property 변경시 어떤 동작 시키기 ( on property in init.rc )

android 에서 property 값 변화시마다 임의의 동작 수행 방밥

init.rc에 on property 사용

아래와 같은 코드가 있음

on property:persist.service.adb.enable=1
    start adbd

설명 : persist.service.adb.enable=1 값이 write 되면 adbd가 실행됨

좀 더 자세한 설명

https://android.googlesource.com/platform/system/core/+/refs/tags/android-9.0.0_r45/init/

Triggers

Triggers are strings which can be used to match certain kinds of events and used to cause an action to occur.

Triggers are subdivided into event triggers and property triggers.

Event triggers are strings triggered by the ‘trigger’ command or by the QueueEventTrigger() function within the init executable. These take the form of a simple string such as ‘boot’ or ‘late-init’.

Property triggers are strings triggered when a named property changes value to a given new value or when a named property changes value to any new value. These take the form of ‘property:=’ and ‘property:=*’ respectively. Property triggers are additionally evaluated and triggered accordingly during the initial boot phase of init.

An Action can have multiple property triggers but may only have one event trigger.

For example: on boot && property:a=b defines an action that is only executed when the ‘boot’ event trigger happens and the property a equals b.

on property:a=b && property:c=d defines an action that is executed at three times:

1. During initial boot if property a=b and property c=d.
2. Any time that property a transitions to value b, while property c already equals d.
3. Any time that property c transitions to value d, while property a already equals b.

만약 값이 변하게 될때마다 처리를 하고 싶을때는 어떻게 할까?
on property:persist.service.adb.enable=*

property 서비스의 전반적인 흐름은 다음과 같습니다.
아래함수에서 init 서비스에서 property 서비스가 만들어지고 소켓 polling을 하고 있다가 커맨드가 도착하면 동작하는 방식입니다.

void start_property_service() {
    selinux_callback cb;
    cb.func_audit = SelinuxAuditCallback;
    selinux_set_callback(SELINUX_CB_AUDIT, cb);

    property_set("ro.property_service.version", "2");

    property_set_fd = CreateSocket(PROP_SERVICE_NAME, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK,
                                   false, 0666, 0, 0, nullptr);
    if (property_set_fd == -1) {
        PLOG(FATAL) << "start_property_service socket creation failed";
    }

    listen(property_set_fd, 8);

    register_epoll_handler(property_set_fd, handle_property_set_fd);
}

static uint32_t PropertySet(const std::string& name, const std::string& value, std::string* error) {
    size_t valuelen = value.size();

    if (!IsLegalPropertyName(name)) {
        *error = "Illegal property name";
        return PROP_ERROR_INVALID_NAME;
    }

    if (valuelen >= PROP_VALUE_MAX && !StartsWith(name, "ro.")) {
        *error = "Property value too long";
        return PROP_ERROR_INVALID_VALUE;
    }

    if (mbstowcs(nullptr, value.data(), 0) == static_cast<std::size_t>(-1)) {
        *error = "Value is not a UTF8 encoded string";
        return PROP_ERROR_INVALID_VALUE;
    }

    prop_info* pi = (prop_info*) __system_property_find(name.c_str());
    if (pi != nullptr) {
        // ro.* properties are actually "write-once".
        if (StartsWith(name, "ro.")) {
            *error = "Read-only property was already set";
            return PROP_ERROR_READ_ONLY_PROPERTY;
        }

        __system_property_update(pi, value.c_str(), valuelen);
    } else {
        int rc = __system_property_add(name.c_str(), name.size(), value.c_str(), valuelen);
        if (rc < 0) {
            *error = "__system_property_add failed";
            return PROP_ERROR_SET_FAILED;
        }
    }

    // Don't write properties to disk until after we have read all default
    // properties to prevent them from being overwritten by default values.
    if (persistent_properties_loaded && StartsWith(name, "persist.")) {
        WritePersistentProperty(name, value);
    }
    property_changed(name, value);
    return PROP_SUCCESS;
}

void property_changed(const std::string& name, const std::string& value) {
    // If the property is sys.powerctl, we bypass the event queue and immediately handle it.
    // This is to ensure that init will always and immediately shutdown/reboot, regardless of
    // if there are other pending events to process or if init is waiting on an exec service or
    // waiting on a property.
    // In non-thermal-shutdown case, 'shutdown' trigger will be fired to let device specific
    // commands to be executed.
    if (name == "sys.powerctl") {
        // Despite the above comment, we can't call HandlePowerctlMessage() in this function,
        // because it modifies the contents of the action queue, which can cause the action queue
        // to get into a bad state if this function is called from a command being executed by the
        // action queue.  Instead we set this flag and ensure that shutdown happens before the next
        // command is run in the main init loop.
        // TODO: once property service is removed from init, this will never happen from a builtin,
        // but rather from a callback from the property service socket, in which case this hack can
        // go away.
        shutdown_command = value;
        do_shutdown = true;
    }

    if (property_triggers_enabled) ActionManager::GetInstance().QueuePropertyChange(name, value);

최종단에서는 property 값이 변할때 trigger 할것이 있는지 검사하게 되는 CheckPropertyTriggers 함수를 이용하게됩니다.

관련소스
trigger_value != "*" 조건이 false가 되므로 found 가 true가 됩니다.

bool Action::CheckPropertyTriggers(const std::string& name,
                                   const std::string& value) const {
    if (property_triggers_.empty()) {
        return true;
    }

    bool found = name.empty();
    for (const auto& [trigger_name, trigger_value] : property_triggers_) {
        if (trigger_name == name) {
            if (trigger_value != "*" && trigger_value != value) {
                return false;
            } else {
                found = true;
            }
        } else {
            std::string prop_val = android::base::GetProperty(trigger_name, "");
            if (prop_val.empty() || (trigger_value != "*" && trigger_value != prop_val)) {
                return false;
            }
        }
    }
    return found;
}

예제)
이미 찾아보니 * 로 사용하는 소스들이 존재 합니다.

# system server cannot write to /proc/sys files,
# and chown/chmod does not work for /proc/sys/ entries.
# So proxy writes through init.
on property:sys.sysctl.extra_free_kbytes=*
    write /proc/sys/vm/extra_free_kbytes ${sys.sysctl.extra_free_kbytes}

# "tcp_default_init_rwnd" Is too long!
on property:sys.sysctl.tcp_def_init_rwnd=*
    write /proc/sys/net/ipv4/tcp_default_init_rwnd ${sys.sysctl.tcp_def_init_rwnd}

추가로 write시 ${sys.sysctl.extra_free_kbytes} 이부분이 있는데 이건 뭘까 검색해 봤습니다.
설명상은 <contents>에 의해 확장될수 있다는 의미로 보입니다.

`write <path> <content>`
> Open the file at _path_ and write a string to it with write(2).
  If the file does not exist, it will be created. If it does exist,
  it will be truncated. Properties are expanded within _content_.

그리고 이것은 ${property.name} 이런 형태로 표현할 수 있다는것이고요.

System properties can be expanded using the syntax
`${property.name}`. This also works in contexts where concatenation is
required, such as `import /init.recovery.${ro.hardware}.rc`.

        {"write",                   {2,     2,    {true,   do_write}}},

static Result<Success> do_write(const BuiltinArguments& args) {
    if (auto result = WriteFile(args[1], args[2]); !result) {
        return Error() << "Unable to write to file '" << args[1] << "': " << result.error();
    }

    return Success();
}

Result<Success> WriteFile(const std::string& path, const std::string& content) {
    android::base::unique_fd fd(TEMP_FAILURE_RETRY(
        OpenFile(path, O_WRONLY | O_CREAT | O_NOFOLLOW | O_TRUNC | O_CLOEXEC, 0600)));
    if (fd == -1) {
        return ErrnoError() << "open() failed";
    }
    if (!android::base::WriteStringToFd(content, fd)) {
        return ErrnoError() << "Unable to write file contents";
    }
    return Success();
}

bool WriteStringToFd(const std::string& content, int fd) {
  const char* p = content.data();
  size_t left = content.size();
  while (left > 0) {
    ssize_t n = TEMP_FAILURE_RETRY(write(fd, p, left));
    if (n == -1) {
      return false;
    }
    p += n;
    left -= n;
  }
  return true;
}

따라가 봤지만 ${} 확장하는 코드는 보이지 않습니다. 그렇다면 do_write 하기전에 이미 확장된것은 아닌지 검토해 보았습니다.

Result<Success> RunBuiltinFunction(const BuiltinFunction& function,
                                   const std::vector<std::string>& args,
                                   const std::string& context) {
    auto builtin_arguments = BuiltinArguments(context);

    builtin_arguments.args.resize(args.size());
    builtin_arguments.args[0] = args[0];
    for (std::size_t i = 1; i < args.size(); ++i) {
        if (!expand_props(args[i], &builtin_arguments.args[i])) {
            return Error() << "cannot expand '" << args[i] << "'";
        }
    }

    return function(builtin_arguments);
}

실제 확장은 expand_props 여기에서 이루어지며 인자를 넘기기 전에 이루어 집니다.

bool expand_props(const std::string& src, std::string* dst) {
    const char* src_ptr = src.c_str();

    if (!dst) {
        return false;
    }

    /* - variables can either be $x.y or ${x.y}, in case they are only part
     *   of the string.
     * - will accept $$ as a literal $.
     * - no nested property expansion, i.e. ${foo.${bar}} is not supported,
     *   bad things will happen
     * - ${x.y:-default} will return default value if property empty.
     */
    while (*src_ptr) {
        const char* c;

        c = strchr(src_ptr, '$');
        if (!c) {
            dst->append(src_ptr);
            return true;
        }

        dst->append(src_ptr, c);
        c++;

        if (*c == '$') {
            dst->push_back(*(c++));
            src_ptr = c;
            continue;
        } else if (*c == '\0') {
            return true;
        }

        std::string prop_name;
        std::string def_val;
        if (*c == '{') {
            c++;
            const char* end = strchr(c, '}');
            if (!end) {
                // failed to find closing brace, abort.
                LOG(ERROR) << "unexpected end of string in '" << src << "', looking for }";
                return false;
            }
            prop_name = std::string(c, end);
            c = end + 1;
            size_t def = prop_name.find(":-");
            if (def < prop_name.size()) {
                def_val = prop_name.substr(def + 2);
                prop_name = prop_name.substr(0, def);
            }
        } else {
            prop_name = c;
            LOG(ERROR) << "using deprecated syntax for specifying property '" << c << "', use ${name} instead";
            c += prop_name.size();
        }

        if (prop_name.empty()) {
            LOG(ERROR) << "invalid zero-length property name in '" << src << "'";
            return false;
        }

        std::string prop_val = android::base::GetProperty(prop_name, "");
        if (prop_val.empty()) {
            if (def_val.empty()) {
                LOG(ERROR) << "property '" << prop_name << "' doesn't exist while expanding '" << src << "'";
                return false;
            }
            prop_val = def_val;
        }

        dst->append(prop_val);
        src_ptr = c;
    }

    return true;
}