本文基于安卓11，propertyService在init进程中启动。
init进程相关代码位于/system/core/init/目录下，其中包括property_service.cpp。
android::properties相关代码位于/system/core/property_service/目录。
底层相关代码/bionic/libc/bionic/, /bionic/libc/system_properties/。
init
main.cpp
int main(int argc, char** argv) {
#if __has_feature(address_sanitizer)
__asan_set_error_report_callback(AsanReportCallback);
#endif
if (!strcmp(basename(argv[0]), "ueventd")) {
return ueventd_main(argc, argv);
}
if (argc > 1) {
if (!strcmp(argv[1], "subcontext")) {
android::base::InitLogging(argv, &android::base::KernelLogger);
const BuiltinFunctionMap& function_map = GetBuiltinFunctionMap();
return SubcontextMain(argc, argv, &function_map);
}
if (!strcmp(argv[1], "selinux_setup")) {
return SetupSelinux(argv);
}
if (!strcmp(argv[1], "second_stage")) {
return SecondStageMain(argc, argv);
}
}
return FirstStageMain(argc, argv);
}
The early init boot sequence is broken up into three stages: first stage init, SELinux setup, and second stage init.
init程序main方法中通过命令行参数判断程序启动时执行的动作，分别执行FirstStageMain(argc, argv),SubcontextMain(argc, argv, &function_map),SetupSelinux(argv),SecondStageMain(argc, argv)方法，在SecondStageMain()方法中执行propertyInit()方法初始化系统属性，启动属性服务。
PropertyInit
void PropertyInit() {
selinux_callback cb;
cb.func_audit = PropertyAuditCallback;
selinux_set_callback(SELINUX_CB_AUDIT, cb);
mkdir("/dev/__properties__", S_IRWXU | S_IXGRP | S_IXOTH);
CreateSerializedPropertyInfo();
if (__system_property_area_init()) {
LOG(FATAL) << "Failed to initialize property area";
}
if (!property_info_area.LoadDefaultPath()) {
LOG(FATAL) << "Failed to load serialized property info file";
}
// If arguments are passed both on the command line and in DT,
// properties set in DT always have priority over the command-line ones.
ProcessKernelDt();
ProcessKernelCmdline();
// Propagate the kernel variables to internal variables
// used by init as well as the current required properties.
ExportKernelBootProps();
PropertyLoadBootDefaults();
}
一开始是和selinux相关的，创建"/dev/_properties_"文件夹，这个路径很关键，和属性系统相关的文件都在这个目录下。
- CreateSerializedPropertyInfo()
读取selinux模块中property相关文件：plat_property_contexts等，将其解析加载到变量property_infos中。property_infos变量为std::vector类型。
void CreateSerializedPropertyInfo() {
auto property_infos = std::vector<PropertyInfoEntry>();
if (access("/system/etc/selinux/plat_property_contexts", R_OK) != -1) {
if (!LoadPropertyInfoFromFile("/system/etc/selinux/plat_property_contexts",
&property_infos)) {
return;
}
// Don't check for failure here, so we always have a sane list of properties.
// E.g. In case of recovery, the vendor partition will not have mounted and we
// still need the system / platform properties to function.
if (access("/system_ext/etc/selinux/system_ext_property_contexts", R_OK) != -1) {
LoadPropertyInfoFromFile("/system_ext/etc/selinux/system_ext_property_contexts",
&property_infos);
}
if (!LoadPropertyInfoFromFile("/vendor/etc/selinux/vendor_property_contexts",
&property_infos)) {
// Fallback to nonplat_* if vendor_* doesn't exist.
LoadPropertyInfoFromFile("/vendor/etc/selinux/nonplat_property_contexts",
&property_infos);
}
if (access("/product/etc/selinux/product_property_contexts", R_OK) != -1) {
LoadPropertyInfoFromFile("/product/etc/selinux/product_property_contexts",
&property_infos);
}
if (access("/odm/etc/selinux/odm_property_contexts", R_OK) != -1) {
LoadPropertyInfoFromFile("/odm/etc/selinux/odm_property_contexts", &property_infos);
}
} else {
if (!LoadPropertyInfoFromFile("/plat_property_contexts", &property_infos)) {
return;
}
LoadPropertyInfoFromFile("/system_ext_property_contexts", &property_infos);
if (!LoadPropertyInfoFromFile("/vendor_property_contexts", &property_infos)) {
// Fallback to nonplat_* if vendor_* doesn't exist.
LoadPropertyInfoFromFile("/nonplat_property_contexts", &property_infos);
}
LoadPropertyInfoFromFile("/product_property_contexts", &property_infos);
LoadPropertyInfoFromFile("/odm_property_contexts", &property_infos);
}
auto serialized_contexts = std::string();
auto error = std::string();
if (!BuildTrie(property_infos, "u:object_r:default_prop:s0", "string", &serialized_contexts,
&error)) {
LOG(ERROR) << "Unable to serialize property contexts: " << error;
return;
}
constexpr static const char kPropertyInfosPath[] = "/dev/__properties__/property_info";
if (!WriteStringToFile(serialized_contexts, kPropertyInfosPath, 0444, 0, 0, false)) {
PLOG(ERROR) << "Unable to write serialized property infos to file";
}
selinux_android_restorecon(kPropertyInfosPath, 0);
}
BuildTrie(property_infos, “u:object_r:default_prop:s0”, “string”, &serialized_contexts, &error))，通过LOG(ERROR)信息应该是说一个将解析得到的property_infos变量进行serialize property contexts的动作，serialize的结果保存在变量serialized_contexts。然后将序列化的结果写入到"/dev/properties/property_info"文件中。selinux_android_restorecon(kPropertyInfosPath, 0)让文件的上下文恢复成file_contexts文件里初始配置。
- __system_property_area_init()
./bionic/libc/bionic/system_property_api.cpp
#define PROP_FILENAME "/dev/__properties__"
int __system_property_area_init() {
bool fsetxattr_failed = false;
return system_properties.AreaInit(PROP_FILENAME, &fsetxattr_failed) && !fsetxattr_failed ? 0 : -1;
}
./bionic/libc/system_properties/system_properties.cpp
bool SystemProperties::AreaInit(const char* filename, bool* fsetxattr_failed) {
if (strlen(filename) >= PROP_FILENAME_MAX) {
return false;
}
strcpy(property_filename_, filename);
contexts_ = new (contexts_data_) ContextsSerialized();
if (!contexts_->Initialize(true, property_filename_, fsetxattr_failed)) {
return false;
}
initialized_ = true;
return true;
}
./bionic/libc/system_properties/contexts_serialized.cpp
bool ContextsSerialized::InitializeProperties() {
if (!property_info_area_file_.LoadDefaultPath()) {
return false;
}
if (!InitializeContextNodes()) {
FreeAndUnmap();
return false;
}
return true;
}
bool ContextsSerialized::Initialize(bool writable, const char* filename, bool* fsetxattr_failed) {
filename_ = filename;
if (!InitializeProperties()) {
return false;
}
if (writable) {
mkdir(filename_, S_IRWXU | S_IXGRP | S_IXOTH);
bool open_failed = false;
if (fsetxattr_failed) {
*fsetxattr_failed = false;
}
for (size_t i = 0; i < num_context_nodes_; ++i) {
if (!context_nodes_[i].Open(true, fsetxattr_failed)) {
open_failed = true;
}
}
if (open_failed || !MapSerialPropertyArea(true, fsetxattr_failed)) {
FreeAndUnmap();
return false;
}
} else {
if (!MapSerialPropertyArea(false, nullptr)) {
FreeAndUnmap();
return false;
}
}
return true;
}
./system/core/property_service/libpropertyinfoparser/property_info_parser.cpp
bool PropertyInfoAreaFile::LoadDefaultPath() {
return LoadPath("/dev/__properties__/property_info");
}
bool PropertyInfoAreaFile::LoadPath(const char* filename) {
int fd = open(filename, O_CLOEXEC | O_NOFOLLOW | O_RDONLY);
struct stat fd_stat;
if (fstat(fd, &fd_stat) < 0) {
close(fd);
return false;
}
if ((fd_stat.st_uid != 0) || (fd_stat.st_gid != 0) ||
((fd_stat.st_mode & (S_IWGRP | S_IWOTH)) != 0) ||
(fd_stat.st_size < static_cast<off_t>(sizeof(PropertyInfoArea)))) {
close(fd);
return false;
}
auto mmap_size = fd_stat.st_size;
void* map_result = mmap(nullptr, mmap_size, PROT_READ, MAP_SHARED, fd, 0);
if (map_result == MAP_FAILED) {
close(fd);
return false;
}
auto property_info_area = reinterpret_cast<PropertyInfoArea*>(map_result);
if (property_info_area->minimum_supported_version() > 1 ||
property_info_area->size() != mmap_size) {
munmap(map_result, mmap_size);
close(fd);
return false;
}
close(fd);
mmap_base_ = map_result;
mmap_size_ = mmap_size;
return true;
}
_system_property_area_init()经过一系列的方法调用，通过mmap()函数将文件"/dev/__properties_/property_info"加载到进程中共享内存。
接下来的判断语句又执行了一遍property_info_area.LoadDefaultPath()，在ContextsSerialized::InitializeProperties()方法中已经做过这个操作了，不是很懂为什么要执行两遍。
- ProcessKernelDt()
const std::string kDefaultAndroidDtDir("/proc/device-tree/firmware/android/");
static void ProcessKernelDt() {
if (!is_android_dt_value_expected("compatible", "android,firmware")) {
return;
}
std::unique_ptr<DIR, int (*)(DIR*)> dir(opendir(get_android_dt_dir().c_str()), closedir);
if (!dir) return;
std::string dt_file;
struct dirent* dp;
while ((dp = readdir(dir.get())) != NULL) {
if (dp->d_type != DT_REG || !strcmp(dp->d_name, "compatible") ||
!strcmp(dp->d_name, "name")) {
continue;
}
std::string file_name = get_android_dt_dir() + dp->d_name;
android::base::ReadFileToString(file_name, &dt_file);
std::replace(dt_file.begin(), dt_file.end(), ',', '.');
InitPropertySet("ro.boot."s + dp->d_name, dt_file);
}
}
读取设备树"/proc/device-tree/firmware/android/"中的"name"和"compatible"节点内容，添加"ro.boot."前缀后加入到property属性系统中。
- ProcessKernelCmdline()
static void ProcessKernelCmdline() {
bool for_emulator = false;
ImportKernelCmdline([&](const std::string& key, const std::string& value) {
if (key == "qemu") {
for_emulator = true;
} else if (StartsWith(key, "androidboot.")) {
InitPropertySet("ro.boot." + key.substr(12), value);
}
});
if (for_emulator) {
ImportKernelCmdline([&](const std::string& key, const std::string& value) {
// In the emulator, export any kernel option with the "ro.kernel." prefix.
InitPropertySet("ro.kernel." + key, value);
});
}
}
读取"/pro/cmdline"，将"androidboot."开头的变量，添加"ro.boot."前缀后加入到property属性系统中。
- ExportKernelBootProps()
static void ExportKernelBootProps() {
constexpr const char* UNSET = "";
struct {
const char* src_prop;
const char* dst_prop;
const char* default_value;
} prop_map[] = {
// clang-format off
{
"ro.boot.serialno", "ro.serialno", UNSET, },
{
"ro.boot.mode", "ro.bootmode", "unknown", },
{
"ro.boot.baseband", "ro.baseband", "unknown", },
{
"ro.boot.bootloader", "ro.bootloader", "unknown", },
{
"ro.boot.hardware", "ro.hardware", "unknown", },
{
"ro.boot.revision", "ro.revision", "0", },
// clang-format on
};
for (const auto& prop : prop_map) {
std::string value = GetProperty(prop.src_prop, prop.default_value);
if (value != UNSET) InitPropertySet(prop.dst_prop, value);
}
}
关注于变量prop_map中列举的几个重要属性，如果之前没有赋值，将其赋值为"unknown"或"0"。
- PropertyLoadBootDefaults()
void PropertyLoadBootDefaults() {
// TODO(b/117892318): merge prop.default and build.prop files into one
// We read the properties and their values into a map, in order to always allow properties
// loaded in the later property files to override the properties in loaded in the earlier
// property files, regardless of if they are "ro." properties or not.
std::map<std::string, std::string> properties;
if (!load_properties_from_file("/system/etc/prop.default", nullptr, &properties)) {
// Try recovery path
if (!load_properties_from_file("/prop.default", nullptr, &properties)) {
// Try legacy path
load_properties_from_file("/default.prop", nullptr, &properties);
}
}
load_properties_from_file("/system/build.prop", nullptr, &properties);
load_properties_from_file("/system_ext/build.prop", nullptr, &properties);
load_properties_from_file("/vendor/default.prop", nullptr, &properties);
load_properties_from_file("/vendor/build.prop", nullptr, &properties);
if (SelinuxGetVendorAndroidVersion() >= __ANDROID_API_Q__) {
load_properties_from_file("/odm/etc/build.prop", nullptr, &properties);
} else {
load_properties_from_file("/odm/default.prop", nullptr, &properties);
load_properties_from_file("/odm/build.prop", nullptr, &properties);
}
load_properties_from_file("/product/build.prop", nullptr, &properties);
load_properties_from_file("/factory/factory.prop", "ro.*", &properties);
if (access(kDebugRamdiskProp, R_OK) == 0) {
LOG(INFO) << "Loading " << kDebugRamdiskProp;
load_properties_from_file(kDebugRamdiskProp, nullptr, &properties);
}
for (const auto& [name, value] : properties) {
std::string error;
if (PropertySet(name, value, &error) != PROP_SUCCESS) {
LOG(ERROR) << "Could not set '" << name << "' to '" << value
<< "' while loading .prop files" << error;
}
}
property_initialize_ro_product_props();
property_derive_build_props();
update_sys_usb_config();
}
PropertyLoadBootDefaults()方法在"/system/etc/prop.default"，"/system/build.prop"文件中加载相关属性到properties变量中，再通过PropertySet()方法添加到属性系统中（下面会重点分析PropertySet()方法）。
property_initialize_ro_product_props()方法初始化"ro.product."为前缀的属性。
property_derive_build_props()初始化一些编译相关的属性。
static void property_derive_build_props() {
property_derive_build_fingerprint();
property_derive_build_product();
property_derive_build_description();
property_derive_build_display_id();
}
update_sys_usb_config()方法设置"persist.sys.usb.config"属性。
PropertySet()
从./system/core/init/property_service.cpp的PropertySet()方法中可以看到，添加属性时是调用的__system_property_add()方法，
./bionic/libc/bionic/system_property_api.cpp
int __system_property_add(const char* name, unsigned int namelen, const char* value,
unsigned int valuelen) {
return system_properties.Add(name, namelen, value, valuelen);
}
./bionic/libc/system_properties/system_properties.cpp
int SystemProperties::Add(const char* name, unsigned int namelen, const char* value,
unsigned int valuelen) {
if (valuelen >= PROP_VALUE_MAX && !is_read_only(name)) {
return -1;
}
if (namelen < 1) {
return -1;
}
if (!initialized_) {
return -1;
}
prop_area* serial_pa = contexts_->GetSerialPropArea();
if (serial_pa == nullptr) {
return -1;
}
prop_area* pa = contexts_->GetPropAreaForName(name);
if (!pa) {
async_safe_format_log(ANDROID_LOG_ERROR, "libc", "Access denied adding property \"%s\"", name);
return -1;
}
bool ret = pa->add(name, namelen, value, valuelen);
if (!ret) {
return -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;
}
initialized_在SystemProperties::Init()方法中被赋值为true，SystemProperties::Add()三个关键方法调用：
-
contexts_->GetSerialPropArea();_
-
contexts->GetPropAreaForName(name);
-
pa->add(name, namelen, value, valuelen);
-
contexts_->GetSerialPropArea();_
./bionic/libc/system_properties/include/system_properties/contexts_serialized.h
virtual prop_area* GetSerialPropArea() override {
return serial_prop_area_;
}
变量serial_prop_area_在 ContextsSerialized::Initialize()方法中通过ContextsSerialized::MapSerialPropertyArea()方法初始化。
./bionic/libc/system_properties/contexts_serialized.cpp
bool ContextsSerialized::MapSerialPropertyArea(bool access_rw, bool* fsetxattr_failed) {
char filename[PROP_FILENAME_MAX];
int len = async_safe_format_buffer(filename, sizeof(filename), "%s/properties_serial", filename_);
if (len < 0 || len >= PROP_FILENAME_MAX) {
serial_prop_area_ = nullptr;
return false;
}
if (access_rw) {
serial_prop_area_ =
prop_area::map_prop_area_rw(filename, "u:object_r:properties_serial:s0", fsetxattr_failed);
} else {
serial_prop_area_ = prop_area::map_prop_area(filename);
}
return serial_prop_area_;
}
./bionic/libc/system_properties/prop_area.cpp
prop_area* prop_area::map_prop_area_rw(const char* filename, const char* context,
bool* fsetxattr_failed) {
/* dev is a tmpfs that we can use to carve a shared workspace
* out of, so let's do that...
*/
const int fd = open(filename, O_RDWR | O_CREAT | O_NOFOLLOW | O_CLOEXEC | O_EXCL, 0444);
if (fd < 0) {
if (errno == EACCES) {
/* for consistency with the case where the process has already
* mapped the page in and segfaults when trying to write to it
*/
abort();
}
return nullptr;
}
if (context) {
if (fsetxattr(fd, XATTR_NAME_SELINUX, context, strlen(context) + 1, 0) != 0) {
async_safe_format_log(ANDROID_LOG_ERROR, "libc",
"fsetxattr failed to set context (%s) for \"%s\"", context, filename);
/*
* fsetxattr() will fail during system properties tests due to selinux policy.
* We do not want to create a custom policy for the tester, so we will continue in
* this function but set a flag that an error has occurred.
* Init, which is the only daemon that should ever call this function will abort
* when this error occurs.
* Otherwise, the tester will ignore it and continue, albeit without any selinux
* property separation.
*/
if (fsetxattr_failed) {
*fsetxattr_failed = true;
}
}
}
if (ftruncate(fd, PA_SIZE) < 0) {
close(fd);
return nullptr;
}
pa_size_ = PA_SIZE;
pa_data_size_ = pa_size_ - sizeof(prop_area);
void* const memory_area = mmap(nullptr, pa_size_, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (memory_area == MAP_FAILED) {
close(fd);
return nullptr;
}
prop_area* pa = new (memory_area) prop_area(PROP_AREA_MAGIC, PROP_AREA_VERSION);
close(fd);
return pa;
}
map_prop_area_rw()通过open()获取文件"/dev/__properties__/properties_serial"的引用，mmap()共享内存，最终返回prop_area对象赋值给变量serial_prop_area_。
- contexts->GetPropAreaForName(name);
./bionic/libc/system_properties/contexts_serialized.cpp
prop_area* ContextsSerialized::GetPropAreaForName(const char* name) {
uint32_t index;
property_info_area_file_->GetPropertyInfoIndexes(name, &index, nullptr);
if (index == ~0u || index >= num_context_nodes_) {
async_safe_format_log(ANDROID_LOG_ERROR, "libc", "Could not find context for property \"%s\"",
name);
return nullptr;
}
auto* context_node = &context_nodes_[index];
if (!context_node->pa()) {
// We explicitly do not check no_access_ in this case because unlike the
// case of foreach(), we want to generate an selinux audit for each
// non-permitted property access in this function.
context_node->Open(false, nullptr);
}
return context_node->pa();
}
通过name找到prop_area。
- pa->add(name, namelen, value, valuelen);
./bionic/libc/system_properties/prop_area.cpp
bool prop_area::add(const char* name, unsigned int namelen, const char* value,
unsigned int valuelen) {
return find_property(root_node(), name, namelen, value, valuelen, true);
}
./bionic/libc/system_properties/prop_area.cpp
const prop_info* prop_area::find_property(prop_bt* const trie, const char* name, uint32_t namelen,
const char* value, uint32_t valuelen,
bool alloc_if_needed) {
if (!trie) return nullptr;
const char* remaining_name = name;
prop_bt* current = trie;
while (true) {
const char* sep = strchr(remaining_name, '.');
const bool want_subtree = (sep != nullptr);
const uint32_t substr_size = (want_subtree) ? sep - remaining_name : strlen(remaining_name);
if (!substr_size) {
return nullptr;
}
prop_bt* root = nullptr;
uint_least32_t children_offset = atomic_load_explicit(¤t->children, memory_order_relaxed);
if (children_offset != 0) {
root = to_prop_bt(¤t->children);
} else if (alloc_if_needed) {
uint_least32_t new_offset;
root = new_prop_bt(remaining_name, substr_size, &new_offset);
if (root) {
atomic_store_explicit(¤t->children, new_offset, memory_order_release);
}
}
if (!root) {
return nullptr;
}
current = find_prop_bt(root, remaining_name, substr_size, alloc_if_needed);
if (!current) {
return nullptr;
}
if (!want_subtree) break;
remaining_name = sep + 1;
}
uint_least32_t prop_offset = atomic_load_explicit(¤t->prop, memory_order_relaxed);
if (prop_offset != 0) {
return to_prop_info(¤t->prop);
} else if (alloc_if_needed) {
uint_least32_t new_offset;
prop_info* new_info = new_prop_info(name, namelen, value, valuelen, &new_offset);
if (new_info) {
atomic_store_explicit(¤t->prop, new_offset, memory_order_release);
}
return new_info;
} else {
return nullptr;
}
}
至此property应该添加到属性系统中了。
StartPropertyService(&property_fd)
void StartPropertyService(int* epoll_socket) {
InitPropertySet("ro.property_service.version", "2");
int sockets[2];
if (socketpair(AF_UNIX, SOCK_SEQPACKET | SOCK_CLOEXEC, 0, sockets) != 0) {
PLOG(FATAL) << "Failed to socketpair() between property_service and init";
}
*epoll_socket = from_init_socket = sockets[0];
init_socket = sockets[1];
StartSendingMessages();
if (auto result = CreateSocket(PROP_SERVICE_NAME, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK,
false, 0666, 0, 0, {
});
result.ok()) {
property_set_fd = *result;
} else {
LOG(FATAL) << "start_property_service socket creation failed: " << result.error();
}
listen(property_set_fd, 8);
auto new_thread = std::thread{
PropertyServiceThread};
property_service_thread.swap(new_thread);
}
std::thread{PropertyServiceThread};
static void PropertyServiceThread() {
Epoll epoll;
if (auto result = epoll.Open(); !result.ok()) {
LOG(FATAL) << result.error();
}
if (auto result = epoll.RegisterHandler(property_set_fd, handle_property_set_fd);
!result.ok()) {
LOG(FATAL) << result.error();
}
if (auto result = epoll.RegisterHandler(init_socket, HandleInitSocket); !result.ok()) {
LOG(FATAL) << result.error();
}
while (true) {
auto pending_functions = epoll.Wait(std::nullopt);
if (!pending_functions.ok()) {
LOG(ERROR) << pending_functions.error();
} else {
for (const auto& function : *pending_functions) {
(*function)();
}
}
}
}
handle_property_set_fd()
static void handle_property_set_fd() {
static constexpr uint32_t kDefaultSocketTimeout = 2000; /* ms */
int s = accept4(property_set_fd, nullptr, nullptr, SOCK_CLOEXEC);
if (s == -1) {
return;
}
ucred cr;
socklen_t cr_size = sizeof(cr);
if (getsockopt(s, SOL_SOCKET, SO_PEERCRED, &cr, &cr_size) < 0) {
close(s);
PLOG(ERROR) << "sys_prop: unable to get SO_PEERCRED";
return;
}
SocketConnection socket(s, cr);
uint32_t timeout_ms = kDefaultSocketTimeout;
uint32_t cmd = 0;
if (!socket.RecvUint32(&cmd, &timeout_ms)) {
PLOG(ERROR) << "sys_prop: error while reading command from the socket";
socket.SendUint32(PROP_ERROR_READ_CMD);
return;
}
switch (cmd) {
case PROP_MSG_SETPROP: {
char prop_name[PROP_NAME_MAX];
char prop_value[PROP_VALUE_MAX];
if (!socket.RecvChars(prop_name, PROP_NAME_MAX, &timeout_ms) ||
!socket.RecvChars(prop_value, PROP_VALUE_MAX, &timeout_ms)) {
PLOG(ERROR) << "sys_prop(PROP_MSG_SETPROP): error while reading name/value from the socket";
return;
}
prop_name[PROP_NAME_MAX-1] = 0;
prop_value[PROP_VALUE_MAX-1] = 0;
std::string source_context;
if (!socket.GetSourceContext(&source_context)) {
PLOG(ERROR) << "Unable to set property '" << prop_name << "': getpeercon() failed";
return;
}
const auto& cr = socket.cred();
std::string error;
uint32_t result =
HandlePropertySet(prop_name, prop_value, source_context, cr, nullptr, &error);
if (result != PROP_SUCCESS) {
LOG(ERROR) << "Unable to set property '" << prop_name << "' from uid:" << cr.uid
<< " gid:" << cr.gid << " pid:" << cr.pid << ": " << error;
}
break;
}
case PROP_MSG_SETPROP2: {
std::string name;
std::string value;
if (!socket.RecvString(&name, &timeout_ms) ||
!socket.RecvString(&value, &timeout_ms)) {
PLOG(ERROR) << "sys_prop(PROP_MSG_SETPROP2): error while reading name/value from the socket";
socket.SendUint32(PROP_ERROR_READ_DATA);
return;
}
std::string source_context;
if (!socket.GetSourceContext(&source_context)) {
PLOG(ERROR) << "Unable to set property '" << name << "': getpeercon() failed";
socket.SendUint32(PROP_ERROR_PERMISSION_DENIED);
return;
}
const auto& cr = socket.cred();
std::string error;
uint32_t result = HandlePropertySet(name, value, source_context, cr, &socket, &error);
if (result != PROP_SUCCESS) {
LOG(ERROR) << "Unable to set property '" << name << "' from uid:" << cr.uid
<< " gid:" << cr.gid << " pid:" << cr.pid << ": " << error;
}
socket.SendUint32(result);
break;
}
default:
LOG(ERROR) << "sys_prop: invalid command " << cmd;
socket.SendUint32(PROP_ERROR_INVALID_CMD);
break;
}
}
总体就是监听socket设备"/dev/socket/property_service"，通过handle_property_set_fd()方法处理socket接收的消息。
Notice
使用属性时应该要注意的地方。
- persist属性：
以"persist."为前缀的属性，设备重启后依然能够查看到，其保存在"dev/socket/property_service"节点中。
- ro属性：
以"ro."为前缀的属性，只读属性，不能修改。read only。
- ctl属性：
"ctl.start"启动服务，"ctl.stop"关闭服务。
- powerctl属性
"sys.powerctl"属性可控制设备重启。
- 权限问题
HandlePropertySet()设置属性时会调用CheckPermissions()方法检查是否有相关权限，
uint32_t CheckPermissions(const std::string& name, const std::string& value,
const std::string& source_context, const ucred& cr, std::string* error) {
if (!IsLegalPropertyName(name)) {
*error = "Illegal property name";
return PROP_ERROR_INVALID_NAME;
}
if (StartsWith(name, "ctl.")) {
if (!CheckControlPropertyPerms(name, value, source_context, cr)) {
*error = StringPrintf("Invalid permissions to perform '%s' on '%s'", name.c_str() + 4,
value.c_str());
return PROP_ERROR_HANDLE_CONTROL_MESSAGE;
}
return PROP_SUCCESS;
}
const char* target_context = nullptr;
const char* type = nullptr;
property_info_area->GetPropertyInfo(name.c_str(), &target_context, &type);
if (!CheckMacPerms(name, target_context, source_context.c_str(), cr)) {
*error = "SELinux permission check failed";
return PROP_ERROR_PERMISSION_DENIED;
}
if (!CheckType(type, value)) {
*error = StringPrintf("Property type check failed, value doesn't match expected type '%s'",
(type ?: "(null)"));
return PROP_ERROR_INVALID_VALUE;
}
return PROP_SUCCESS;
}
CheckMacPerms()方法通过socket客户端的source_context和目标的target_context检查SELinux相关权限。