         atomic_load_explicit(serial_pa->serial(), memory_order_relaxed) + 1,
                         memory_order_release);
   if (have_override) {
     atomic_store_explicit(override_serial_pa->serial(),
                           atomic_load_explicit(serial_pa->serial(), memory_order_relaxed) + 1,
                           memory_order_release);
   }
   __futex_wake(serial_pa->serial(), INT32_MAX);

   return 0;
 }

 int SystemProperties::Add(const char* name, unsigned int namelen, const char* value,
                           unsigned int valuelen) {
   if (namelen < 1) {
     async_safe_format_log(ANDROID_LOG_ERROR, "libc",
                           "__system_property_add failed: name length 0");
     return -1;
   }

   if (valuelen >= PROP_VALUE_MAX && !is_read_only(name)) {
     async_safe_format_log(ANDROID_LOG_ERROR, "libc",
                           "__system_property_add failed: \"%s\" value too long: %d >= PROP_VALUE_MAX",
                           name, valuelen);
     return -1;
   }

   if (!initialized_) {
     async_safe_format_log(ANDROID_LOG_ERROR, "libc",
                           "__system_property_add failed: properties not initialized");
     return -1;
   }

   prop_area* serial_pa = contexts_->GetSerialPropArea();
   if (serial_pa == nullptr) {
     async_safe_format_log(ANDROID_LOG_ERROR, "libc",
                           "__system_property_add failed: property area not found");
     return -1;
   }

   prop_area* pa = contexts_->GetPropAreaForName(name);
   if (!pa) {
     async_safe_format_log(ANDROID_LOG_ERROR, "libc",
                           "__system_property_add failed: access denied for \"%s\"", name);
     return -1;
   }

   if (!pa->add(name, namelen, value, valuelen)) {
     async_safe_format_log(ANDROID_LOG_ERROR, "libc",
                           "__system_property_add failed: add failed for \"%s\"", name);
     return -1;
   }

   if (appcompat_override_contexts_ != nullptr) {
     bool is_override = is_appcompat_override(name);
     const char* override_name = name;
     if (is_override) override_name += strlen(APPCOMPAT_PREFIX);
     prop_area* other_pa = appcompat_override_contexts_->GetPropAreaForName(override_name);
     prop_area* other_serial_pa = appcompat_override_contexts_->GetSerialPropArea();
     CHECK(other_pa && other_serial_pa);
     // We may write a property twice to overrides, once for the ro.*, and again for the
     // ro.appcompat_override.ro.* property. If we've already written, then we should essentially
     // perform an Update, not an Add.
     auto other_pi = const_cast<prop_info*>(other_pa->find(override_name));
     if (!other_pi) {
       if (other_pa->add(override_name, strlen(override_name), value, valuelen)) {
         atomic_store_explicit(
             other_serial_pa->serial(),
             atomic_load_explicit(other_serial_pa->serial(), memory_order_relaxed) + 1,
             memory_order_release);
       }
     } else if (is_override) {
       // We already wrote the ro.*, but appcompat_override.ro.* should override that. We don't
       // need to do the usual dirty bit setting, as this only happens during the init process,
       // before any readers are started. Check that only init or root can write appcompat props.
       CHECK(getpid() == 1 || getuid() == 0);
       atomic_thread_fence(memory_order_release);
       strlcpy(other_pi->value, value, valuelen + 1);
     }
   }

   // There is only a single mutator, but we want to make sure that
   // updates are visible to a reader waiting for the update.
   atomic_store_explicit(serial_pa->serial(),
                         atomic_load_explicit(serial_pa->serial(), memory_order_relaxed) + 1,
                         memory_order_release);
   __futex_wake(serial_pa->serial(), INT32_MAX);
   return 0;
 }

 uint32_t SystemProperties::WaitAny(uint32_t old_serial) {
   uint32_t new_serial;
   Wait(nullptr, old_serial, &new_serial, nullptr);
   return new_serial;
 }

 bool SystemProperties::Wait(const prop_info* pi, uint32_t old_serial, uint32_t* new_serial_ptr,
                             const timespec* relative_timeout) {
   // Are we waiting on the global serial or a specific serial?
   atomic_uint_least32_t* serial_ptr;
   if (pi == nullptr) {
     if (!initialized_) {
       return -1;
     }

     prop_area* serial_pa = contexts_->GetSerialPropArea();
     if (serial_pa == nullptr) {
       return -1;
     }

     serial_ptr = serial_pa->serial();
   } else {
     serial_ptr = const_cast<atomic_uint_least32_t*>(&pi->serial);
   }

   uint32_t new_serial;
   do {
     int rc;
     if ((rc = __futex_wait(serial_ptr, old_serial, relative_timeout)) != 0 && rc == -ETIMEDOUT) {
       return false;
     }
     new_serial = load_const_atomic(serial_ptr, memory_order_acquire);
   } while (new_serial == old_serial);

   *new_serial_ptr = new_serial;
   return true;
 }

 const prop_info* SystemProperties::FindNth(unsigned n) {
   struct find_nth {
     const uint32_t sought;
     uint32_t current;
     const prop_info* result;

     explicit find_nth(uint32_t n) : sought(n), current(0), result(nullptr) {
     }
     static void fn(const prop_info* pi, void* ptr) {
       find_nth* self = reinterpret_cast<find_nth*>(ptr);
       if (self->current++ == self->sought) self->result = pi;
     }
   } state(n);
   Foreach(find_nth::fn, &state);
   return state.result;
 }

 int SystemProperties::Foreach(void (*propfn)(const prop_info* pi, void* cookie), void* cookie) {
   if (!initialized_) {
     return -1;
   }

   contexts_->ForEach(propfn, cookie);

   return 0;
 }
	/*
	* 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,
	* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
	* OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	* SUCH DAMAGE.
	*/

	#include "system_properties/system_properties.h"

	#include <errno.h>
	#include <private/android_filesystem_config.h>
	#include <stdatomic.h>
	#include <stdlib.h>
	#include <string.h>
	#include <sys/stat.h>
	#include <sys/types.h>
	#include <unistd.h>

	#include <new>

	#include <async_safe/CHECK.h>
	#include <async_safe/log.h>

	#include "private/ErrnoRestorer.h"
	#include "private/bionic_futex.h"

	#include "system_properties/context_node.h"
	#include "system_properties/prop_area.h"
	#include "system_properties/prop_info.h"

	#define SERIAL_DIRTY(serial) ((serial)&1)
	#define SERIAL_VALUE_LEN(serial) ((serial) >> 24)
	#define APPCOMPAT_PREFIX "ro.appcompat_override."

	static bool is_dir(const char* pathname) {
	struct stat info;
	if (stat(pathname, &info) == -1) {
	return false;
	}
	return S_ISDIR(info.st_mode);
	}

	bool SystemProperties::Init(const char* filename) {
	// This is called from __libc_init_common, and should leave errno at 0 (http://b/37248982).
	ErrnoRestorer errno_restorer;

	if (initialized_) {
	contexts_->ResetAccess();
	return true;
	}

	properties_filename_ = filename;

	if (!InitContexts(false)) {
	return false;
	}

	initialized_ = true;
	return true;
	}

	bool SystemProperties::InitContexts(bool load_default_path) {
	if (is_dir(properties_filename_.c_str())) {
	if (access(PROP_TREE_FILE, R_OK) == 0) {
	auto serial_contexts = new (contexts_data_) ContextsSerialized();
	contexts_ = serial_contexts;
	if (!serial_contexts->Initialize(false, properties_filename_.c_str(), nullptr,
	load_default_path)) {
	return false;
	}
	} else {
	contexts_ = new (contexts_data_) ContextsSplit();
	if (!contexts_->Initialize(false, properties_filename_.c_str(), nullptr)) {
	return false;
	}
	}
	} else {
	contexts_ = new (contexts_data_) ContextsPreSplit();
	if (!contexts_->Initialize(false, properties_filename_.c_str(), nullptr)) {
	return false;
	}
	}
	return true;
	}

	bool SystemProperties::AreaInit(const char* filename, bool* fsetxattr_failed) {
	return AreaInit(filename, fsetxattr_failed, false);
	}

	// Note: load_default_path is only used for testing, as it will cause properties to be loaded from
	// one file (specified by PropertyInfoAreaFile.LoadDefaultPath), but be written to "filename".
	bool SystemProperties::AreaInit(const char* filename, bool* fsetxattr_failed,
	bool load_default_path) {
	properties_filename_ = filename;
	auto serial_contexts = new (contexts_data_) ContextsSerialized();
	contexts_ = serial_contexts;
	if (!serial_contexts->Initialize(true, properties_filename_.c_str(), fsetxattr_failed,
	load_default_path)) {
	return false;
	}

	appcompat_filename_ = PropertiesFilename(properties_filename_.c_str(), "appcompat_override");
	appcompat_override_contexts_ = nullptr;
	if (access(appcompat_filename_.c_str(), F_OK) != -1) {
	auto* appcompat_contexts = new (appcompat_override_contexts_data_) ContextsSerialized();
	if (!appcompat_contexts->Initialize(true, appcompat_filename_.c_str(), fsetxattr_failed,
	load_default_path)) {
	// The appcompat folder exists, but initializing it failed
	return false;
	} else {
	appcompat_override_contexts_ = appcompat_contexts;
	}
	}

	initialized_ = true;
	return true;
	}

	bool SystemProperties::Reload(bool load_default_path) {
	if (!initialized_) {
	return true;
	}

	return InitContexts(load_default_path);
	}

	uint32_t SystemProperties::AreaSerial() {
	if (!initialized_) {
	return -1;
	}

	prop_area* pa = contexts_->GetSerialPropArea();
	if (!pa) {
	return -1;
	}

	// Make sure this read fulfilled before __system_property_serial
	return atomic_load_explicit(pa->serial(), memory_order_acquire);
	}

	const prop_info* SystemProperties::Find(const char* name) {
	if (!initialized_) {
	return nullptr;
	}

	prop_area* pa = contexts_->GetPropAreaForName(name);
	if (!pa) {
	async_safe_format_log(ANDROID_LOG_WARN, "libc", "Access denied finding property \"%s\"", name);
	return nullptr;
	}

	return pa->find(name);
	}

	static bool is_appcompat_override(const char* name) {
	return strncmp(name, APPCOMPAT_PREFIX, strlen(APPCOMPAT_PREFIX)) == 0;
	}

	static bool is_read_only(const char* name) {
	return strncmp(name, "ro.", 3) == 0;
	}

	uint32_t SystemProperties::ReadMutablePropertyValue(const prop_info* pi, char* value) {
	// We assume the memcpy below gets serialized by the acquire fence.
	uint32_t new_serial = load_const_atomic(&pi->serial, memory_order_acquire);
	uint32_t serial;
	unsigned int len;
	for (;;) {
	serial = new_serial;
	len = SERIAL_VALUE_LEN(serial);
	if (__predict_false(SERIAL_DIRTY(serial))) {
	// See the comment in the prop_area constructor.
	prop_area* pa = contexts_->GetPropAreaForName(pi->name);
	memcpy(value, pa->dirty_backup_area(), len + 1);
	} else {
	memcpy(value, pi->value, len + 1);
	}
	atomic_thread_fence(memory_order_acquire);
	new_serial = load_const_atomic(&pi->serial, memory_order_relaxed);
	if (__predict_true(serial == new_serial)) {
	break;
	}
	// We need another fence here because we want to ensure that the memcpy in the
	// next iteration of the loop occurs after the load of new_serial above. We could
	// get this guarantee by making the load_const_atomic of new_serial
	// memory_order_acquire instead of memory_order_relaxed, but then we'd pay the
	// penalty of the memory_order_acquire even in the overwhelmingly common case
	// that the serial number didn't change.
	atomic_thread_fence(memory_order_acquire);
	}
	return serial;
	}

	int SystemProperties::Read(const prop_info* pi, char* name, char* value) {
	uint32_t serial = ReadMutablePropertyValue(pi, value);
	if (name != nullptr) {
	size_t namelen = strlcpy(name, pi->name, PROP_NAME_MAX);
	if (namelen >= PROP_NAME_MAX) {
	async_safe_format_log(ANDROID_LOG_ERROR, "libc",
	"The property name length for \"%s\" is >= %d;"
	" please use __system_property_read_callback"
	" to read this property. (the name is truncated to \"%s\")",
	pi->name, PROP_NAME_MAX - 1, name);
	}
	}
	if (is_read_only(pi->name) && pi->is_long()) {
	async_safe_format_log(
	ANDROID_LOG_ERROR, "libc",
	"The property \"%s\" has a value with length %zu that is too large for"
	" __system_property_get()/__system_property_read(); use"
	" __system_property_read_callback() instead.",
	pi->name, strlen(pi->long_value()));
	}
	return SERIAL_VALUE_LEN(serial);
	}

	void SystemProperties::ReadCallback(const prop_info* pi,
	void (callback)(void cookie, const char* name,
	const char* value, uint32_t serial),
	void* cookie) {
	// Read only properties don't need to copy the value to a temporary buffer, since it can never
	// change. We use relaxed memory order on the serial load for the same reason.
	if (is_read_only(pi->name)) {
	uint32_t serial = load_const_atomic(&pi->serial, memory_order_relaxed);
	if (pi->is_long()) {
	callback(cookie, pi->name, pi->long_value(), serial);
	} else {
	callback(cookie, pi->name, pi->value, serial);
	}
	return;
	}

	char value_buf[PROP_VALUE_MAX];
	uint32_t serial = ReadMutablePropertyValue(pi, value_buf);
	callback(cookie, pi->name, value_buf, serial);
	}

	int SystemProperties::Get(const char* name, char* value) {
	const prop_info* pi = Find(name);

	if (pi != nullptr) {
	return Read(pi, nullptr, value);
	} else {
	value[0] = 0;
	return 0;
	}
	}

	int SystemProperties::Update(prop_info* pi, const char* value, unsigned int len) {
	if (len >= PROP_VALUE_MAX) {
	return -1;
	}

	if (!initialized_) {
	return -1;
	}
	bool have_override = appcompat_override_contexts_ != nullptr;

	prop_area* serial_pa = contexts_->GetSerialPropArea();
	prop_area* override_serial_pa =
	have_override ? appcompat_override_contexts_->GetSerialPropArea() : nullptr;
	if (!serial_pa) {
	return -1;
	}
	prop_area* pa = contexts_->GetPropAreaForName(pi->name);
	prop_area* override_pa =
	have_override ? appcompat_override_contexts_->GetPropAreaForName(pi->name) : nullptr;
	if (__predict_false(!pa)) {
	async_safe_format_log(ANDROID_LOG_ERROR, "libc", "Could not find area for \"%s\"", pi->name);
	return -1;
	}
	CHECK(!have_override \|\| (override_pa && override_serial_pa));

	auto* override_pi = const_cast<prop_info*>(have_override ? override_pa->find(pi->name) : nullptr);

	uint32_t serial = atomic_load_explicit(&pi->serial, memory_order_relaxed);
	unsigned int old_len = SERIAL_VALUE_LEN(serial);

	// The contract with readers is that whenever the dirty bit is set, an undamaged copy
	// of the pre-dirty value is available in the dirty backup area. The fence ensures
	// that we publish our dirty area update before allowing readers to see a
	// dirty serial.
	memcpy(pa->dirty_backup_area(), pi->value, old_len + 1);
	if (have_override) {
	memcpy(override_pa->dirty_backup_area(), override_pi->value, old_len + 1);
	}
	atomic_thread_fence(memory_order_release);
	serial \|= 1;
	atomic_store_explicit(&pi->serial, serial, memory_order_relaxed);
	strlcpy(pi->value, value, len + 1);
	if (have_override) {
	atomic_store_explicit(&override_pi->serial, serial, memory_order_relaxed);
	strlcpy(override_pi->value, value, len + 1);
	}
	// Now the primary value property area is up-to-date. Let readers know that they should
	// look at the property value instead of the backup area.
	atomic_thread_fence(memory_order_release);
	int new_serial = (len << 24) \| ((serial + 1) & 0xffffff);
	atomic_store_explicit(&pi->serial, new_serial, memory_order_relaxed);
	if (have_override) {
	atomic_store_explicit(&override_pi->serial, new_serial, memory_order_relaxed);
	}
	__futex_wake(&pi->serial, INT32_MAX); // Fence by side effect
	atomic_store_explicit(serial_pa->serial(),
	atomic_load_explicit(serial_pa->serial(), memory_order_relaxed) + 1,
	memory_order_release);
	if (have_override) {
	atomic_store_explicit(override_serial_pa->serial(),
	atomic_load_explicit(serial_pa->serial(), memory_order_relaxed) + 1,
	memory_order_release);
	}
	__futex_wake(serial_pa->serial(), INT32_MAX);

	return 0;
	}

	int SystemProperties::Add(const char* name, unsigned int namelen, const char* value,
	unsigned int valuelen) {
	if (namelen < 1) {
	async_safe_format_log(ANDROID_LOG_ERROR, "libc",
	"__system_property_add failed: name length 0");
	return -1;
	}

	if (valuelen >= PROP_VALUE_MAX && !is_read_only(name)) {
	async_safe_format_log(ANDROID_LOG_ERROR, "libc",
	"__system_property_add failed: \"%s\" value too long: %d >= PROP_VALUE_MAX",
	name, valuelen);
	return -1;
	}

	if (!initialized_) {
	async_safe_format_log(ANDROID_LOG_ERROR, "libc",
	"__system_property_add failed: properties not initialized");
	return -1;
	}

	prop_area* serial_pa = contexts_->GetSerialPropArea();
	if (serial_pa == nullptr) {
	async_safe_format_log(ANDROID_LOG_ERROR, "libc",
	"__system_property_add failed: property area not found");
	return -1;
	}

	prop_area* pa = contexts_->GetPropAreaForName(name);
	if (!pa) {
	async_safe_format_log(ANDROID_LOG_ERROR, "libc",
	"__system_property_add failed: access denied for \"%s\"", name);
	return -1;
	}

	if (!pa->add(name, namelen, value, valuelen)) {
	async_safe_format_log(ANDROID_LOG_ERROR, "libc",
	"__system_property_add failed: add failed for \"%s\"", name);
	return -1;
	}

	if (appcompat_override_contexts_ != nullptr) {
	bool is_override = is_appcompat_override(name);
	const char* override_name = name;
	if (is_override) override_name += strlen(APPCOMPAT_PREFIX);
	prop_area* other_pa = appcompat_override_contexts_->GetPropAreaForName(override_name);
	prop_area* other_serial_pa = appcompat_override_contexts_->GetSerialPropArea();
	CHECK(other_pa && other_serial_pa);
	// We may write a property twice to overrides, once for the ro.*, and again for the
	// ro.appcompat_override.ro.* property. If we've already written, then we should essentially
	// perform an Update, not an Add.
	auto other_pi = const_cast<prop_info*>(other_pa->find(override_name));
	if (!other_pi) {
	if (other_pa->add(override_name, strlen(override_name), value, valuelen)) {
	atomic_store_explicit(
	other_serial_pa->serial(),
	atomic_load_explicit(other_serial_pa->serial(), memory_order_relaxed) + 1,
	memory_order_release);
	}
	} else if (is_override) {
	// We already wrote the ro., but appcompat_override.ro. should override that. We don't
	// need to do the usual dirty bit setting, as this only happens during the init process,
	// before any readers are started. Check that only init or root can write appcompat props.
	CHECK(getpid() == 1 \|\| getuid() == 0);
	atomic_thread_fence(memory_order_release);
	strlcpy(other_pi->value, value, valuelen + 1);
	}
	}

	// There is only a single mutator, but we want to make sure that
	// updates are visible to a reader waiting for the update.
	atomic_store_explicit(serial_pa->serial(),
	atomic_load_explicit(serial_pa->serial(), memory_order_relaxed) + 1,
	memory_order_release);
	__futex_wake(serial_pa->serial(), INT32_MAX);
	return 0;
	}

	uint32_t SystemProperties::WaitAny(uint32_t old_serial) {
	uint32_t new_serial;
	Wait(nullptr, old_serial, &new_serial, nullptr);
	return new_serial;
	}

	bool SystemProperties::Wait(const prop_info* pi, uint32_t old_serial, uint32_t* new_serial_ptr,
	const timespec* relative_timeout) {
	// Are we waiting on the global serial or a specific serial?
	atomic_uint_least32_t* serial_ptr;
	if (pi == nullptr) {
	if (!initialized_) {
	return -1;
	}

	prop_area* serial_pa = contexts_->GetSerialPropArea();
	if (serial_pa == nullptr) {
	return -1;
	}

	serial_ptr = serial_pa->serial();
	} else {
	serial_ptr = const_cast<atomic_uint_least32_t*>(&pi->serial);
	}

	uint32_t new_serial;
	do {
	int rc;
	if ((rc = __futex_wait(serial_ptr, old_serial, relative_timeout)) != 0 && rc == -ETIMEDOUT) {
	return false;
	}
	new_serial = load_const_atomic(serial_ptr, memory_order_acquire);
	} while (new_serial == old_serial);

	*new_serial_ptr = new_serial;
	return true;
	}

	const prop_info* SystemProperties::FindNth(unsigned n) {
	struct find_nth {
	const uint32_t sought;
	uint32_t current;
	const prop_info* result;

	explicit find_nth(uint32_t n) : sought(n), current(0), result(nullptr) {
	}
	static void fn(const prop_info* pi, void* ptr) {
	find_nth* self = reinterpret_cast<find_nth*>(ptr);
	if (self->current++ == self->sought) self->result = pi;
	}
	} state(n);
	Foreach(find_nth::fn, &state);
	return state.result;
	}

	int SystemProperties::Foreach(void (propfn)(const prop_info pi, void* cookie), void* cookie) {
	if (!initialized_) {
	return -1;
	}

	contexts_->ForEach(propfn, cookie);

	return 0;
	}