问题：
项目中有一下情况：进程A调用另一进程的Ｂ ContentProvider,Ｂ在该此次query中需要在query另一个 Ｃ ContentProvider:
class BContentProvider extends ContentProvider {
Context mContext;
...
@Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
...
try {
// query C ContentProvider：
Cursor cursor = mContext.getContentResolver().query(...);
if (cursor != null) {
try {
//do something;
} finally {
cursor.close();
}
}
Cursor cursor = mContext.getContentResolver().query(...);
...
...
}
}
}
在这种情况下，系统抛出Exception如下：
1-11 16:04:51.867 2633 3557 W AppOps : Bad call: specified package com.providers.xxx under uid 10032 but it is really 10001
01-11 16:04:51.867 2633 3557 W AppOps : java.lang.RuntimeException: here
01-11 16:04:51.867 2633 3557 W AppOps : at com.android.server.AppOpsService.getOpsRawLocked(AppOpsService.java:1399)
01-11 16:04:51.867 2633 3557 W AppOps : at com.android.server.AppOpsService.noteOperationUnchecked(AppOpsService.java:1115)
01-11 16:04:51.867 2633 3557 W AppOps : at com.android.server.AppOpsService.noteProxyOperation(AppOpsService.java:1093)
01-11 16:04:51.867 2633 3557 W AppOps : at com.android.internal.app.IAppOpsService$Stub.onTransact(IAppOpsService.java:157)
01-11 16:04:51.867 2633 3557 W AppOps : at android.os.BinderInjector.onTransact(BinderInjector.java:30)
01-11 16:04:51.867 2633 3557 W AppOps : at android.os.Binder.execTransact(Binder.java:569)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: Writing exception to parcel
01-11 16:04:51.868 4659 6791 E DatabaseUtils: java.lang.SecurityException: Proxy package com.providers.xxx from uid 10001 or calling package com.providers.xxx from uid 10032 not allowed to perform READ_PROVIDER_C
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.app.AppOpsManager.noteProxyOp(AppOpsManager.java:1834)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.content.ContentProvider.checkPermissionAndAppOp(ContentProvider.java:538)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.content.ContentProvider.enforceReadPermissionInner(ContentProvider.java:560)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.content.ContentProvider$Transport.enforceReadPermission(ContentProvider.java:483)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.content.ContentProvider$Transport.query(ContentProvider.java:212)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.content.ContentResolver.query(ContentResolver.java:532)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.content.ContentResolver.query(ContentResolver.java:473)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at com.android.providers.xxx.BDatabaseHelper.query(BDatabaseHelper.java:7238)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.content.ContentProvider$Transport.query(ContentProvider.java:239)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.content.ContentProviderNative.onTransact(ContentProviderNative.java:112)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.os.BinderInjector.onTransact(BinderInjector.java:30)
01-11 16:04:51.868 4659 6791 E DatabaseUtils: at android.os.Binder.execTransact(Binder.java:569)
分析：
由于错误log首先反应了没有C ContentProvider的权限，但检查A应用是有C的读写权限的。所以排除了A的权限问题。
继续分析：
通过log可以看到确实是ContentProvider在做权限检查时出错。通过log中对应的源码进行分析：
首先可以看到ContentProvider.query()的时候做了权限检查，注意，传入的enforceReadPermission()的callingPkg是调用方的包名，以上面为例，就是B的包名。
ContentProvider.query():
@Override
public Cursor query(String callingPkg, Uri uri, @Nullable String[] projection,
@Nullable Bundle queryArgs, @Nullable ICancellationSignal cancellationSignal) {
validateIncomingUri(uri);
uri = maybeGetUriWithoutUserId(uri);
if (enforceReadPermission(callingPkg, uri, null) != AppOpsManager.MODE_ALLOWED) {
enforceReadPermission()调用了.checkPermissionAndAppOp()方法，ContentProvider.checkPermissionAndAppOp()调用了AppOpsManager.noteProxyOp()去做检查出了异常。
AppOpsManager.noteProxyOp()：
public int noteProxyOp(int op, String proxiedPackageName) {
int mode = noteProxyOpNoThrow(op, proxiedPackageName);
if (mode == MODE_ERRORED) {
throw new SecurityException("Proxy package " + mContext.getOpPackageName()
+ " from uid " + Process.myUid() + " or calling package "
+ proxiedPackageName + " from uid " + Binder.getCallingUid()
+ " not allowed to perform " + sOpNames[op]);
}
return mode;
}
noteProxyOpNoThrow()又做了什么呢？
AppOpsManager.noteProxyOpNoThrow():
/**
* Like {@link #noteProxyOp(int, String)} but instead
* of throwing a {@link SecurityException} it returns {@link #MODE_ERRORED}.
* @hide
*/
public int noteProxyOpNoThrow(int op, String proxiedPackageName) {
try {
return mService.noteProxyOperation(op, mContext.getOpPackageName(),
Binder.getCallingUid(), proxiedPackageName);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
可见noteProxyOpNoThrow()是通过binder调用到了AppOpsService.noteProxyOperation()方法，注意，这里传入的是AppOpsService.noteProxyOperation()的后两个参数为Binder.getCallingUid()和之前层层传入的调用方的包名，也就是上面例子的B的包名。
下面，继续看binder另一侧的AppOpsService.noteProxyOperation()方法,我们结合log中AppOps的输出log：
AppOpsService.noteProxyOperation():
@Override
public int noteProxyOperation(int code, String proxyPackageName,
int proxiedUid, String proxiedPackageName) {
verifyIncomingOp(code);
final int proxyUid = Binder.getCallingUid();
String resolveProxyPackageName = resolvePackageName(proxyUid, proxyPackageName);
if (resolveProxyPackageName == null) {
return AppOpsManager.MODE_IGNORED;
}
final int proxyMode = noteOperationUnchecked(code, proxyUid,
resolveProxyPackageName, -1, null);
if (proxyMode != AppOpsManager.MODE_ALLOWED || Binder.getCallingUid() == proxiedUid) {
return proxyMode;
}
String resolveProxiedPackageName = resolvePackageName(proxiedUid, proxiedPackageName);
if (resolveProxiedPackageName == null) {
return AppOpsManager.MODE_IGNORED;
}
return noteOperationUnchecked(code, proxiedUid, resolveProxiedPackageName,
proxyMode, resolveProxyPackageName);
}
AppOpsService.noteOperationUnchecked():
private int noteOperationUnchecked(int code, int uid, String packageName,
int proxyUid, String proxyPackageName) {
Op op = null;
Op switchOp = null;
int switchCode;
int resultMode = AppOpsManager.MODE_ALLOWED;
synchronized (this) {
Ops ops = getOpsRawLocked(uid, packageName, true);
...
}
...
}
AppOpsService.getOpsRawLocked():
private Ops getOpsRawLocked(int uid, String packageName, boolean edit) {
...
Ops ops = uidState.pkgOps.get(packageName);
if (ops == null) {
if (!edit) {
return null;
}
boolean isPrivileged = false;
// This is the first time we have seen this package name under this uid,
// so let's make sure it is valid.
if (uid != 0) {
final long ident = Binder.clearCallingIdentity();
try {
int pkgUid = -1;
try {
ApplicationInfo appInfo = ActivityThread.getPackageManager()
.getApplicationInfo(packageName,
PackageManager.MATCH_DEBUG_TRIAGED_MISSING,
UserHandle.getUserId(uid));
if (appInfo != null) {
pkgUid = appInfo.uid;
isPrivileged = (appInfo.privateFlags
& ApplicationInfo.PRIVATE_FLAG_PRIVILEGED) != 0;
}
...
}
...
if (pkgUid != uid) {
// Oops! The package name is not valid for the uid they are calling
// under. Abort.
RuntimeException ex = new RuntimeException("here");
ex.fillInStackTrace();
Slog.w(TAG, "Bad call: specified package " + packageName
+ " under uid " + uid + " but it is really " + pkgUid, ex);
return null;
}
} finally {
Binder.restoreCallingIdentity(ident);
}
}
ops = new Ops(packageName, uidState, isPrivileged);
uidState.pkgOps.put(packageName, ops);
}
return ops;
}
这里主要的操作就是将传入的uid和包名进行判断：比对该包对应的uid和传入的uid比较，如果不一致就报错。错误信息和log中的一致：
Bad call: specified package com.providers.xxx under uid 10032 but it is really 10001
上文提到了，这个包名是传入的ContentProvider的调用方的包名，也就是例子中的B的包名。而uid是在AppOpsManager中通过Binder.getCallingUid()获得的。log中显示，此uid并不是B的uid，而是其上游调用者A的uid。
为什么在C中调用Binder.getCallingUid()得到的是A进程的呢？我找到了袁辉辉大神的一片博客： Binder IPC的权限控制
“线程B通过Binder调用当前线程的某个组件：此时线程B是线程B某个组件的调用端，则mCallingUid和mCallingPid应该保存当前线程B的PID和UID，故需要调用clearCallingIdentity()方法完成这个功能。当线程B调用完某个组件，由于线程B仍然处于线程A的被调用端，因此mCallingUid和mCallingPid需要恢复成线程A的UID和PID，这是调用restoreCallingIdentity()即可完成。”
Binder的机制就是这么设计的，所以需要在B进行下一次Binder调用(也就是query ContentProvider)之前调用clearCallingIdentity()来将B的
PID和UID附给mCallingUid和mCallingPid。Binder调用结束后在restoreCallingIdentity()来将其恢复成其原本调用方的PID和UID。这样在C里就会用B的相关信息进行权限校验，在AppOpsService.getOpsRawLocked()，UID和包名都是B的，是一致的，就不会报错。
解决办法：
其实上文也已经提到了，参考 Binder IPC的权限控制，在B进行Query前后分别调用clearCallingIdentity()
//作用是清空远程调用端的uid和pid，用当前本地进程的uid和pid替代，这样在之后的调用方去进行权限校验时会以B的信息为主，不会出现包名和UID不一致的情况。
最后修改过的调用方式如下：
long token = Binder.clearCallingIdentity();
try {
Cursor cursor = mContext.getContentResolver().query(...);
if (cursor != null) {
try {
//do something;
} finally {
cursor.close();
}
}
} finally {
Binder.restoreCallingIdentity(token);
}
总结：
1.ContentProvider是用Binder实现的，查询的过程其实就是一次Binder调用，所以想深入了解ContentProvider一定要会一些Binder相关的知识。
2.ContentProvider在接受一次查询前会调用AppOpsManager(其会通过Binder再由AppOpsService完成)进行权限校验，其中会校验调用方的UID和包名是否一致，其相关功能可见文章： Android 权限管理 —— AppOps。
2.Binder调用时候可以通过Binder.getCallingPid()和Binder.getCallingUid()来获取调用方的PID和UID，而如果A通过Binder调用B，B又Binder调用了C，那么在C中Binder.getCallingPid()和Binder.getCallingUid()得到的是A的PID和UID，这种情况下需要在B调用C的前后用Binder.clearCallingIdentity()和Binder.restoreCallingIdentity()使其带上B的PID和UID，从而在C中进行权限校验时候用B的信息进行校验，当然这也符合逻辑，B调用的C，应该B需要有相应权限。
3.Binder.clearCallingIdentity()和Binder.restoreCallingIdentity()的实现原理 Binder IPC的权限控制也有介绍，是通过移位实现的。