高版本的android对文件权限的管控抓的很严格,理论上两个应用之间的文件传递现在都应该是用FileProvider去实现,这篇博客来一起了解下它的实现原理。
首先我们要明确一点,FileProvider就是一个ContentProvider,所以需要在AndroidManifest.xml里面对它进行声明:
<provider
android:name="androidx.core.content.FileProvider"
android:authorities="me.linjw.demo.fileprovider.provider"
android:exported="false"
android:grantUriPermissions="true">
<meta-data
android:name="android.support.FILE_PROVIDER_PATHS"
android:resource="@xml/file_path" />
</provider>
和普通的ContentProvider不一样的是他多了一个android.support.FILE_PROVIDER_PATHS的meta-data指定了一个xml资源:
<?xml version="1.0" encoding="utf-8"?>
<paths xmlns:android="http://schemas.android.com/apk/res/android">
<root-path name="root" path="" />
<files-path name="files" path="images/" />
<cache-path name="cache" path="" />
<external-path name="external" path="" />
<external-files-path name="external-files" path="" />
<external-cache-path name="external-cache" path="" />
<external-media-path name="external-media" path="" />
</paths>
文件URI
这个xml的作用在于为文件生成URI,root-path、files-path、cache-path这些标签代表父路径:
- root-path : File("/")
- files-path : Context.getFilesDir()
- cache-path : context.getCacheDir()
- external-path : Environment.getExternalStorageDirectory()
- external-files-path : ContextCompat.getExternalFilesDirs(context, null)[0]
- external-cache-path : ContextCompat.getExternalCacheDirs(context)[0]
- external-media-path : context.getExternalMediaDirs()[0]
path属性代表子路径,name代表为"父路径/子路径"起的名字,
<files-path name="files" path="images/" />
例如上面配置代表的就是我们为 new File(context.getFilesDir(), "images/") 这个路径起了个名字叫做files
val filesDir = File(context.getFilesDir(), "images/")
val uri = FileProvider.getUriForFile(this, "me.linjw.demo.fileprovider.provider", File(filesDir, "test.jpg"))
// uri就是把filesDir的路径转换"files",然后加上content://me.linjw.demo.fileprovider.provider
// 即 "content://me.linjw.demo.fileprovider.provider/files/test.jpg"
从FileProvider的源码里面就能看到这部分的转换逻辑:
private static final String TAG_ROOT_PATH = "root-path";
private static final String TAG_FILES_PATH = "files-path";
private static final String TAG_CACHE_PATH = "cache-path";
private static final String TAG_EXTERNAL = "external-path";
private static final String TAG_EXTERNAL_FILES = "external-files-path";
private static final String TAG_EXTERNAL_CACHE = "external-cache-path";
private static final String TAG_EXTERNAL_MEDIA = "external-media-path";
...
int type;
while ((type = in.next()) != END_DOCUMENT) {
if (type == START_TAG) {
final String tag = in.getName();
final String name = in.getAttributeValue(null, ATTR_NAME);
String path = in.getAttributeValue(null, ATTR_PATH);
File target = null;
if (TAG_ROOT_PATH.equals(tag)) {
target = DEVICE_ROOT;
} else if (TAG_FILES_PATH.equals(tag)) {
target = context.getFilesDir();
} else if (TAG_CACHE_PATH.equals(tag)) {
target = context.getCacheDir();
} else if (TAG_EXTERNAL.equals(tag)) {
target = Environment.getExternalStorageDirectory();
} else if (TAG_EXTERNAL_FILES.equals(tag)) {
File[] externalFilesDirs = ContextCompat.getExternalFilesDirs(context, null);
if (externalFilesDirs.length > 0) {
target = externalFilesDirs[0];
}
} else if (TAG_EXTERNAL_CACHE.equals(tag)) {
File[] externalCacheDirs = ContextCompat.getExternalCacheDirs(context);
if (externalCacheDirs.length > 0) {
target = externalCacheDirs[0];
}
} else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP
&& TAG_EXTERNAL_MEDIA.equals(tag)) {
File[] externalMediaDirs = context.getExternalMediaDirs();
if (externalMediaDirs.length > 0) {
target = externalMediaDirs[0];
}
}
if (target != null) {
strat.addRoot(name, buildPath(target, path));
}
}
}
...
private static File buildPath(File base, String... segments) {
File cur = base;
for (String segment : segments) {
if (segment != null) {
cur = new File(cur, segment);
}
}
return cur;
}
查询的时候就只需要从strat里面找到文件路径最匹配的name即可。
打开文件
有了这个uri之后我们就能通过Intent将它传给其他应用,并配置Intent.FLAG_GRANT_READ_URI_PERMISSION或者Intent.FLAG_GRANT_WRITE_URI_PERMISSION为其他应用设置读写权限:
val uri = FileProvider.getUriForFile(this, "me.linjw.demo.fileprovider.provider", file)
val intent = Intent()
intent.data = uri
intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION)
intent.setClassName("me.linjw.demo.fileprovider.recv", "me.linjw.demo.fileprovider.recv.MainActivity")
startActivity(intent)
其他应用拿到这个uri就可以通过ContentResolver.openInputStream打开文件流:
val inputStream = intent.data?.let { contentResolver.openInputStream(it) }
或者有时候我们希望通过String传递uri的时候可以提前使用Context.grantUriPermission为指定的包名申请权限,然后接收端Uri.parse去解析出Uri来操作文件:
// 发送端
val uri = FileProvider.getUriForFile(this, "me.linjw.demo.fileprovider.provider", file)
grantUriPermission("me.linjw.demo.fileprovider.recv", uri, Intent.FLAG_GRANT_READ_URI_PERMISSION)
val intent = Intent()
intent.putExtra("uri", uri.toString())
intent.setClassName("me.linjw.demo.fileprovider.recv", "me.linjw.demo.fileprovider.recv.MainActivity")
startActivity(intent)
// 接收端
val uri = Uri.parse(intent.getStringExtra("uri"))
val inputStream = contentResolver.openInputStream(uri)
Uri操作文件的原理实际上就是通过请求我们之前声明的me.linjw.demo.fileprovider.provider这个ContentProvider,让它给我们去打开文件:
// FileProvider.java
public ParcelFileDescriptor openFile(@NonNull Uri uri, @NonNull String mode)
throws FileNotFoundException {
// ContentProvider has already checked granted permissions
final File file = mStrategy.getFileForUri(uri);
final int fileMode = modeToMode(mode);
return ParcelFileDescriptor.open(file, fileMode);
}
也就是说文件权限的校验实际上只发生在打开的阶段.其他应用虽然没有权限打开我们的文件,但是我们可以在ContentProvider里面帮它打开然后返回文件描述符,给其他应用去读写。
image.png
系统应用使用FileProvider的坑
项目中有个系统应用需要向其他应用传的文件,于是把FileProvider加上,然后发现其他应用还是没有权限。从日志里面看是说这个FileProvider并没有从UID 1000里暴露出来:
02-13 06:52:28.921 4292 4292 E AndroidRuntime: Caused by: java.lang.SecurityException: Permission Denial: opening provider androidx.core.content.FileProvider from ProcessRecord{806d30d 4292:me.linjw.demo.fileprovider.recv/u0a53} (pid=4292, uid=10053) that is not exported from UID 1000
由于这个UID 1000太显眼，所以尝试将系统签名去掉发现权限就正常了,实锤是系统签名的原因。
查看出现异常的时候的日志,发现了下面的打印:
02-13 06:52:28.486 863 1393 W UriGrantsManagerService: For security reasons, the system cannot issue a Uri permission grant to content://me.linjw.demo.fileprovider.provider/root/data/user/0/me.linjw.demo.fileprovider/files/test.txt [user 0]; use startActivityAsCaller() instead
在代码里面搜索关键字,发现系统应用需要在源码里面配置FileProvider的authorities:
// https://cs.android.com/android/platform/superproject/+/android-13.0.0_r29:frameworks/base/services/core/java/com/android/server/uri/UriGrantsManagerService.java
// Bail early if system is trying to hand out permissions directly; it
// must always grant permissions on behalf of someone explicit.
final int callingAppId = UserHandle.getAppId(callingUid);
if ((callingAppId == SYSTEM_UID) || (callingAppId == ROOT_UID)) {
if ("com.android.settings.files".equals(grantUri.uri.getAuthority())
|| "com.android.settings.module_licenses".equals(grantUri.uri.getAuthority())) {
// Exempted authority for
// 1. cropping user photos and sharing a generated license html
// file in Settings app
// 2. sharing a generated license html file in TvSettings app
// 3. Sharing module license files from Settings app
} else {
Slog.w(TAG, "For security reasons, the system cannot issue a Uri permission"
+ " grant to " + grantUri + "; use startActivityAsCaller() instead");
return -1;
}
}
直接传递ParcelFileDescriptor
从原理上看FileProvider实际就是打开文件的ParcelFileDescriptor传给其他应用使用,那我们能不能直接打开文件然后将ParcelFileDescriptor直接通过Intent传给其他应用呢?
val intent = Intent()
intent.putExtra("fd" , ParcelFileDescriptor.open(file, MODE_READ_ONLY))
intent.setClassName("me.linjw.demo.fileprovider.recv", "me.linjw.demo.fileprovider.recv.MainActivity")
startActivity(intent)
答案是不行:
02-15 20:27:24.200 16968 16968 E AndroidRuntime: Process: me.linjw.demo.fileprovider, PID: 16968
02-15 20:27:24.200 16968 16968 E AndroidRuntime: java.lang.RuntimeException: Not allowed to write file descriptors here
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.os.Parcel.nativeWriteFileDescriptor(Native Method)
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.os.Parcel.writeFileDescriptor(Parcel.java:922)
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.os.ParcelFileDescriptor.writeToParcel(ParcelFileDescriptor.java:1110)
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.os.Parcel.writeParcelable(Parcel.java:1953)
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.os.Parcel.writeValue(Parcel.java:1859)
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.os.Parcel.writeArrayMapInternal(Parcel.java:1024)
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.os.BaseBundle.writeToParcelInner(BaseBundle.java:1620)
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.os.Bundle.writeToParcel(Bundle.java:1304)
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.os.Parcel.writeBundle(Parcel.java:1093)
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.content.Intent.writeToParcel(Intent.java:11123)
02-15 20:27:24.200 16968 16968 E AndroidRuntime: at android.app.IActivityTaskManager$Stub$Proxy.startActivity(IActivityTaskManager.java:
2298)
原因在于Instrumentation的execStartActivity启动Activity前会调用Intent.prepareToLeaveProcess最终调用到Bundle.setAllowFds(false)不允许传递ParcelFileDescriptor:
// https://cs.android.com/android/platform/superproject/+/android-13.0.0_r29:frameworks/base/core/java/android/app/Instrumentation.java
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
...
intent.prepareToLeaveProcess(who);
...
}
// https://cs.android.com/android/platform/superproject/+/android-13.0.0_r29:frameworks/base/core/java/android/content/Intent.java
public void prepareToLeaveProcess(Context context) {
final boolean leavingPackage;
if (mComponent != null) {
leavingPackage = !Objects.equals(mComponent.getPackageName(), context.getPackageName());
} else if (mPackage != null) {
leavingPackage = !Objects.equals(mPackage, context.getPackageName());
} else {
leavingPackage = true;
}
prepareToLeaveProcess(leavingPackage);
}
/**
* Prepare this {@link Intent} to leave an app process.
*
* @hide
*/
public void prepareToLeaveProcess(boolean leavingPackage) {
setAllowFds(false);
...
}
public void setAllowFds(boolean allowFds) {
if (mExtras != null) {
mExtras.setAllowFds(allowFds);
}
}
一开始我想通过反射去强行调用setAllowFds(true),但是发现这个方法被限制了,需要系统权限才能调用:
Accessing hidden method Landroid/os/Bundle;->setAllowFds(Z)Z (max-target-o, reflection, denied)
只能另谋出路,由于ParcelFileDescriptor实现了Parcelable,所以我们可以通过传递Binder的方式迂回的去传递:
// aidl
interface IFileDescriptorsProvider {
ParcelFileDescriptor get();
}
// 发送端
val fileProvider = object : IFileDescriptorsProvider.Stub() {
override fun get(): ParcelFileDescriptor {
return ParcelFileDescriptor.open(file, MODE_READ_ONLY)
}
}
val intent = Intent()
val bundle = Bundle().apply { putBinder("fileProvider", fileProvider) }
intent.putExtras(bundle)
intent.setClassName("me.linjw.demo.fileprovider.recv", "me.linjw.demo.fileprovider.recv.MainActivity")
startActivity(intent)
// 接收端
val text = intent.extras?.getBinder("fileProvider")?.let { it ->
val fd = IFileDescriptorsProvider.Stub.asInterface(it).get()
AssetFileDescriptor(fd, 0, -1)
.createInputStream()
.use { it.bufferedReader().readLine() }
}
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