方法:

修改./default.prop

把ro.secure设为0,persist.service.adb.enable设为1,adbd进程就会以root用户的身份启动。

 

原理:

可以看一下Android系统根目录下的/init.rc的片段:

… …

# adbd is controlled by the persist.service.adb.enable system property

service adbd /sbin/adbd

disabled

# adbd on at boot in emulator

on property:ro.kernel.qemu=1

start adbd

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

start adbd

on property:persist.service.adb.enable=0

stop adbd

… …

这里定义了一个触发器,只要persist.service.adb.enable值被置为1,就会启动/sbin/adbd。

 

在build目录下搜索一下,发现了build/core/main.mk中有这样的代码片段

## user/userdebug ##

 

user_variant := $(filter userdebug user,$(TARGET_BUILD_VARIANT))

enable_target_debugging := true

ifneq (,$(user_variant))

# Target is secure in user builds.

ADDITIONAL_DEFAULT_PROPERTIES += ro.secure=1

 

tags_to_install := user

ifeq ($(user_variant),userdebug)

# Pick up some extra useful tools

tags_to_install += debug

else

# Disable debugging in plain user builds.

enable_target_debugging :=

endif

 

# TODO: Always set WITH_DEXPREOPT (for user builds) once it works on OSX.

# Also, remove the corresponding block in config/product_config.make.

ifeq ($(HOST_OS)-$(WITH_DEXPREOPT_buildbot),linux-true)

WITH_DEXPREOPT := true

endif

 

# Disallow mock locations by default for user builds

ADDITIONAL_DEFAULT_PROPERTIES += ro.allow.mock.location=0

 

else # !user_variant

# Turn on checkjni for non-user builds.

ADDITIONAL_BUILD_PROPERTIES += ro.kernel.android.checkjni=1

# Set device insecure for non-user builds.

ADDITIONAL_DEFAULT_PROPERTIES += ro.secure=0

# Allow mock locations by default for non user builds

ADDITIONAL_DEFAULT_PROPERTIES += ro.allow.mock.location=1

endif # !user_variant

 

ifeq (true,$(strip $(enable_target_debugging)))

# Target is more debuggable and adbd is on by default

ADDITIONAL_DEFAULT_PROPERTIES += ro.debuggable=1 persist.service.adb.enable=1

# Include the debugging/testing OTA keys in this build.

INCLUDE_TEST_OTA_KEYS := true

else # !enable_target_debugging

# Target is less debuggable and adbd is off by default

ADDITIONAL_DEFAULT_PROPERTIES += ro.debuggable=0 persist.service.adb.enable=0

endif # !enable_target_debugging

这段代码我大致解释一下:

主要通过判断当前的编译模式来给几个属性赋予不同的值,然后把属性存储在ADDITIONAL_DEFAULT_PROPERTIES这个变量中, 这个变量在后面是要写到根目录下的/default.prop中去,在系统启动时被属**加载的。也就是说我们在/default.prop中看到的几 个属性的值是在这里设置的。

只看两个属性ro.secure,persist.service.adb.enable。当前是user模式的话,编译系统会把 ro.secure置为1,把persist.service.adb.enable置为0.也就是说,用user模式编译出来的系统运行在安全模式 下,adbd默认关闭。即使通过设置属性的方式打开,adbd进程的用户也是shell,不具有root权限。这样,普通用户或者开发者拿到一个机器后, 通过PC运行adb shell时,是以shell用户登录机器的。

好了,现在把ro.secure置为0,再重新编译,只要设置属性persist.service.adb.enable的值为1,adbd进程就会以root用户的身份启动。