Android:Static storage paths aren’t available from AID_SYSTEM 错误分析及解决方法

适用于 Android 官方模拟器的 platform.x509.pem 和 platform.pk8 这两个文件可以在GitHub上下载：https://github.com/android/platform_build/tree/master/target/product/security
签名工具 signapk.jar 下载：http://url.cn/Pmgjeq 用到的命令如下；

// 给未签名的apk文件用系统对应的 platform.x509.pem 和 platform.pk8 两个文件签名
java -jar signapk.jar platform.x509.pem platform.pk8 debug.apk debug_signed.apk

// ADB 安装和卸载的命令
adb install debug_signed.apk
adb uninstall com.ifeegoo.debug

A/Environment(1261): Static storage paths aren't available from AID_SYSTEM
A/Environment(1261): java.lang.Throwable
A/Environment(1261): at android.os.Environment.throwIfSystem(Environment.java:637)
A/Environment(1261): at android.os.Environment.getExternalStorageDirectory(Environment.java:316)
A/Environment(1261): at com.ifeegoo.debug.utils.FileManager.saveToExternalStorageDirectory(FileManager.java:18)
A/Environment(1261): at com.ifeegoo.debug.activities.MainActivity$1.run(MainActivity.java:23)
A/Environment(1261): at java.lang.Thread.run(Thread.java:856)
W/System.err(1261): java.io.FileNotFoundException: /mnt/sdcard/ifeegoo.txt: open failed: EACCES (Permission denied)
W/System.err(1261): at libcore.io.IoBridge.open(IoBridge.java:416)
W/System.err(1261): at java.io.FileOutputStream.<init>(FileOutputStream.java:88)
W/System.err(1261): at java.io.FileOutputStream.<init>(FileOutputStream.java:73)
W/System.err(1261): at com.ifeegoo.debug.utils.FileManager.saveToExternalStorageDirectory(FileManager.java:17)
W/System.err(1261): at com.ifeegoo.debug.activities.MainActivity$1.run(MainActivity.java:23)
W/System.err(1261): at java.lang.Thread.run(Thread.java:856)
W/System.err(1261): Caused by: libcore.io.ErrnoException: open failed: EACCES (Permission denied)
W/System.err(1261): at libcore.io.Posix.open(Native Method)
I/Choreographer(1261): Skipped 63 frames!  The application may be doing too much work on its main thread.
W/System.err(1261): at libcore.io.BlockGuardOs.open(BlockGuardOs.java:110)
W/System.err(1261): at libcore.io.IoBridge.open(IoBridge.java:400)
W/System.err(1261): ... 5 more

应用通过系统对应的 platform.x509.pem 和 platform.pk8 文件签名，AndroidManifest.xml 中有声明 android:sharedUserId=”android.uid.system” 和写入 Micro SD卡权限：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.ifeegoo.debug"
    android:sharedUserId="android.uid.system"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="19" />

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/icon_launcher" >
        <activity
            android:name="com.ifeegoo.debug.activities.MainActivity" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

通过错误日志第3行的提示，查找源代码，发现在 android.os.Environment 这个类中，有以下方法产生以上错误：

	private static void throwIfSystem()
	{
		if (Process.myUid() == Process.SYSTEM_UID)
		{
			Log.wtf(TAG,
					"Static storage paths aren't available from AID_SYSTEM",
					new Throwable());
		}
	}

当 Process.myUid() 的值与 Process.SYSTEM_UID 的值相等时，会抛出异常！查看 android.os.Process 这个类，有如下说明，由于个人英语水平和Linux知识有限，不能理解。但是可以通过另外的方法来验证以上条件是否成立。

    /**
     * Defines the UID/GID under which system code runs.
     */
    public static final int SYSTEM_UID = 1000;

    /**
     * Returns the identifier of this process's uid.  This is the kernel uid
     * that the process is running under, which is the identity of its
     * app-specific sandbox.  It is different from {@link #myUserHandle} in that
     * a uid identifies a specific app sandbox in a specific user.
     */
    public static final native int myUid();

通过应用内打印日志的方式，发现如下结果：

D/Process(1806): Process.myUid() = 1000

凭借着自己的对进程ID的一点认识，觉得这个值会不会变呢？会不会这次恰巧是1000呢，通过5次的验证，发现如下结果：

备注：每一次都是卸载重新安装重启之后打开应用测试的结果！

08-28 09:31:26.149: D/Process(995): Process.myUid() = 1000
08-28 09:35:18.577: D/Process(718): Process.myUid() = 1000
08-28 09:38:43.600: D/Process(722): Process.myUid() = 1000
08-28 09:41:40.382: D/Process(730): Process.myUid() = 1000
08-28 09:44:17.761: D/Process(724): Process.myUid() = 1000

通过 adb shell 的 ps 命令，查看到本应用 com.ifeegoo.debug 的 USER 标识为 system ：

C:\>adb shell
root@android:/ # ps
ps
USER     PID   PPID  VSIZE  RSS     WCHAN    PC         NAME
root      1     0     296    208   c0098770 0000e840 S /init
root      2     0     0      0     c005048c 00000000 S kthreadd
root      3     2     0      0     c0042268 00000000 S ksoftirqd/0
root      4     2     0      0     c004ce30 00000000 S events/0
root      5     2     0      0     c004ce30 00000000 S khelper
root      6     2     0      0     c004ce30 00000000 S suspend
root      7     2     0      0     c004ce30 00000000 S kblockd/0
root      8     2     0      0     c004ce30 00000000 S cqueue
root      9     2     0      0     c016f7c4 00000000 S kseriod
root      10    2     0      0     c004ce30 00000000 S kmmcd
root      11    2     0      0     c006f36c 00000000 S pdflush
root      12    2     0      0     c006f36c 00000000 S pdflush
root      13    2     0      0     c007340c 00000000 S kswapd0
root      14    2     0      0     c004ce30 00000000 S aio/0
root      25    2     0      0     c016d0f8 00000000 S mtdblockd
root      26    2     0      0     c004ce30 00000000 S kstriped
root      27    2     0      0     c004ce30 00000000 S hid_compat
root      28    2     0      0     c004ce30 00000000 S rpciod/0
root      29    2     0      0     c0189ddc 00000000 S mmcqd
root      30    1     292    172   c0098770 0000e840 S /sbin/ueventd
system    31    1     836    348   c0195c08 40036fc0 S /system/bin/servicemanager
root      32    1     4008   860   ffffffff 4003e76c S /system/bin/vold
root      34    1     8632   1252  ffffffff 4006a76c S /system/bin/netd
root      35    1     880    388   c01a10a0 40037a70 S /system/bin/debuggerd
radio     36    1     5468   836   ffffffff 4003776c S /system/bin/rild
system    37    1     13388  3196  ffffffff 4006bfc0 S /system/bin/surfaceflinger
root      38    1     165936 33672 ffffffff 400370e4 S zygote
drm       39    1     6564   2320  ffffffff 400befc0 S /system/bin/drmserver
media     40    1     23040  6404  ffffffff 4008cfc0 S /system/bin/mediaserver
install   41    1     848    472   c021db90 40036d50 S /system/bin/installd
keystore  42    1     1796   892   c01a10a0 40037a70 S /system/bin/keystore
root      43    1     828    372   c00b4eb0 40037ebc S /system/bin/qemud
shell     46    1     764    460   c0148178 40031d50 S /system/bin/sh
root      47    1     5524   300   ffffffff 00015ef0 S /sbin/adbd
system    278   38    248768 43040 ffffffff 40036fc0 S system_server
u0_a23    399   38    182520 33800 ffffffff 40037ebc S com.android.systemui
u0_a24    426   38    177720 20876 ffffffff 40037ebc S com.android.inputmethod.latin
radio     444   38    197980 26020 ffffffff 40037ebc S com.android.phone
system    458   38    183816 19280 ffffffff 40037ebc S com.android.settings
u0_a4     503   38    201724 33892 ffffffff 40037ebc S android.process.acore
u0_a5     519   38    194748 35200 ffffffff 40037ebc S com.android.launcher
u0_a32    528   38    176204 17928 ffffffff 40037ebc S com.android.music
u0_a10    558   38    180700 21524 ffffffff 40037ebc S android.process.media
u0_a1     573   38    177700 17544 ffffffff 40037ebc S com.android.quicksearchbox
u0_a4     620   38    183928 21000 ffffffff 40037ebc S com.android.contacts
u0_a16    642   38    174268 16404 ffffffff 40037ebc S com.android.location.fused
u0_a3     657   38    180948 20592 ffffffff 40037ebc S com.android.mms
u0_a6     681   38    178692 19916 ffffffff 40037ebc S com.android.deskclock
u0_a28    703   38    183548 17936 ffffffff 40037ebc S com.android.exchange
u0_a33    725   38    180920 19316 ffffffff 40037ebc S com.android.providers.calendar
u0_a26    741   38    186104 20064 ffffffff 40037ebc S com.android.calendar
u0_a9     933   38    176340 16664 ffffffff 40037ebc S com.android.defcontainer
u0_a14    951   38    174256 16012 ffffffff 40037ebc S com.svox.pico
root      984   47    752    432   c002a7a0 4003294c S /system/bin/sh
root      986   984   720    412   c0098770 400370e4 S logcat
system    995   38    175676 20056 ffffffff 40037ebc S com.ifeegoo.debug
root      1014  47    764    480   c002a7a0 4003294c S /system/bin/sh
root      1019  1014  1092   436   00000000 40036d50 R ps

其它4次的结果：

C:\>adb shell
root@android:/ # ps
ps
USER     PID   PPID  VSIZE  RSS     WCHAN    PC         NAME
***
system    718   38    175676 20060 ffffffff 40037ebc S com.ifeegoo.debug
system    722   38    175676 20060 ffffffff 40037ebc S com.ifeegoo.debug
system    730   38    175676 20060 ffffffff 40037ebc S com.ifeegoo.debug
system    724   38    175676 20068 ffffffff 40037ebc S com.ifeegoo.debug

去掉应用 AndroidManifest.xml 文件中的 android:sharedUserId=”android.uid.system” 的声明，还是用系统对应的 platform.x509.pem, platform.pk8 文件对应用进行签名，得到的测试结果：

08-28 10:19:01.193: D/Process(1448): Process.myUid() = 10046
08-28 10:22:45.724: D/Process(705):  Process.myUid() = 10046
08-28 10:24:54.461: D/Process(697):  Process.myUid() = 10046
08-28 10:27:49.170: D/Process(699):  Process.myUid() = 10046
08-28 10:29:59.600: D/Process(708):  Process.myUid() = 10046

C:\>adb shell
root@android:/ # ps
ps
USER     PID   PPID  VSIZE  RSS     WCHAN    PC         NAME
***
u0_a46    1448  38    175676 19956 ffffffff 40037ebc S com.ifeegoo.debug
u0_a46    705   38    175676 19968 ffffffff 40037ebc S com.ifeegoo.debug
u0_a46    697   38    175676 19968 ffffffff 40037ebc S com.ifeegoo.debug
u0_a46    699   38    175676 19960 ffffffff 40037ebc S com.ifeegoo.debug
u0_a46    773   38    175676 19968 ffffffff 40037ebc S com.ifeegoo.debug

从以上结果可以看到，去掉应用 AndroidManifest.xml 文件中的 android:sharedUserId=”android.uid.system” 的声明，然后 Process.myUid() 的值也不等于 1000，同时 USER 标识也变成了 u0_a46，再者，android.os.Environment 这个类中的 throwIfSystem()
方法是在以下方法中调用：

	public static File getExternalStorageDirectory()
	{
		throwIfSystem();
		return sCurrentUser.getExternalStorageDirectory();
	}

同时，通过查看源代码和逐一测试得到以下结果：

—Android 4.2.2 以下版本是不会报 “Static storage paths aren’t available from AID_SYSTEM” 错误，但是会报文件写入 Permission denied 。
—Android 4.3 不会报 “Static storage paths aren’t available from AID_SYSTEM” 错误，但是会报文件写入 Permission denied 。
—Android 4.4.2 不会报 “Static storage paths aren’t available from AID_SYSTEM” 错误，并且能够写入成功！！！！！！

/**
 * Android 4.2.2 以下
 * 
 */
public class Environment
{
    public static File getExternalStorageDirectory() 
    {
        return EXTERNAL_STORAGE_DIRECTORY;
    }
}

/**
 * Android 4.2.2
 * 
 */
public class Environment
{
	public static File getExternalStorageDirectory()
	{
		throwIfSystem();
		return sCurrentUser.getExternalStorageDirectory();
	}

	private static void throwIfSystem()
	{
		if (Process.myUid() == Process.SYSTEM_UID)
		{
			Log.wtf(TAG,
					"Static storage paths aren't available from AID_SYSTEM",
					new Throwable());
		}
	}
}

/**
* Android 4.3 / Android 4.4.2
*/
public class Environment
{
	private static boolean sUserRequired;

	public static File getExternalStorageDirectory()
	{
		throwIfUserRequired();
		return sCurrentUser.getExternalStorageDirectory();
	}

	/** {@hide} */
	public static void setUserRequired(boolean userRequired)
	{
		sUserRequired = userRequired;
	}

	private static void throwIfUserRequired()
	{
		if (sUserRequired)
		{
			Log.wtf(TAG,
					"Path requests must specify a user by using UserEnvironment",
					new Throwable());
		}
	}
}

从以上 Android 4.3 / Android 4.4.2 代码可以看到，如果不调用隐藏方法 setUserRequired(boolean userRequired) 的话，就不会抛出异常。由此可以得到以下结论：

探讨解决方法：

1. 由于涉及到向 Micro SD 卡写入数据，是调用 android.os.Environment 类中的 getExternalStorageDirectory() 方法得到外部存储路径的，我们可不可以采用 “hardcode” 方式直接写出已知外部存储路径（备注：此应用运行在定制的 Android Pad 上！），这样我们是不是就可以解决这个问题呢？

/**
 * @author ifeegoo www.ifeegoo.com
 */
public static void saveToExternalStorageDirectory(String filename,
		String content)
{
	if ((filename == null) || (content == null))
	{
		return;
	}

	FileOutputStream fileOutputStream = null;
	try
	{
		// 将 Environment.getExternalStorageDirectory().getAbsolutePath() 替换成 "mnt/sdcard"
		fileOutputStream = new FileOutputStream(new File("mnt/sdcard",
				filename));
		fileOutputStream.write(content.getBytes());
	} catch (FileNotFoundException e)
	{
		e.printStackTrace();
	} catch (IOException e)
	{
		e.printStackTrace();
	} finally
	{
		try
		{
			if (fileOutputStream != null)
			{
				fileOutputStream.close();
			}
		} catch (IOException e)
		{
			e.printStackTrace();
		}
	}
}

W/System.err(5089): java.io.FileNotFoundException: /mnt/sdcard/ifeegoo.txt: open failed: EACCES (Permission denied)
W/System.err(5089): at libcore.io.IoBridge.open(IoBridge.java:416)
W/System.err(5089): at java.io.FileOutputStream.<init>(FileOutputStream.java:88)
W/System.err(5089): at java.io.FileOutputStream.<init>(FileOutputStream.java:73)
W/System.err(5089): at com.ifeegoo.debug.utils.FileManager.saveToExternalStorageDirectory(FileManager.java:28)
W/System.err(5089): at com.ifeegoo.debug.activities.MainActivity$1.run(MainActivity.java:23)
W/System.err(5089): at java.lang.Thread.run(Thread.java:856)
W/System.err(5089): Caused by: libcore.io.ErrnoException: open failed: EACCES (Permission denied)
W/System.err(5089): at libcore.io.Posix.open(Native Method)

在Android 4.2.2 中测试，从以上日志可以看到，虽然没有报 “Static storage paths aren’t available from AID_SYSTEM” 错误，但是依然会有无法写入文件，Permission denied 。检查是否写入文件：

C:\>adb shell
root@android:/ # cd /mnt/sdcard
cd /mnt/sdcard
root@android:/mnt/sdcard # ls
ls
Alarms
Android
DCIM
Download
LOST.DIR
Movies
Music
Notifications
Pictures
Podcasts
Ringtones
www
root@android:/mnt/sdcard #

2. 由于涉及到向 Micro SD 卡写入数据，如果说应用的数据不必非要写入 Micro SD 卡的话，我们尝试写入到应用内部存储目录：

/**
 * @author ifeegoo www.ifeegoo.com
 */
public static void saveToInternalFilesDirectory(Context context,
		String filename, String content)
{
	if ((context == null) || (filename == null) || (content == null))
	{
		return;
	}

	FileOutputStream fileOutputStream = null;
	try
	{
		fileOutputStream = new FileOutputStream(new File(context
				.getFilesDir().getAbsolutePath(), filename));
		fileOutputStream.write(content.getBytes());
	} catch (FileNotFoundException e)
	{
		e.printStackTrace();
	} catch (IOException e)
	{
		e.printStackTrace();
	} finally
	{
		try
		{
			if (fileOutputStream != null)
			{
				fileOutputStream.close();
			}
		} catch (IOException e)
		{
			e.printStackTrace();
		}
	}
}

检查是否写入到内部存储目录：

C:\>adb shell
root@android:/ # cd /data/data/com.ifeegoo.debug/files
cd /data/data/com.ifeegoo.debug/files
root@android:/data/data/com.ifeegoo.debug/files # ls
ls
ifeegoo.txt
root@android:/data/data/com.ifeegoo.debug/files # cat ifeegoo.txt
cat ifeegoo.txt
www.ifeegoo.com
root@android:/data/data/com.ifeegoo.debug/files #

3. Android 4.2.2 API 中，从 android.os.Environment 这个类中，发现有一个隐藏的静态内部类，里面也有一个方法为 getExternalStorageDirectory() ，同时这个方法并没有调用 throwIfSystem() 方法，如果说能够用反射来调用这个里面的方法，是不是就可以解决这个问题呢？

public class Environment
{
	/** {@hide} */
	public static class UserEnvironment
	{
		public File getExternalStorageDirectory()
		{
			return mExternalStorage;
		}
	}
}

/**
 * UserEnvironment 类的构造方法中需要传入 userId，不明白这个 userId，暂且传入一个int数值0
 * 
 * @author ifeegoo www.ifeegoo.com
 * 
 */
public class ReflectManager
{
	public static String getExternalStoragePath()
	{
		try
		{
			Class<?> klass = Class
					.forName("android.os.Environment$UserEnvironment");

			Method method = klass
					.getDeclaredMethod("getExternalStorageDirectory");
			Object object = method.invoke(klass.getConstructor(int.class)
					.newInstance(new Object[] { 0 }));

			if (object != null)
			{
				return object.toString();
			}
		} catch (ClassNotFoundException e)
		{
			e.printStackTrace();
		} catch (NoSuchMethodException e)
		{
			e.printStackTrace();
		} catch (IllegalAccessException e)
		{
			e.printStackTrace();
		} catch (IllegalArgumentException e)
		{
			e.printStackTrace();
		} catch (InvocationTargetException e)
		{
			e.printStackTrace();
		} catch (InstantiationException e)
		{
			e.printStackTrace();
		}

		return null;
	}
}

这样是可以避免抛出异常，但是以上方法其实和第一种方法一样，虽然避免了采用那种 “hardcode” 方式来获取外部存储路径，但是依然写入 Micro SD 卡，出现 Permission denied ，依然是权限问题！

4. 从上面的研究看来，即使不报 “Static storage paths aren’t available from AID_SYSTEM” 错误，也会出现读写权限的问题，那可不可以自己在应用中，自己更改 Micro SD 卡的读写权限呢？

/**
 * 
 * @author ifeegoo www.ifeegoo.com
 *
 */
public class AdbShellManager
{
	public static String chmod777(String path)
	{
		String[] args = { "chmod", "777", path, "\n" };
		ProcessBuilder processBuilder = new ProcessBuilder(args);
		
		String result = null;
		Process process = null;
		InputStream standardErrorInputStream = null;
		InputStream standardOutInputStream = null;
		
		try
		{
			ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
			int read = -1;
			if (processBuilder != null)
			{
				process = processBuilder.start();
			}
			
			standardErrorInputStream = process.getErrorStream();
			if (standardErrorInputStream != null)
			{
				while ((read = standardErrorInputStream.read()) != -1)
				{
					byteArrayOutputStream.write(read);
				}
			}
			
			byteArrayOutputStream.write('\n');
			
			standardOutInputStream = process.getInputStream();
			if (standardOutInputStream != null)
			{
				while ((read = standardOutInputStream.read()) != -1)
				{
					byteArrayOutputStream.write(read);
				}
			}
			
			result = new String(byteArrayOutputStream.toByteArray());
		} catch (IOException e)
		{
			e.printStackTrace();
		} catch (Exception e)
		{
			e.printStackTrace();
		} finally
		{
			try
			{
				if (standardErrorInputStream != null)
				{
					standardErrorInputStream.close();
				}
				if (standardOutInputStream != null)
				{
					standardOutInputStream.close();
				}
			} catch (IOException e)
			{
				e.printStackTrace();
			}
			
			if (process != null)
			{
				process.destroy();
			}
		}
		
		return result;
	}
}

在以上 AdbShellManager 类中的 chmod777(String path) 方法传入 “mnt/sdcard” ,在 Android 4.2.2 系统中的测试结果如下：

08-31 23:04:25.404: D/chmod777(971): Unable to chmod 
08-31 23:04:25.404: D/chmod777(971): : No such file or directory

以上结果提示 “Unable to chmod” , “No such file or directory”。我们查看目录是否存在：

C:\>adb shell
root@android:/mnt/sdcard # cd /mnt/sdcard
cd /mnt/sdcard
root@android:/mnt/sdcard # ls
ls
Alarms
Android
DCIM
Download
LOST.DIR
Movies
Music
Notifications
Pictures
Podcasts
Ringtones
www

可以看到 “mnt/sdcard” 目录是存在的，但是不知道为什么提示“无此文件或目录”，难道是因为更改权限没有成功或者访问权限的问题？尝试先请求 root 权限，将 AdbShellManager 类中的 chmod777(String path)
方法中的 String[] args = { “chmod”, “777”, path, “\n” };
更改成 String[] args = { “su”, “\n”, “chmod”, “777”, path, “\n” }; 测试结果如下：

08-31 23:22:13.765: D/chmod777(1337): uid 1000 not allowed to su

以上结果提示拒绝获得 root 权限。

对比模拟器和定制的 Android Pad 两个版本的 Android 4.2.2 中 Micro SD 卡写入权限：

// 在当前路径下用 ls -l 可以查看当前路径的读写权限
// Android Simulator
d---rwxr-x system   sdcard_rw          1970-01-01 00:00 sdcard
// Android Pad
drwxrwxr-x system   sdcard_rw          2014-09-01 19:07 sdcard

5. 既然本应用是运行在定制的 Android Pad 上，通过源代码可以看到，只有 Android 4.2.2 才会出现报 “Static storage paths aren’t available from AID_SYSTEM” 错误，那么我们可以将当前运行在 Android Pad 上的 Android 4.2.2 系统换成 Android 4.4.2，这样是否就可以解决这个问题了呢？

C:\>adb shell
root@android:/mnt/sdcard # cd /mnt/sdcard
cd /mnt/sdcard
root@generic:/mnt/sdcard # ls
ls
Alarms
Android
DCIM
Download
LOST.DIR
Movies
Music
Notifications
Pictures
Podcasts
Ringtones
ifeegoo.txt
www
root@generic:/mnt/sdcard # cat ifeegoo.txt
cat ifeegoo.txt
www.ifeegoo.com
root@generic:/mnt/sdcard #

再确认下应用的 USER 标识：

C:\>adb shell
root@generic:/ # ps
ps
USER     PID   PPID  VSIZE  RSS     WCHAN    PC         NAME
***
system    1270  54    214952 20884 ffffffff b6efe5cc S com.ifeegoo.debug

6. 既然本应用是运行在定制的 Android Pad 上，因为在 Android 4.2.2 系统中的 android.os.Environment 类中的 getExternalStorageDirectory() 方法调用了 throwIfSystem() 方法，导致了报 “Static storage paths aren’t available from AID_SYSTEM” 错误，同时由于系统限制了通过声明 android:sharedUserId=”android.uid.system”，并且通过系统的 platform.x509.pem 和 platform.pk8 文件签名的应用读写 Micro SD 卡权限，那么我们可否通过更改框架层的代码，并且更改应用对 Micro SD 卡读写权限来解决以上问题呢？

答案是肯定的！

备注：由于我的应用运行在定制的 Android Pad 上，并且这个 Android Pad 是提供解决方案的公司做 Framework 层的工程师来修改源代码的，我本人是做应用层的，对这个修改框架层的代码也不懂，但是在 CSDN 上发现一篇作者 ID 为 echojiangyq_fight 写的一篇文章，恰好是我这个问题，我将文章内容转载过来，以供参考（以下内容中没有看到作者提到是否处理 throwIfSystem()方法）：

2015-12-03(四)新增思考：

******************************************************

移植原来v210（三星平台，android2.3系统）的老程序到mtk6575 android4.2上，遇到的一个问题，因为要读写settings的共享数据库，必须要获得system uid，但是这时向sdcard写log时就会遇到权限问题，陷入两者不能兼得的尴尬境地。因为有源码，选择了修改void从而对system uid开放scard写权限的方式。原来的sdcard权限：

root@android:/system/bin # ls -l /storage/sdcard0
ls -l /storage/sdcard0
----rwxr-x system  sdcard_rw 7390845 2012-12-15 10:43 22.mp4
d---rwxr-x system  sdcard_rw         2013-05-29 00:00 Alarms
d---rwxr-x system  sdcard_rw         2013-05-29 00:00 Android
d---rwxr-x system  sdcard_rw         2013-05-29 00:00 DCIM
d---rwxr-x system  sdcard_rw         2013-05-29 00:00 Download
d---rwxr-x system  sdcard_rw         2013-05-29 00:10 LOST.DIR
d---rwxr-x system  sdcard_rw         2014-05-30 14:57 MTK
d---rwxr-x system  sdcard_rw         2013-05-29 00:00 Movies
d---rwxr-x system  sdcard_rw         2014-05-30 14:08 Music
d---rwxr-x system  sdcard_rw         2013-05-29 00:00 Notifications
d---rwxr-x system  sdcard_rw         2013-05-29 00:00 Pictures
d---rwxr-x system  sdcard_rw         2013-05-29 00:00 Podcasts
d---rwxr-x system  sdcard_rw         2014-05-29 19:29 Recording
d---rwxr-x system  sdcard_rw         2013-05-29 00:00 Ringtones
d---rwxr-x system  sdcard_rw         2014-05-30 17:51 TestDetailLogs
d---rwxr-x system  sdcard_rw         2013-05-29 01:02 mtklog
d---rwxr-x system  sdcard_rw         2014-05-30 17:35 test

修改源码 /system/vold/Volume.cpp

#ifdef MTK_EMMC_DISCARD
	// 0702 --> 0002
	if (Fat::doMount(devicePath, "mnt/secure/staging", false, false, false,
		AID_SYSTEM, gid, 0002, true, IsEmmcStorage()))
	{
		SLOGE("%s failed to mount via VFAT (%s)\n", devicePath, strerror(errno));
		continue;
	}
#else //MTK_EMMC_DISCARD
	// 0702 --> 0002
	if (Fat::doMount(devicePath, "mnt/secure/staging", false, false, false,
		AID_SYSTEM, gid, 0002, true))
	{
		SLOGE("%s failed to mount via VFAT (%s)\n", devicePath, strerror(errno));
		continue;
	}
#endif //MTK_EMMC_DISCARD

修改后编译产生新的void可执行文件，adb push到/system/bin，加上可执行权限，关机重新开机，权限开放！再次查看 SD 卡读写权限：

root@android:/system/bin # ls -l /storage/sdcard0
ls -l /storage/sdcard0
-rwxrwxr-x system  sdcard_rw 7390845 2012-12-15 10:43 22.mp4
drwxrwxr-x system  sdcard_rw         2013-05-29 00:00 Alarms
drwxrwxr-x system  sdcard_rw         2013-05-29 00:00 Android
drwxrwxr-x system  sdcard_rw         2013-05-29 00:00 DCIM
drwxrwxr-x system  sdcard_rw         2013-05-29 00:00 Download
drwxrwxr-x system  sdcard_rw         2013-05-29 00:10 LOST.DIR
drwxrwxr-x system  sdcard_rw         2014-05-30 14:57 MTK
drwxrwxr-x system  sdcard_rw         2013-05-29 00:00 Movies
drwxrwxr-x system  sdcard_rw         2014-05-30 14:08 Music
drwxrwxr-x system  sdcard_rw         2013-05-29 00:00 Notifications
drwxrwxr-x system  sdcard_rw         2013-05-29 00:00 Pictures
drwxrwxr-x system  sdcard_rw         2013-05-29 00:00 Podcasts
drwxrwxr-x system  sdcard_rw         2014-05-29 19:29 Recording
drwxrwxr-x system  sdcard_rw         2013-05-29 00:00 Ringtones
drwxrwxr-x system  sdcard_rw         2014-05-30 17:51 TestDetailLogs
drwxrwxr-x system  sdcard_rw         2013-05-29 01:02 mtklog
drwxrwxr-x system  sdcard_rw         2014-05-30 17:35 test

******************************************************

说明：以上两行”*”号中间的内容来自 CSDN 作者 ID 为 echojiangyq_fight的博客文章：
http://blog.csdn.net/echojiangyq_fight/article/details/28232953，感谢他给予的参考，谢谢！

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