关于Android的浅杀

android.app.ActivityManager类提供了removeTask方法来清理任务。
在API21及之前，removeTask中提供一个参数来指定是否要连同任务所运行的进程一起杀掉。
这个方法的第二个参数就是用来指定是否要连同进程一起杀掉，可以为0或者REMOVE_TASK_KILL_PROCESS。
我们先看在4.4上的实现：

    /**
     * Completely remove the given task.
     *
     * @param taskId Identifier of the task to be removed.
     * @param flags Additional operational flags.  May be 0 or
     * {@link #REMOVE_TASK_KILL_PROCESS}.
     * @return Returns true if the given task was found and removed.
     *
     * @hide
     */
    public boolean removeTask(int taskId, int flags)
            throws SecurityException {
        try {
            return ActivityManagerNative.getDefault().removeTask(taskId, flags);
        } catch (RemoteException e) {
            // System dead, we will be dead too soon!
            return false;
        }
    }

在ActivityManager中提供的只是一个接口，真正的实现要通过IPC去调用ActivityManagerService的removeTask. 主要的逻辑在cleanUpRemovedTaskLocked中实现。

    @Override
    public boolean removeTask(int taskId, int flags) {
...
            try {
                TaskRecord tr = recentTaskForIdLocked(taskId);
                if (tr != null) {
                    ActivityRecord r = tr.removeTaskActivitiesLocked(-1, false);
                    if (r != null) {
                        cleanUpRemovedTaskLocked(tr, flags);
                        return true;
                    }
...
    }

我们再往下看cleanUpRemovedTaskLocked的逻辑，如果flags&ActivityManager.REMOVE_TASK_KILL_PROCESS就杀进程。

    private void cleanUpRemovedTaskLocked(TaskRecord tr, int flags) {
...
        final boolean killProcesses = (flags&ActivityManager.REMOVE_TASK_KILL_PROCESS) != 0;

...

        if (killProcesses) {
...
                    killUnneededProcessLocked(pr, "remove task");
                } else {
                    pr.waitingToKill = "remove task";
                }
            }
        }
    }

杀进程的逻辑在killUnneededProcessLocked中，接着调用android.os.Process.killProcessQuiet方法来杀进程。然后调用bionic中的函数来做系统调用。

    private void killUnneededProcessLocked(ProcessRecord pr, String reason) {
        if (!pr.killedByAm) {
            Slog.i(TAG, "Killing " + pr.toShortString() + " (adj " + pr.setAdj + "): " + reason);
            EventLog.writeEvent(EventLogTags.AM_KILL, pr.userId, pr.pid,
                    pr.processName, pr.setAdj, reason);
            pr.killedByAm = true;
            Process.killProcessQuiet(pr.pid);
        }
    }

比如ARM的kill调用是这样的：

ENTRY(kill)
    mov     ip, r7
    ldr     r7, =__NR_kill
    swi     #0
    mov     r7, ip
    cmn     r0, #(MAX_ERRNO + 1)
    bxls    lr
    neg     r0, r0
    b       __set_errno_internal
END(kill)

在AArch64状态下，swi已经失效了，替换成svc指令。

ENTRY(kill)
    mov     x8, __NR_kill
    svc     #0

    cmn     x0, #(MAX_ERRNO + 1)
    cneg    x0, x0, hi
    b.hi    __set_errno_internal

    ret
END(kill)

在Intel x86芯片上，系统调用要用int 0x80软中断。

ENTRY(kill)
    pushl   %ebx
    pushl   %ecx
    mov     12(%esp), %ebx
    mov     16(%esp), %ecx
    movl    $__NR_kill, %eax
    int     $0x80
    cmpl    $-MAX_ERRNO, %eax
    jb      1f
    negl    %eax
    pushl   %eax
    call    __set_errno
    addl    $4, %esp
    orl     $-1, %eax
1:
    popl    %ecx
    popl    %ebx
    ret
END(kill)

但是在x86_64指令集中，系统调用已经变成基本指令了，不再需要int 0x80了。

ENTRY(kill)
    movl    $__NR_kill, %eax
    syscall
    cmpq    $-MAX_ERRNO, %rax
    jb      1f
    negl    %eax
    movl    %eax, %edi
    call    __set_errno_internal
1:
    ret
END(kill)

    /**
     * Completely remove the given task.
     *
     * @param taskId Identifier of the task to be removed.
     * @return Returns true if the given task was found and removed.
     *
     * @hide
     */
    public boolean removeTask(int taskId) throws SecurityException {
        try {
            return ActivityManagerNative.getDefault().removeTask(taskId);
        } catch (RemoteException e) {
            // System dead, we will be dead too soon!
            return false;
        }
    }

经过IPC，走到ActivityManagerService中，默认调用removeTaskByIdLocked时就杀进程。

    @Override
    public boolean removeTask(int taskId) {
        synchronized (this) {
            enforceCallingPermission(android.Manifest.permission.REMOVE_TASKS,
                    "removeTask()");
            long ident = Binder.clearCallingIdentity();
            try {
                return removeTaskByIdLocked(taskId, true);
            } finally {
                Binder.restoreCallingIdentity(ident);
            }
        }
    }

我们继续看removeTaskByIdLocked，还是会调用cleanUpRemovedTaskLocked，这两个方法都是可以控制是否要杀进程的。

    /**
     * Removes the task with the specified task id.
     *
     * @param taskId Identifier of the task to be removed.
     * @param killProcess Kill any process associated with the task if possible.
     * @return Returns true if the given task was found and removed.
     */
    private boolean removeTaskByIdLocked(int taskId, boolean killProcess) {
        TaskRecord tr = mStackSupervisor.anyTaskForIdLocked(taskId);
        if (tr != null) {
            tr.removeTaskActivitiesLocked();
            cleanUpRemovedTaskLocked(tr, killProcess);
            if (tr.isPersistable) {
                notifyTaskPersisterLocked(null, true);
            }
            return true;
        }
        Slog.w(TAG, "Request to remove task ignored for non-existent task " + taskId);
        return false;
    }

    private void cleanUpRemovedTaskLocked(TaskRecord tr, boolean killProcess) {
        mRecentTasks.remove(tr);
        tr.removedFromRecents();

......

        if (!killProcess) {
            return;
        }
......