Getting start with dbus in systemd (01)
基本概念
幾個概念
dbus name:
connetion: 如下，第一行，看到的就是 "dbus name", 有一個中心 dbus name (org.freedesktop.DBus) , 其他的每個app和 dbus-daemon 創建一個連接，就是：“connection”， 所以，我們常見的“org.freedesktop.systemd1” 不僅是一個dbus name, 也是一個 “connection”. 所以，其實，我們需要看一下systemd的代碼，看看systemd的connection (org.freedesktop.systemd1) 是如何 和 “dbus-daemon” 建立連接的？
path(object): path 也是object，一個app 可能有多個object，org.freedesktop.DBus 的path就是 “/”
interface： 每個 “connection” 都有很多的 “interface”
method：每個“interface” 都有很多的 “method”
properties: 每個“interface” 都有很多的 “properties”（屬性）
格式：
dbus name path(object) interface.method argument
#dbus-send --system --print-reply --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.DBus.Properties.Get string:'org.freedesktop.systemd1.Manager' string:'Version'
下面的輸出，有一點需要注意： 第一列雖然都是bus name，但是也表示了connection含義，前面數字部分代表：建立的鏈接，后面的 well-know name 不代表connection，比如，systemd-但是這個app 只有在啟動的時候，才會和dbus-daemon 創建一個 connection
下面的，systemd-logind 沒有創建 connection:
#busctl
NAME PID PROCESS USER CONNECTION UNIT SESSION DESCRIPTION
:1.1 1 systemd root :1.1 - - -
:1.3 109116 dbus-monitor root :1.3 sshd.service - -
:1.31 30273 busctl root :1.31 sshd.service - -
net.reactivated.Fprint - - - (activatable) - -
org.freedesktop.DBus - - - - - - -
org.freedesktop.PolicyKit1 - - - (activatable) - -
org.freedesktop.hostname1 - - - (activatable) - -
org.freedesktop.import1 - - - (activatable) - -
org.freedesktop.locale1 - - - (activatable) - -
org.freedesktop.login1 - - - (activatable) - -
org.freedesktop.machine1 - - - (activatable) - -
org.freedesktop.systemd1 1 systemd root :1.1 - - -
org.freedesktop.timedate1 - - - (activatable) - -
下面的systemd 創建了connection: (connection name 就是1.130 ）
#busctl
NAME PID PROCESS USER CONNECTION UNIT SESSION DESCRIPTION
:1.130 115271 systemd-logind root :1.130 systemd-logind.service - -
:1.131 115289 busctl root :1.131 sshd.service - -
:1.15 1 systemd root :1.15 - - -
:1.4 1571 libvirtd root :1.4 libvirtd.service - -
:1.6 89749 polkitd polkitd :1.6 polkit.service - -
net.reactivated.Fprint - - - (activatable) - -
org.freedesktop.DBus - - - - - - -
org.freedesktop.PolicyKit1 89749 polkitd polkitd :1.6 polkit.service - -
org.freedesktop.hostname1 - - - (activatable) - -
org.freedesktop.import1 - - - (activatable) - -
org.freedesktop.locale1 - - - (activatable) - -
org.freedesktop.login1 115271 systemd-logind root :1.130 systemd-logind.service - -
org.freedesktop.machine1 - - - (activatable) - -
org.freedesktop.systemd1 1 systemd root :1.15 - - -
org.freedesktop.timedate1 - - - (activatable) - -
在： https://dbus.freedesktop.org/doc/dbus-specification.html 中 有一段介紹：
Message Bus Overview
The message bus accepts connections from one or more applications. Once connected, applications can exchange messages with other applications that are also connected to the bus.
In order to route messages among connections, the message bus keeps a mapping from names to connections. Each connection has one unique-for-the-lifetime-of-the-bus name automatically assigned. Applications may request additional names for a connection. Additional names are usually "well-known names" such as "com.example.TextEditor1". When a name is bound to a connection, that connection is said to own the name.
The bus itself owns a special name, org.freedesktop.DBus, with an object located at /org/freedesktop/DBus that implements the org.freedesktop.DBus interface. This service allows applications to make administrative requests of the bus itself. For example, applications can ask the bus to assign a name to a connection.
Each name may have queued owners. When an application requests a name for a connection and the name is already in use, the bus will optionally add the connection to a queue waiting for the name. If the current owner of the name disconnects or releases the name, the next connection in the queue will become the new owner.
This feature causes the right thing to happen if you start two text editors for example; the first one may request "com.example.TextEditor1", and the second will be queued as a possible owner of that name. When the first exits, the second will take over.
Applications may send unicast messages to a specific recipient or to the message bus itself, or broadcast messages to all interested recipients. See the section called “Message Bus Message Routing” for details.
配置文件
/etc/dbus-1/system.conf 這個
/etc/dbus-1/session.conf
看到這個xml配置： https://thebigdoc.readthedocs.io/en/latest/dbus/system-dbus.html
監聽socket
/var/run/dbus/system_bus_socket
當關閉對 “/var/run/dbus/system_bus_socket” 的監聽后， dbus-send 就會失敗，但是： “#systemctl list-unit-files” 依然可以執行成功，這個也是我奇怪的地方
#systemctl stop dbus.socket
#systemctl stop dbus.service
#dbus-send --system --print-reply --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.DBus.Properties.Get string:'org.freedesktop.systemd1.Manager' string:'Version'
Failed to open connection to "system" message bus: Failed to connect to socket /var/run/dbus/system_bus_socket: Connection refused
systemd(org.freedesktop.systemd1)
查看connection：“org.freedesktop.systemd1” 有多少method？
#dbus-send --system --dest=org.freedesktop.systemd1 --type=method_call /org/freedesktop/systemd1 --print-reply org.freedesktop.DBus.Introspectable.Introspect
--
<method name="GetUnit">
<arg type="s" direction="in"/>
<arg type="o" direction="out"/>
</method>
<method name="GetUnitByPID">
<arg type="u" direction="in"/>
<arg type="o" direction="out"/>
</method>
<method name="LoadUnit">
<arg type="s" direction="in"/>
<arg type="o" direction="out"/>
</method>
<method name="StartUnit">
<arg type="s" direction="in"/>
<arg type="s" direction="in"/>
--
</method>
<method name="StartUnitReplace">
<arg type="s" direction="in"/>
<arg type="s" direction="in"/>
--
</method>
<method name="StopUnit">
<arg type="s" direction="in"/>
<arg type="s" direction="in"/>
--
除了“org.freedesktop.systemd1” 還有 “org.freedesktop.login1”:
# dbus-send --system --dest=org.freedesktop.login1 --type=method_call /org/freedesktop/login1 --print-reply org.freedesktop.DBus.Introspectable.Introspect
查看connection：“org.freedesktop.systemd1” 有多少interface?
其中： “org.freedesktop.DBus.Introspectable” ， “org.freedesktop.DBus.Properties” 來自於 Dbus，是一種通用的 interface
systemd 功能相關的，全部實現在 interface： “org.freedesktop.systemd1.Manager” 下的 method中.
#dbus-send --system --dest=org.freedesktop.systemd1 --type=method_call /org/freedesktop/systemd1 --print-reply org.freedesktop.DBus.Introspectable.Introspect | grep -i interface
<interface name="org.freedesktop.DBus.Peer">
</interface>
<interface name="org.freedesktop.DBus.Introspectable">
</interface>
<interface name="org.freedesktop.DBus.Properties">
</interface>
<interface name="org.freedesktop.systemd1.Manager">
</interface>
org.freedesktop.DBus.Properties 用法：
”org.freedesktop.DBus.Properties” 是一個通用的interface，其實，只要有這個interface，就會有這些method, 這個interface，是用來看interface的屬性的.
“org.freedesktop.DBus.Properties” 提供了一些方法：“Get”， “GetAll”， “Set”， “PropertiesChanged”，用來專門操作 interface的 "屬性"的:
#dbus-send --system --dest=org.freedesktop.systemd1 --type=method_call /org/freedesktop/systemd1 --print-reply org.freedesktop.DBus.Introspectable.Introspect
<interface name="org.freedesktop.DBus.Properties">
<method name="Get">
<arg name="interface" direction="in" type="s"/>
<arg name="property" direction="in" type="s"/>
<arg name="value" direction="out" type="v"/>
</method>
<method name="GetAll">
<arg name="interface" direction="in" type="s"/>
<arg name="properties" direction="out" type="a{sv}"/>
</method>
<method name="Set">
<arg name="interface" direction="in" type="s"/>
<arg name="property" direction="in" type="s"/>
<arg name="value" direction="in" type="v"/>
</method>
<signal name="PropertiesChanged">
<arg type="s" name="interface"/>
<arg type="a{sv}" name="changed_properties"/>
<arg type="as" name="invalidated_properties"/>
</signal>
</interface>
每個interface，可能會有“property”：
<interface name="org.freedesktop.systemd1.Manager">
<property name="Version" type="s" access="read">
<annotation name="org.freedesktop.DBus.Property.EmitsChangedSignal" value="const"/>
</property>
比如： 我們來看看 “org.freedesktop.systemd1” 的 “org.freedesktop.systemd1.Manager” 的幾個屬性吧：
#dbus-send --system --print-reply --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.DBus.Properties.Get string:'org.freedesktop.systemd1.Manager' string:'Version'
method return sender=:1.15 -> dest=:1.104 reply_serial=2
variant string "219"
#dbus-send --system --dest=org.freedesktop.systemd1 --type=method_call /org/freedesktop/systemd1 --print-reply org.freedesktop.DBus.Properties.Get string:"org.freedesktop.systemd1.Manager" string:"UnitPath"
method return sender=:1.1 -> dest=:1.30 reply_serial=2
variant array [
string "/mnt/systemd_build/etc/systemd/system"
string "/etc/systemd/system"
string "/run/systemd/system"
string "/run/systemd/generator"
string "/usr/local/lib/systemd/system"
string "/usr/lib/systemd/system"
string "/run/systemd/generator.late"
]
org.freedesktop.DBus.Manager 用法：
調用method的時候，必須要指定“interface”和“method”, 指定“路徑”， 指定“--type”.
如果，你調用的是“org.freedesktop.DBus.Properties” 這個interface，可能還需要指定“參數”（指定參數的時候，還需要指定參數的 “類型:string, int...”）
比如：
#dbus-send --system --dest=org.freedesktop.systemd1 --type=method_call /org/freedesktop/systemd1 --print-reply org.freedesktop.systemd1.Manager.ListUnits
#dbus-send --system --print-reply --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.DBus.Properties.Get string:'org.freedesktop.systemd1.Manager' string:'Version'
method return sender=:1.15 -> dest=:1.104 reply_serial=2
variant string "219"
#dbus-send --system --print-reply --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.DBus.Properties.Get string:'org.freedesktop.systemd1.Manager' string:'Features'
method return sender=:1.15 -> dest=:1.105 reply_serial=2
variant string "+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 -SECCOMP +BLKID +ELFUTILS +KMOD +IDN"
busctl
refs: http://0pointer.net/blog/the-new-sd-bus-api-of-systemd.html
#busctl tree org.freedesktop.login1
└─/org/freedesktop/login1
├─/org/freedesktop/login1/seat
│ └─/org/freedesktop/login1/seat/seat0
├─/org/freedesktop/login1/session
└─/org/freedesktop/login1/user
# busctl introspect org.freedesktop.login1 /org/freedesktop/login1/session
NAME TYPE SIGNATURE RESULT/VALUE FLAGS
org.freedesktop.DBus.Introspectable interface - - -
.Introspect method - s -
org.freedesktop.DBus.Peer interface - - -
.GetMachineId method - s -
.Ping method - - -
org.freedesktop.DBus.Properties interface - - -
.Get method ss v -
.GetAll method s a{sv} -
.Set method ssv - -
.PropertiesChanged signal sa{sv}as - -
interface method
#busctl call org.freedesktop.login1 /org/freedesktop/login1/session org.freedesktop.DBus.Properties GetAll
Expected interface parameter
dbus(org.freedesktop.DBus)
dbus-daemon
首先，我們知道： dbus-send, dbus-daemon, dbus-monitor 都來自dbus這個包。
所以，dbus-send , dbus-monitor --system 監控的就是 dbus-daemon。
dbus-daemon的啟動方式：
#systemctl cat dbus
# /usr/lib/systemd/system/dbus.service
[Unit]
Description=D-Bus System Message Bus
Requires=dbus.socket
After=syslog.target
[Service]
ExecStart=/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation
ExecReload=/bin/dbus-send --print-reply --system --type=method_call --dest=org.freedesktop.DBus / org.freedesktop.DBus.ReloadConfig
OOMScoreAdjust=-900
dbus-daemon 和 org.freedesktop.DBus 之間有關系嗎？
有的
dbus-daemon reload 方式：
#/bin/dbus-send --print-reply --system --type=method_call --dest=org.freedesktop.DBus / org.freedesktop.DBus.ReloadConfig
method return sender=org.freedesktop.DBus -> dest=:1.4 reply_serial=2
org.freedesktop.DBus 有哪些interface:
#/bin/dbus-send --print-reply --system --type=method_call --dest=org.freedesktop.DBus / org.freedesktop.DBus.Introspectable.Introspect | grep interface
<interface name="org.freedesktop.DBus">
</interface>
<interface name="org.freedesktop.DBus.Introspectable">
</interface>
org.freedesktop.DBus 有哪些method:
看上去，大部分都在 interface: org.freedesktop.DBus 下面:
#/bin/dbus-send --print-reply --system --type=method_call --dest=org.freedesktop.DBus / org.freedesktop.DBus.Introspectable.Introspect
method return sender=org.freedesktop.DBus -> dest=:1.18 reply_serial=2
string "<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node>
<interface name="org.freedesktop.DBus">
<method name="Hello">
<arg direction="out" type="s"/>
</method>
<method name="RequestName">
<arg direction="in" type="s"/>
<arg direction="in" type="u"/>
<arg direction="out" type="u"/>
</method>
<method name="ReleaseName">
<arg direction="in" type="s"/>
<arg direction="out" type="u"/>
</method>
<method name="StartServiceByName">
<arg direction="in" type="s"/>
<arg direction="in" type="u"/>
<arg direction="out" type="u"/>
</method>
<method name="UpdateActivationEnvironment">
<arg direction="in" type="a{ss}"/>
</method>
<method name="NameHasOwner">
<arg direction="in" type="s"/>
<arg direction="out" type="b"/>
</method>
<method name="ListNames">
<arg direction="out" type="as"/>
</method>
<method name="ListActivatableNames">
<arg direction="out" type="as"/>
</method>
<method name="AddMatch">
<arg direction="in" type="s"/>
</method>
<method name="RemoveMatch">
<arg direction="in" type="s"/>
</method>
<method name="GetNameOwner">
<arg direction="in" type="s"/>
<arg direction="out" type="s"/>
</method>
執行一個method：
報錯是因為， Hello method只有在connection建立的時候才會執行；
但是，這里還有一個問題，為什么dest: "org.freedesktop.DBus" 對應的path是:"/" 呢？
#/bin/dbus-send --print-reply --system --type=method_call --dest=org.freedesktop.DBus / org.freedesktop.DBus.Hello
Error org.freedesktop.DBus.Error.Failed: Already handled an Hello message
gdbus
來自： glib2
#gdbus introspect --system --dest org.freedesktop.systemd1 --object-path /org/freedesktop/systemd1
node /org/freedesktop/systemd1 {
interface org.freedesktop.DBus.Peer {
methods:
Ping();
GetMachineId(out s machine_uuid);
signals:
properties:
};
interface org.freedesktop.DBus.Introspectable {
methods:
Introspect(out s data);
signals:
properties:
};
interface org.freedesktop.DBus.Properties {
methods:
Get(in s interface,
in s property,
out v value);
GetAll(in s interface,
out a{sv} properties);
Set(in s interface,
in s property,
in v value);
signals:
PropertiesChanged(s interface,
a{sv} changed_properties,
as invalidated_properties);
properties:
};
interface org.freedesktop.systemd1.Manager {
methods:
GetUnit(in s arg_0,
out o arg_1);
Debug
#busctl
NAME PID PROCESS USER CONNECTION UNIT SESSION DESCRIPTION
:1.1 1 systemd root :1.1 - - -
:1.33 33465 dbus-monitor root :1.33 sshd.service - -
:1.36 33689 busctl root :1.36 sshd.service - -
net.reactivated.Fprint - - - (activatable) - -
org.freedesktop.DBus - - - - - - -
org.freedesktop.PolicyKit1 - - - (activatable) - -
org.freedesktop.hostname1 - - - (activatable) - -
org.freedesktop.import1 - - - (activatable) - -
org.freedesktop.locale1 - - - (activatable) - -
org.freedesktop.login1 - - - (activatable) - -
org.freedesktop.machine1 - - - (activatable) - -
org.freedesktop.systemd1 1 systemd root :1.1 - - -
org.freedesktop.timedate1 - - - (activatable) - -
當dbus-send 發送給 “org.freedesktop.systemd1” 一個 “method_call” 的時候：
注意：“1.1” 這個connection 就是 “org.freedesktop.systemd1” 這個connection，
你會注意到， “1.1” 返回 給 “1.38” 一個reply， 但是 "buctl" 確沒有看到 “1.38”， 這是因為dbus-send 執行的太快，消失了。
#dbus-send --system --dest=org.freedesktop.systemd1 --type=method_call /org/freedesktop/systemd1 --print-reply org.freedesktop.DBus.Properties.Get string:"org.freedesktop.systemd1.Manager" string:"Features"
method return sender=:1.1 -> dest=:1.38 reply_serial=2
variant string "-PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP -LIBCRYPTSETUP -GCRYPT -GNUTLS -ACL -XZ -LZ4 -SECCOMP +BLKID -ELFUTILS -KMOD -IDN"
#dbus-monitor --system
signal sender=org.freedesktop.DBus -> dest=(null destination) serial=82 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameOwnerChanged
string ":1.38"
string ""
string ":1.38"
signal sender=org.freedesktop.DBus -> dest=(null destination) serial=83 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameOwnerChanged
string ":1.38"
string ":1.38"
string ""
busctl高級用法
（首先，你需要知道，busctl來自systemd，有更好的耦合，不像dbus-send, dbus-monitor）
這個和 dbus-monitor 差不多：
#busctl monitor org.freedesktop.DBus
#busctl status $pid
#busctl status 1
Refs
dbus手冊（必讀）： https://dbus.freedesktop.org/doc/dbus-specification.html
https://blog.csdn.net/linweig/article/details/5068183
https://blog.fpmurphy.com/2013/05/exploring-systemd-d-bus-interface.html
systemd dbus: https://www.freedesktop.org/wiki/Software/systemd/dbus/
不錯的中文博客： http://www.fmddlmyy.cn/text52.html