从 am start 的 --user 参数说到 Android 多用户

java.lang.SecurityException: Permission Denial: startActivity asks to run as user -2 but is calling from user 0; this requires android.permission.INTERACT_ACROSS_USERS_FULL
    at android.os.Parcel.readException(Parcel.java:1425)
    at android.os.Parcel.readException(Parcel.java:1379)
    at android.app.ActivityManagerProxy.startActivityAsUser(ActivityManagerNative.java:1921)
    at com.android.commands.am.Am.runStart(Am.java:494)
    at com.android.commands.am.Am.run(Am.java:109)
    at com.android.commands.am.Am.main(Am.java:82)
    at com.android.internal.os.RuntimeInit.nativeFinishInit(Native Method)
    at com.android.internal.os.RuntimeInit.main(RuntimeInit.java:235)

/**
 * Representation of a user on the device.
 */
public final class UserHandle implements Parcelable {
    /**
     * @hide Range of uids allocated for a user.
     */
    public static final int PER_USER_RANGE = 100000;
    ......
    /**
     * @hide Enable multi-user related side effects. Set this to false if
     * there are problems with single user use-cases.
     */
    public static final boolean MU_ENABLED = true;
    ......
    /**
     * Returns the user id for a given uid.
     * @hide
     */
    public static final int getUserId(int uid) {
        if (MU_ENABLED) {
            return uid / PER_USER_RANGE;
        } else {
            return 0;
        }
    }
    ......
}

/**
 * Tools for managing OS processes.
 */
public class Process {
    ......
    /**
     * Defines the root UID.
     * @hide
     */
    public static final int ROOT_UID = 0;
    ......
    /**
     * Defines the start of a range of UIDs (and GIDs), going from this
     * number to {@link #LAST_APPLICATION_UID} that are reserved for assigning
     * to applications.
     */
    public static final int FIRST_APPLICATION_UID = 10000;

    /**
     * Last of application-specific UIDs starting at
     * {@link #FIRST_APPLICATION_UID}.
     */
    public static final int LAST_APPLICATION_UID = 19999;
    ......        
}

public static void main(String[] args) {
    (new Am()).run(args);
}

public void run(String[] args) {
    ......
    mArgs = args;
    ......

    try {
        onRun();
        ......
    }
    ......
}

@Override
public void onRun() throws Exception {

    mAm = ActivityManagerNative.getDefault();
    ......

    String op = nextArgRequired();

    if (op.equals("start")) {
        runStart();
    }
    ......
}

public class Am extends BaseCommand {
    ......
    private boolean mWaitOption = false;
    ......
    private int mUserId;
    ......
    private Intent makeIntent(int defUser) throws URISyntaxException {
        ......
        mWaitOption = false;
        ......
        mUserId = defUser;
        ......
        while ((opt=nextOption()) != null) {
            if (opt.equals("-a")) {
                ......
            } else if (opt.equals("-W")) {
                mWaitOption = true;
                ......
            } else if (opt.equals("--user")) {
                mUserId = parseUserArg(nextArgRequired());
            }
            ......
    }

    private void runStart() throws Exception {
        Intent intent = makeIntent(UserHandle.USER_CURRENT);
        ......
        if (mWaitOption) {
            result = mAm.startActivityAndWait(null, null, intent, mimeType,
                        null, null, 0, mStartFlags, profilerInfo, null, mUserId);
            res = result.result;
        } else {
            res = mAm.startActivityAsUser(null, null, intent, mimeType,
                    null, null, 0, mStartFlags, profilerInfo, null, mUserId);
        }
        ......
        switch (res) {
            // 启动 Activity 的结果提示
        }
        if (mWaitOption && launched) {
            // 输出启动 Activity 的结果状态，起止时间等
        }
        ......
    }
    ......
}

@Override
public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
        Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
        int startFlags, ProfilerInfo profilerInfo, Bundle options, int userId) {
    enforceNotIsolatedCaller("startActivity");
    userId = handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(), userId,
            false, ALLOW_FULL_ONLY, "startActivity", null);
    // TODO: Switch to user app stacks here.
    return mStackSupervisor.startActivityMayWait(caller, -1, callingPackage, intent,
            resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
            profilerInfo, null, null, options, false, userId, null, null);
}

/**
 * Return the Linux uid assigned to the process that sent you the
 * current transaction that is being processed.  This uid can be used with
 * higher-level system services to determine its identity and check
 * permissions.  If the current thread is not currently executing an
 * incoming transaction, then its own uid is returned.
 */
public static final native int getCallingUid();

int handleIncomingUser(int callingPid, int callingUid, int userId, boolean allowAll,
        int allowMode, String name, String callerPackage) {
    final int callingUserId = UserHandle.getUserId(callingUid);
    if (callingUserId == userId) {
        return userId;
    }
    ......
    if (callingUid != 0 && callingUid != Process.SYSTEM_UID) {
        final boolean allow;
        if (checkComponentPermission(INTERACT_ACROSS_USERS_FULL, callingPid,
                callingUid, -1, true) == PackageManager.PERMISSION_GRANTED) {
            // If the caller has this permission, they always pass go.  And collect $200.
            allow = true;
        } else if (allowMode == ALLOW_FULL_ONLY) {
            // We require full access, sucks to be you.
            allow = false;
        } else if (......) {
            ......
        }
        if (!allow) {
            if (userId == UserHandle.USER_CURRENT_OR_SELF) {
                // In this case, they would like to just execute as their
                // owner user instead of failing.
                targetUserId = callingUserId;
            } else {
                StringBuilder builder = new StringBuilder(128);
                builder.append("Permission Denial: ");
                builder.append(name);
                if (callerPackage != null) {
                    builder.append(" from ");
                    builder.append(callerPackage);
                }
                builder.append(" asks to run as user ");
                builder.append(userId);
                builder.append(" but is calling from user ");
                builder.append(UserHandle.getUserId(callingUid));
                builder.append("; this requires ");
                builder.append(INTERACT_ACROSS_USERS_FULL);
                if (allowMode != ALLOW_FULL_ONLY) {
                    builder.append(" or ");
                    builder.append(INTERACT_ACROSS_USERS);
                }
                String msg = builder.toString();
                Slog.w(TAG, msg);
                throw new SecurityException(msg);
            }
        }
    }
    ......
}

/**
 * Returns this process's user handle.  This is the
 * user the process is running under.  It is distinct from
 * {@link #myUid()} in that a particular user will have multiple
 * distinct apps running under it each with their own uid.
 */
public static final UserHandle myUserHandle() {
    return new UserHandle(UserHandle.getUserId(myUid()));
}

public final class UserHandle implements Parcelable {
    ......
    /** @hide */
    public UserHandle(int h) {
        mHandle = h;
    }

    /**
     * Returns the userId stored in this UserHandle.
     * @hide
     */
    @SystemApi
    public int getIdentifier() {
        return mHandle;
    }
    ......
    @Override
    public int hashCode() {
        return mHandle;
    }
    ......

@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
        Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
        int startFlags, ProfilerInfo profilerInfo, Bundle options) {
    return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
        resultWho, requestCode, startFlags, profilerInfo, options,
        UserHandle.getCallingUserId());
}