4.2.2(Jelly Bean)上root的方法比较简单，调用su命令就可以获取到root权限并执行一些命令。但是在Android 4.3+到5.0，Google为这种root方法设置了层层障碍：
1. su命令源码中添加了uid检验，只允许shell/root用户进行调用
2. Zygote源码中添加了添加DropCapabilitiesBoundingSet屏蔽APP了setuid的功能
3. adb源码中添加了添加should_drop_privileges屏蔽adb了setuid的功能(对于userdebug/eng版本该函数未被调用)
4. 开启了SELinux安全模块，1,2条都满足情况下也会被中断su。
具体解决方法：
针对前三项的解决方法是按照以下diff文件进行修改代码并重新编译Android系统：
su命令源码中添加了uid检验，只允许shell/root用户进行调用
Zygote源码中添加了添加DropCapabilitiesBoundingSet屏蔽APP了setuid的功能
adb源码中添加了添加should_drop_privileges屏蔽adb了setuid的功能(对于userdebug/eng版本该函数未被调用)
以上三项需要改动代码并重新编译Android系统。
1，2，3代码修改的diff文件：
project frameworks/base/
diff --git a/cmds/app_process/app_main.cpp b/cmds/app_process/app_main.cpp
index 1bb28c3..3e69750 100644
--- a/cmds/app_process/app_main.cpp
+++ b/cmds/app_process/app_main.cpp
@@ -185,6 +185,7 @@ static const char ZYGOTE_NICE_NAME[] = "zygote";
int main(int argc, char* const argv[])
{
+/*
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
// Older kernels don't understand PR_SET_NO_NEW_PRIVS and return
// EINVAL. Don't die on such kernels.
@@ -193,6 +194,7 @@ int main(int argc, char* const argv[])
return 12;
}
}
+*/
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
// Process command line arguments
diff --git a/core/jni/com_android_internal_os_Zygote.cpp b/core/jni/com_android_internal_os_Zygote.cpp
index 4f5e08b..8b136bd 100644
--- a/core/jni/com_android_internal_os_Zygote.cpp
+++ b/core/jni/com_android_internal_os_Zygote.cpp
@@ -208,6 +208,7 @@ static void EnableKeepCapabilities(JNIEnv* env) {
}
static void DropCapabilitiesBoundingSet(JNIEnv* env) {
+/*
for (int i = 0; prctl(PR_CAPBSET_READ, i, 0, 0, 0) >= 0; i++) {
int rc = prctl(PR_CAPBSET_DROP, i, 0, 0, 0);
if (rc == -1) {
@@ -220,6 +221,7 @@ static void DropCapabilitiesBoundingSet(JNIEnv* env) {
}
}
}
+*/
}
static void SetCapabilities(JNIEnv* env, int64_t permitted, int64_t effective) {
project system/core/
diff --git a/adb/adb.c b/adb/adb.c
index 10a1e0d..2cd4f97 100644
--- a/adb/adb.c
+++ b/adb/adb.c
@@ -1261,6 +1261,7 @@ static void drop_capabilities_bounding_set_if_needed() {
}
static int should_drop_privileges() {
+ return 0;
#ifndef ALLOW_ADBD_ROOT
return 1;
#else /* ALLOW_ADBD_ROOT */
diff --git a/include/private/android_filesystem_config.h b/include/private/android_filesystem_config.h
index 2f528b9..1223b45 100644
--- a/include/private/android_filesystem_config.h
+++ b/include/private/android_filesystem_config.h
@@ -244,7 +244,7 @@ static const struct fs_path_config android_files[] = {
/* the following five files are INTENTIONALLY set-uid, but they
* are NOT included on user builds. */
- { 04750, AID_ROOT, AID_SHELL, 0, "system/xbin/su" },
+ { 06755, AID_ROOT, AID_SHELL, 0, "system/xbin/su" },
{ 06755, AID_ROOT, AID_ROOT, 0, "system/xbin/librank" },
{ 06755, AID_ROOT, AID_ROOT, 0, "system/xbin/procrank" },
{ 06755, AID_ROOT, AID_ROOT, 0, "system/xbin/procmem" },
@@ -255,6 +255,7 @@ static const struct fs_path_config android_files[] = {
{ 00750, AID_ROOT, AID_ROOT, 0, "system/bin/uncrypt" },
{ 00750, AID_ROOT, AID_ROOT, 0, "system/bin/install-recovery.sh" },
+ { 06755, AID_ROOT, AID_ROOT, 0, "system/bin/su" },
{ 00755, AID_ROOT, AID_SHELL, 0, "system/bin/*" },
{ 00755, AID_ROOT, AID_ROOT, 0, "system/lib/valgrind/*" },
{ 00755, AID_ROOT, AID_ROOT, 0, "system/lib64/valgrind/*" },
project system/extras/
diff --git a/su/su.c b/su/su.c
index 8365379..826acfc 100644
--- a/su/su.c
+++ b/su/su.c
@@ -107,11 +107,12 @@ int main(int argc, char **argv)
/* Until we have something better, only root and the shell can use su. */
myuid = getuid();
+ /*
if (myuid != AID_ROOT && myuid != AID_SHELL) {
fprintf(stderr,"su: uid %d not allowed to su\n", myuid);
return 1;
}
-
+ */
if(argc < 2) {
uid = gid = 0;
} else {
设置SELinux权限为Permissive
我是尽量把事情简单化，这次这里讨论的是SELinux，其实现原理会比较复杂，我这里只是简单记录在Android 5.0 + Linux 3.4如何将SELinux降级，这么做的原因是实现：APP可以调用su命令最终实现运行一些命令。
SELinux常用状态有两个Permissive 和 Enforcing，后者会使APP无法调用su命令，前者可以。
1. 查询当前SELinux状态：
adb shell getenforce
运行该命令后，会输出当前系统的SELinux权限Permissive 和 Enforcing。
2. 在eng/userdebug版本 将SELinux 模式调整到Permissive mode
临时方案：运行如下命令（无需重启系统，即刻见效）
adb shell setenforce 0
长久方案:在启动参数中添加
androidboot.selinux=permissive
ROOT不成功问题解决方法
root不成功主要是因为没有改全这些项，有其中一项没有改好都会造成root不成功。这里把每项没有改好会造成的问题列出来以供参考。
安装运行 HelloRoot App，这是一个协助查看是否能获取root权限的小工具。
查看HelloRoot的Application user id
root@nanopi2:/ # ps | grep com.example.helloroot
u0_a50 1283 121 1233532 40640 ffffffff b6e791c4 S com.example.helloroot
root@nanopi2:/ #
其中u0_a50就是HelloRoot的Application user id了，这个id在下面步骤中会用到；
切换到u0_a50用户下并运行su命令
root@nanopi2:/ # su u0_a50
root@nanopi2:/ $ id
uid=10050(u0_a50) gid=10050(u0_a50) groups=1003(graphics),1004(input),1007(log),1011(adb),1015(sdcard_rw),1028(sdcard_r),3001(net_bt_admin),3002(net_bt),3003(inet),3006(net_bw_stats) context=u:r:su:s0
# 以u0_a50身份运行su命令，正常情况下会再切到root权限下
root@nanopi2:/ $ su
root@nanopi2:/ #
不正常的话会出现如下错误：
su: can't execute: Permission denied
这个属于u0_a50对su无可执行权限，说明include/private/android_filesystem_config.h中需要改的内容没有改或者是没有改成功。
su: uid 10050 not allowed to su
这个属于su不允许su和shell以外的用户（比如u0_a50）调用，说明su.c中需要改的内容没有改或者是没有改成功。
点击CheckRoot按钮，判断App是否可以root，如果Log信息如下：
System.err: java.io.IOException: Error running exec(). Command: [su] Working Directory: null Environment: null
System.err: at java.lang.ProcessManager.exec(ProcessManager.java:211)
System.err: at java.lang.Runtime.exec(Runtime.java:173)
System.err: at java.lang.Runtime.exec(Runtime.java:246)
System.err: at java.lang.Runtime.exec(Runtime.java:189)
System.err: at cn.trinea.android.common.util.ShellUtils.execCommand(ShellUtils.java:135)
System.err: at cn.trinea.android.common.util.ShellUtils.execCommand(ShellUtils.java:93)
System.err: at cn.trinea.android.common.util.ShellUtils.checkRootPermission(ShellUtils.java:44)
System.err: at com.example.helloroot.MainActivity.onCreate(MainActivity.java:25)
System.err: at android.app.Activity.performCreate(Activity.java:5990)
System.err: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
System.err: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
System.err: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
System.err: at android.app.ActivityThread.access$800(ActivityThread.java:151)
System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
System.err: at android.os.Handler.dispatchMessage(Handler.java:102)
System.err: at android.os.Looper.loop(Looper.java:135)
System.err: at android.app.ActivityThread.main(ActivityThread.java:5254)
System.err: at java.lang.reflect.Method.invoke(Native Method)
System.err: at java.lang.reflect.Method.invoke(Method.java:372)
System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
System.err: Caused by: java.io.IOException: Permission denied
System.err: at java.lang.ProcessManager.exec(Native Method)
System.err: at java.lang.ProcessManager.exec(ProcessManager.java:209)
System.err: ... 20 more
System.err: java.io.IOException: Error running exec(). Command: [su] Working Directory: null Environment: null
System.err: at java.lang.ProcessManager.exec(ProcessManager.java:211)
System.err: at java.lang.Runtime.exec(Runtime.java:173)
System.err: at java.lang.Runtime.exec(Runtime.java:246)
System.err: at java.lang.Runtime.exec(Runtime.java:189)
System.err: at cn.trinea.android.common.util.ShellUtils.execCommand(ShellUtils.java:135)
System.err: at cn.trinea.android.common.util.ShellUtils.execCommand(ShellUtils.java:56)
System.err: at com.example.helloroot.MainActivity.onCreate(MainActivity.java:26)
System.err: at android.app.Activity.performCreate(Activity.java:5990)
System.err: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
System.err: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
System.err: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
System.err: at android.app.ActivityThread.access$800(ActivityThread.java:151)
System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
System.err: at android.os.Handler.dispatchMessage(Handler.java:102)
System.err: at android.os.Looper.loop(Looper.java:135)
System.err: at android.app.ActivityThread.main(ActivityThread.java:5254)
System.err: at java.lang.reflect.Method.invoke(Native Method)
System.err: at java.lang.reflect.Method.invoke(Method.java:372)
System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
System.err: Caused by: java.io.IOException: Permission denied
System.err: at java.lang.ProcessManager.exec(Native Method)
System.err: at ... 19 more
A. 出现这个问题是由于没有将SELinux关闭，需要按照以下方法查看
# 通过查看SELinux状态
root@nanopi2:/ # getenforce
Enforcing
需要按照「设置SELinux权限为Permissive」进行设置。
B. 无法setuid的限制修改
core/jni/com_android_internal_os_Zygote.cpp和cmds/app_process/app_main.cpp修改。
参考文档：
1. How to set SELinux to 0 or permissive mode in android 4.4.4 and above? by user1147688
2. user版本如何打开root权限