Android 的系统属性(SystemProperties)设置分析

Android 的系统属性(SystemProperties)设置分析

作者：徐建祥(netpirate@gmail.com)

日期：2009/11/11

网址：http://www.anymobile.org

Android 的系统属性包括两部分：文件保存的持久属性和每次开机导入的cache属性。前者主要保存在下面几个文件中：

bionic
/
libc
/
include
/
sys
/
_system_properties.h

1

#define
PROP_SERVICE_NAME "property_service"

2

#define
PROP_PATH_RAMDISK_DEFAULT "/default.prop"

3

#define
PROP_PATH_SYSTEM_BUILD "/system/build.prop"

4

#define
PROP_PATH_SYSTEM_DEFAULT "/system/default.prop"

5

#define
PROP_PATH_LOCAL_OVERRIDE "/data/local.prop"

后者则通过frameworks/base/core/java/android/os/SystemProperties.java的接口定义，

1

private

static

native
String native_get(String key);

2

private

static

native
String native_get(String key, String def);

3

private

static

native

void
native_set(String key, String def);

4

public

static

void
set(String key, String val) {

5

if
(key.length()
>
PROP_NAME_MAX) {

6

throw

new
IllegalArgumentException(
"
key.length >
"

+
PROP_NAME_MAX);

7
}

8

if
(val
!=

null

&&
val.length()
>
PROP_VALUE_MAX) {

9

throw

new
IllegalArgumentException(
"
val.length >
"

+

10
PROP_VALUE_MAX);

11
}

12
native_set(key, val);

13
}

该接口类在初始化运行环境中注册对应的cpp接口android_os_SystemProperties.cpp，实际操作通过JNI调用的是cpp文件对应的接口：

frameworks/base/core/jni/AndroidRuntime.cpp

1

namespace
android {

2

extern

int
register_android_os_SystemProperties(JNIEnv
*
env);

3
}

frameworks/base/core/jni/android_os_SystemProperties.cpp

1

static

void
SystemProperties_set(JNIEnv
*
env, jobject clazz, jstring keyJ, jstring valJ)

2
{

3

int
err;

4

const

char
*
key;

5

const

char
*
val;

6
key
=
env
->
GetStringUTFChars(keyJ, NULL);

7

if
(valJ
==
NULL) {

8
val
=

""
;
/*
NULL pointer not allowed here
*/

9
}
else
{

10
val
=
env
->
GetStringUTFChars(valJ, NULL);

11
}

12
err
=
property_set(key, val);

13
env
->
ReleaseStringUTFChars(keyJ, key);

14

if
(valJ
!=
NULL) {

15
env
->
ReleaseStringUTFChars(valJ, val);

16
}

17
}

设置key的value时，需要作鉴权，根据设置程序所在进程的fd获知uid值，比如system server进程可以设置net打头的key，不可以设置gsm打头的key，相关的定义如下：

system/core/include/private/android_filesystem_config.h

1

#define
AID_ROOT 0 /* traditional unix root user */

2

#define
AID_SYSTEM 1000 /* system server */

3

#define
AID_RADIO 1001 /* telephony subsystem, RIL */

4

#define
AID_DHCP 1014 /* dhcp client */

5

#define
AID_SHELL 2000 /* adb and debug shell user */

6

#define
AID_CACHE 2001 /* cache access */

7

#define
AID_APP 10000 /* first app user */

system/core/init/property_service.c

1

#define
PERSISTENT_PROPERTY_DIR "/data/property"

2

struct
{

3

const

char

*
prefix;

4
unsigned
int
uid;

5
} property_perms[]
=
{

6
{
"
net.rmnet0.
"
, AID_RADIO },

7
{
"
net.gprs.
"
, AID_RADIO },

8
{
"
ril.
"
, AID_RADIO },

9
{
"
gsm.
"
, AID_RADIO },

10
{
"
net.dns
"
, AID_RADIO },

11
{
"
net.usb0
"
, AID_RADIO },

12
{
"
net.
"
, AID_SYSTEM },

13
{
"
dev.
"
, AID_SYSTEM },

14
{
"
runtime.
"
, AID_SYSTEM },

15
{
"
hw.
"
, AID_SYSTEM },

16
{
"
sys.
"
, AID_SYSTEM },

17
{
"
service.
"
, AID_SYSTEM },

18
{
"
wlan.
"
, AID_SYSTEM },

19
{
"
dhcp.
"
, AID_SYSTEM },

20
{
"
dhcp.
"
, AID_DHCP },

21
{
"
debug.
"
, AID_SHELL },

22
{
"
log.
"
, AID_SHELL },

23
{
"
service.adb.root
"
, AID_SHELL },

24
{
"
persist.sys.
"
, AID_SYSTEM },

25
{
"
persist.service.
"
, AID_SYSTEM },

26
{ NULL,
0
}

27
};

28

int
property_set(
const

char

*
name,
const

char

*
value)

29
{

30
property_changed(name, value);

31

return

0
;

32
}

33

int
start_property_service(
void
)

34
{

35

int
fd;

36

37
load_properties_from_file(PROP_PATH_SYSTEM_BUILD);

38
load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);

39
load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE);

40

/*
Read persistent properties after all default values have been loaded.
*/

41
load_persistent_properties();

42

43
fd
=
create_socket(PROP_SERVICE_NAME, SOCK_STREAM,
0666
,
0
,
0
);

44

if
(fd
<

0
)
return

-
1
;

45
fcntl(fd, F_SETFD, FD_CLOEXEC);

46
fcntl(fd, F_SETFL, O_NONBLOCK);

47

48
listen(fd,
8
);

49

return
fd;

50
}

51

void
handle_property_set_fd(
int
fd)

52
{

53

switch
(msg.cmd) {

54

case
PROP_MSG_SETPROP:

55
msg.name[PROP_NAME_MAX
-
1
]
=

0
;

56
msg.value[PROP_VALUE_MAX
-
1
]
=

0
;

57

58

if
(memcmp(msg.name,
"
ctl.
"
,
4
)
==

0
) {

59

if
(check_control_perms(msg.value, cr.uid)) {

60
handle_control_message((
char
*
) msg.name
+

4
, (
char
*
) msg.value);

61
}
else
{

62
ERROR(
"
sys_prop: Unable to %s service ctl [%s] uid: %d pid:%d\n
"
,

63
msg.name
+

4
, msg.value, cr.uid, cr.pid);

64
}

65
}
else
{

66

if
(check_perms(msg.name, cr.uid)) {

67
property_set((
char
*
) msg.name, (
char
*
) msg.value);

68
}
else
{

69
ERROR(
"
sys_prop: permission denied uid:%d name:%s\n
"
,

70
cr.uid, msg.name);

71
}

72
}

73

break
;

74

75

default
:

76

break
;

77
}

78
}

在开机启动后的init操作中，会执行一个loop循环，当检测到有新的设置时，进入设置流程，鉴权失败会提示相关的异常，如sys_prop: permission denied uid:1000 name:gsm.phone.id

system/core/init/init.c

1

void
property_changed(
const

char

*
name,
const

char

*
value)

2
{

3

if
(property_triggers_enabled) {

4
queue_property_triggers(name, value);

5
drain_action_queue();

6
}

7
}

8

int
main(
int
argc,
char

**
argv)

9
{

10
parse_config_file(
"
/init.rc
"
);

11
qemu_init();

12
device_fd
=
device_init();

13
property_init();

14
fd
=
open(console_name, O_RDWR);

15
property_set_fd
=
start_property_service();

16
ufds[
0
].fd
=
device_fd;

17
ufds[
0
].events
=
POLLIN;

18
ufds[
1
].fd
=
property_set_fd;

19
ufds[
1
].events
=
POLLIN;

20
ufds[
2
].fd
=
signal_recv_fd;

21
ufds[
2
].events
=
POLLIN;

22
fd_count
=

3
;

23

for
(;;) {

24

if
(ufds[
0
].revents
==
POLLIN)

25
handle_device_fd(device_fd);

26

27

if
(ufds[
1
].revents
==
POLLIN)

28
handle_property_set_fd(property_set_fd);

29

if
(ufds[
3
].revents
==
POLLIN)

30
handle_keychord(keychord_fd);

31
}

32

return

0
;

33
}

OVER！