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。