Android安全问题 抢先接收广播

导读：本文说明系统发送广播的部分流程，如何利用Intent查找到对应接收器。我们依然只关注接收器的排序问题

这篇文章主要是针对我前两篇文章

android安全问题（四） 抢先开机启动 - 结果篇

android安全问题（五） 抢先拦截短信 - 结果篇

现在给出第二步分的分析

下面就来看看发送广播的流程

Context中的sendBroadCast函数的实现是在ContextImpl中，和发送广播相关的有如下六个函数

void android.app.ContextImpl.sendBroadcast(Intent intent)

void android.app.ContextImpl.sendBroadcast(Intent intent, String receiverPermission)

void android.app.ContextImpl.sendOrderedBroadcast(Intent intent, String receiverPermission)

void android.app.ContextImpl.sendOrderedBroadcast(Intent intent, String receiverPermission, BroadcastReceiver resultReceiver, Handler scheduler, int initialCode, String initialData, Bundle initialExtras)

void android.app.ContextImpl.sendStickyBroadcast(Intent intent)

void android.app.ContextImpl.sendStickyOrderedBroadcast(Intent intent, BroadcastReceiver resultReceiver, Handler scheduler, int initialCode, String initialData, Bundle initialExtras)

可以分为3组：1普通广播；2Ordered广播；3Sticky广播

不论哪种，最后都会由ActivityManagerService处理

private final int broadcastIntentLocked(ProcessRecord callerApp,
        String callerPackage, Intent intent, String resolvedType,
        IIntentReceiver resultTo, int resultCode, String resultData,
        Bundle map, String requiredPermission,
        boolean ordered, boolean sticky, int callingPid, int callingUid)

以第一种情况为例，流程图大概是这个样子的

ordered和sticky用来区分上面3组广播

下面我们仔细看看这个方法都干了些什么

删减了一些代码

private final int broadcastIntentLocked(ProcessRecord callerApp,
        String callerPackage, Intent intent, String resolvedType,
        IIntentReceiver resultTo, int resultCode, String resultData,
        Bundle map, String requiredPermission,
        boolean ordered, boolean sticky, int callingPid, int callingUid) {
    ...//处理特殊intent
    // Add to the sticky list if requested.
    ...//处理sticky广播
    // Figure out who all will receive this broadcast.
    List receivers = null;
    List<BroadcastFilter> registeredReceivers = null;
    try {
        if (intent.getComponent() != null) {
            // Broadcast is going to one specific receiver class...
            ActivityInfo ai = AppGlobals.getPackageManager().
                getReceiverInfo(intent.getComponent(), STOCK_PM_FLAGS);
            if (ai != null) {
                receivers = new ArrayList();
                ResolveInfo ri = new ResolveInfo();
                ri.activityInfo = ai;
                receivers.add(ri);
            }
        } else {
            // Need to resolve the intent to interested receivers...
            if ((intent.getFlags()&Intent.FLAG_RECEIVER_REGISTERED_ONLY)
                     == 0) {
                receivers =
                    AppGlobals.getPackageManager().queryIntentReceivers(
                            intent, resolvedType, STOCK_PM_FLAGS);
            }
            registeredReceivers = mReceiverResolver.queryIntent(intent, resolvedType, false);
        }
    } catch (RemoteException ex) {
        // pm is in same process, this will never happen.
    }

    final boolean replacePending =
            (intent.getFlags()&Intent.FLAG_RECEIVER_REPLACE_PENDING) != 0;
    ...
    int NR = registeredReceivers != null ? registeredReceivers.size() : 0;
    ...//如果广播是非ordered，先处理动态注册的接收器
    if (!ordered && NR > 0) {
        // If we are not serializing this broadcast, then send the
        // registered receivers separately so they don't wait for the
        // components to be launched.
        BroadcastRecord r = new BroadcastRecord(intent, callerApp,
                callerPackage, callingPid, callingUid, requiredPermission,
                registeredReceivers, resultTo, resultCode, resultData, map,
                ordered, sticky, false);
        ...
    //mParallelBroadcasts只含有动态注册的receiver
        boolean replaced = false;
        if (replacePending) {
            for (int i=mParallelBroadcasts.size()-1; i>=0; i--) {
                if (intent.filterEquals(mParallelBroadcasts.get(i).intent)) {
                    if (DEBUG_BROADCAST) Slog.v(TAG,
                            "***** DROPPING PARALLEL: " + intent);
                    mParallelBroadcasts.set(i, r);
                    replaced = true;
                    break;
                }
            }
        }
        if (!replaced) {
            mParallelBroadcasts.add(r);
            scheduleBroadcastsLocked();
        }
        registeredReceivers = null;
        NR = 0;
    }

    // Merge into one list.
    //如果广播是ordered，合并静态、动态接收器
    //否则之前处理过动态接收器，这里registeredReceivers为null
    int ir = 0;
    if (receivers != null) {
        ...
        //合并的过程，注意顺序
        int NT = receivers != null ? receivers.size() : 0;
        int it = 0;
        ResolveInfo curt = null;
        BroadcastFilter curr = null;
        while (it < NT && ir < NR) {
            if (curt == null) {
                curt = (ResolveInfo)receivers.get(it);
            }
            if (curr == null) {
                curr = registeredReceivers.get(ir);
            }
            //如果动态接收器优先级高，那么就插到前面
            //否则进入else，然后进行下一轮比较，拿下一个静态接收器与之前的动态接收器比较，直到找到自己的位置才插入进列表中
            //在这里，调整好接收器的顺序，同等优先级的，显然动态的要在静态的前面
            if (curr.getPriority() >= curt.priority) {
                // Insert this broadcast record into the final list.
                receivers.add(it, curr);
                ir++;
                curr = null;
                it++;
                NT++;
            } else {
                // Skip to the next ResolveInfo in the final list.
                it++;
                curt = null;
            }
        }
    }
    while (ir < NR) {
        if (receivers == null) {
            receivers = new ArrayList();
        }
        receivers.add(registeredReceivers.get(ir));
        ir++;
    }

    if ((receivers != null && receivers.size() > 0)
            || resultTo != null) {
        BroadcastRecord r = new BroadcastRecord(intent, callerApp,
                callerPackage, callingPid, callingUid, requiredPermission,
                receivers, resultTo, resultCode, resultData, map, ordered,
                sticky, false);
        ...
        if (!replaced) {
            mOrderedBroadcasts.add(r);
            scheduleBroadcastsLocked();
        }
    }

    return BROADCAST_SUCCESS;
}

注意上面函数中提到的其中两个成员变量

/**
 * List of all active broadcasts that are to be executed immediately
 * (without waiting for another broadcast to finish).  Currently this only
 * contains broadcasts to registered receivers, to avoid spinning up
 * a bunch of processes to execute IntentReceiver components.
 */
final ArrayList<BroadcastRecord> mParallelBroadcasts = new ArrayList<BroadcastRecord>();


/**
 * List of all active broadcasts that are to be executed one at a time.
 * The object at the top of the list is the currently activity broadcasts;
 * those after it are waiting for the top to finish..
 */
final ArrayList<BroadcastRecord> mOrderedBroadcasts = new ArrayList<BroadcastRecord>();

如果是非ordered广播，那么mParallelBroadcasts将存储所有动态接收器，然后合并的时候，mParallelBroadcasts设置为null，所以不会合并到receivers中

如果是ordered广播，那么mParallelBroadcasts将合并到receivers中

然后，不管是哪种广播，最后都是调用scheduleBroadcastsLocked继续处理

最终到processNextBroadcast函数上

在看processNextBroadcast函数之前，还有个问题需要解决

receivers = AppGlobals.getPackageManager().queryIntentReceivers(intent, resolvedType, STOCK_PM_FLAGS);

registeredReceivers = mReceiverResolver.queryIntent(intent, resolvedType, false);

那就是查询出来的List是什么顺序，之后的处理过程上面都已经看到了，但是这里面是什么顺序我们还没有看到

先看看动态注册的Receiver情况

public List<R> queryIntent(Intent intent, String resolvedType, boolean defaultOnly) {
    String scheme = intent.getScheme();

    ArrayList<R> finalList = new ArrayList<R>();
    ...
    ArrayList<F> firstTypeCut = null;
    ArrayList<F> secondTypeCut = null;
    ArrayList<F> thirdTypeCut = null;
    ArrayList<F> schemeCut = null;

    //下面是一些匹配的细节，我们先不关注
    // If the intent includes a MIME type, then we want to collect all of
    // the filters that match that MIME type.
    //这里不涉及MIME，所以这里resolvedType=null
    if (resolvedType != null) {
        ...
    }

    // If the intent includes a data URI, then we want to collect all of
    // the filters that match its scheme (we will further refine matches
    // on the authority and path by directly matching each resulting filter).
    if (scheme != null) {
        schemeCut = mSchemeToFilter.get(scheme);
        if (debug) Slog.v(TAG, "Scheme list: " + schemeCut);
    }

    // If the intent does not specify any data -- either a MIME type or
    // a URI -- then we will only be looking for matches against empty
    // data.
    if (resolvedType == null && scheme == null && intent.getAction() != null) {
        firstTypeCut = mActionToFilter.get(intent.getAction());//在addFilter的时候将其add进去，见上一篇博客
        if (debug) Slog.v(TAG, "Action list: " + firstTypeCut);
    }

    FastImmutableArraySet<String> categories = getFastIntentCategories(intent);
    //finalList就是我们的结果集
    if (firstTypeCut != null) {
        //buildResolveList循环List，检查是否符合条件，然后复制到finalList中
        buildResolveList(intent, categories, debug, defaultOnly,
                resolvedType, scheme, firstTypeCut, finalList);
    }
    if (secondTypeCut != null) {
        buildResolveList(intent, categories, debug, defaultOnly,
                resolvedType, scheme, secondTypeCut, finalList);
    }
    if (thirdTypeCut != null) {
        buildResolveList(intent, categories, debug, defaultOnly,
                resolvedType, scheme, thirdTypeCut, finalList);
    }
    if (schemeCut != null) {
        buildResolveList(intent, categories, debug, defaultOnly,
                resolvedType, scheme, schemeCut, finalList);
    }
    sortResults(finalList);
    ...
    return finalList;
}

这里我们也看到关于排序代码，具体如下，很简单

@SuppressWarnings("unchecked")
protected void sortResults(List<R> results) {
    Collections.sort(results, mResolvePrioritySorter);
}

// Sorts a List of IntentFilter objects into descending priority order.
@SuppressWarnings("rawtypes")
private static final Comparator mResolvePrioritySorter = new Comparator() {
    public int compare(Object o1, Object o2) {
        final int q1 = ((IntentFilter) o1).getPriority();
        final int q2 = ((IntentFilter) o2).getPriority();
        return (q1 > q2) ? -1 : ((q1 < q2) ? 1 : 0);
    }
};

上面就是简单的按优先级排序，大的放在前面，否则位置不动

下面看看静态注册的Receiver情况，最终也会和动态注册的Receiver一样，调用同一个函数

public List<ResolveInfo> queryIntentReceivers(Intent intent, String resolvedType, int flags) {
    //ComponentName之后应该均为null，我们不讨论只发给特定组件的情况，因为那样不涉及优先级和顺序的问题
    ComponentName comp = intent.getComponent();
    if (comp == null) {
        if (intent.getSelector() != null) {
            intent = intent.getSelector(); 
            comp = intent.getComponent();
        }
    }
    if (comp != null) {
        List<ResolveInfo> list = new ArrayList<ResolveInfo>(1);
        ActivityInfo ai = getReceiverInfo(comp, flags);
        if (ai != null) {
            ResolveInfo ri = new ResolveInfo();
            ri.activityInfo = ai;
            list.add(ri);
        }
        return list;
    }

    // reader
    //ComponentName=null，所以会执行下面代码
    synchronized (mPackages) {
        String pkgName = intent.getPackage();
        //只考虑pkgName=null的情况，同一个package中，哪个receiver先接收到广播暂时不关心
        if (pkgName == null) {
            return mReceivers.queryIntent(intent, resolvedType, flags);//最终会调用IntentResolver.queryIntent，上面已经分析过
        }
        final PackageParser.Package pkg = mPackages.get(pkgName);
        if (pkg != null) {
            return mReceivers.queryIntentForPackage(intent, resolvedType, flags, pkg.receivers);
        }
        return null;
    }
}

现在看来，上面两个查询都是按优先级从高到低排序的，如果优先级相同，顺序则保持不变

之后是调用scheduleBroadcastsLocked来发广播给每一个receiver

至于广播后续如何处理，我们就不再深究了

到这里已经能看到应用中的接收顺序了

总结：

情况分为两种（scheduleBroadcastsLocked），ordered广播和非ordered广播

非ordered广播

先处理动接收器，然后处理静态接收器

ordered广播

同时处理动态接收器和静态接收器

先将动态接收器与静态接收器合并，保持着与优先级相同的顺序，优先级高的在前面，否则顺序不变。静态接收器与动态接收器优先级相同的话，动态接收器在前