Android源碼分析（十二）ServiceManager服務分析

一.啟動過程分析

基於 binder 機制實現通信，添加服務，查詢服務，獲取服務。查詢，獲取服務時候需要檢查權限，android是基於Linux底層，所以也很好的實現了linux多用戶管理。

frameworks\native\cmds\servicemanager\servicemanager.rc
service servicemanager /system/bin/servicemanager
    class core animation
    user system
    group system readproc
    critical
    onrestart restart healthd
    onrestart restart zygote
    onrestart restart audioserver
    onrestart restart media
    onrestart restart surfaceflinger
    onrestart restart inputflinger
    onrestart restart drm
    onrestart restart cameraserver
    writepid /dev/cpuset/system-background/tasks

從上面可知，如果ServiceManager服務異常退出的話，系統會重啟。

frameworks\native\cmds\servicemanager\service_manager.c
int main(int argc, char** argv)
{
    struct binder_state *bs;
    union selinux_callback cb;
    char *driver;

    if (argc > 1) {
        driver = argv[1];
    } else {
        driver = "/dev/binder";
    }

    bs = binder_open(driver, 128*1024);
    if (!bs) {
    #ifdef VENDORSERVICEMANAGER
        ALOGW("failed to open binder driver %s\n", driver);
        while (true) {
            sleep(UINT_MAX);
        }
    #else
        ALOGE("failed to open binder driver %s\n", driver);
    #endif
        return -1;
    }

    if (binder_become_context_manager(bs)) {
        ALOGE("cannot become context manager (%s)\n", strerror(errno));
        return -1;
    }

    cb.func_audit = audit_callback;
    selinux_set_callback(SELINUX_CB_AUDIT, cb);
    cb.func_log = selinux_log_callback;
    selinux_set_callback(SELINUX_CB_LOG, cb);

    #ifdef VENDORSERVICEMANAGER
    sehandle = selinux_android_vendor_service_context_handle();
    #else
    sehandle = selinux_android_service_context_handle();
    #endif
    selinux_status_open(true);

    if (sehandle == NULL) {
        ALOGE("SELinux: Failed to acquire sehandle. Aborting.\n");
        abort();
    }

    if (getcon(&service_manager_context) != 0) {
        ALOGE("SELinux: Failed to acquire service_manager context. Aborting.\n");
        abort();
    }


    //loop 接受消息。並將binder解析完的消息返回給svcmgr_handler處理。
    binder_loop(bs, svcmgr_handler);

    return 0;
}

從這我們知道，ServiceManager是基於binder機制實現的。進入binder.c中了解下binder_open,binder_loop，然后binder將解析完的消息，返回給svcmag_handler處理

frameworks\native\cmds\servicemanager\binder.c

    struct binder_state *binder_open(const char* driver, size_t mapsize)
{
    struct binder_state *bs;
    struct binder_version vers;

    bs = malloc(sizeof(*bs));
    if (!bs) {
        errno = ENOMEM;
        return NULL;
    }

    bs->fd = open(driver, O_RDWR | O_CLOEXEC);
    if (bs->fd < 0) {
        fprintf(stderr,"binder: cannot open %s (%s)\n",
                driver, strerror(errno));
        goto fail_open;
    }

    if ((ioctl(bs->fd, BINDER_VERSION, &vers) == -1) ||
        (vers.protocol_version != BINDER_CURRENT_PROTOCOL_VERSION)) {
        fprintf(stderr,
                "binder: kernel driver version (%d) differs from user space version (%d)\n",
                vers.protocol_version, BINDER_CURRENT_PROTOCOL_VERSION);
        goto fail_open;
    }

    bs->mapsize = mapsize;
    bs->mapped = mmap(NULL, mapsize, PROT_READ, MAP_PRIVATE, bs->fd, 0);
    if (bs->mapped == MAP_FAILED) {
        fprintf(stderr,"binder: cannot map device (%s)\n",
                strerror(errno));
        goto fail_map;
    }

    return bs;
}

void binder_loop(struct binder_state *bs, binder_handler func)
{
    int res;
    struct binder_write_read bwr;
    uint32_t readbuf[32];

    bwr.write_size = 0;
    bwr.write_consumed = 0;
    bwr.write_buffer = 0;

    readbuf[0] = BC_ENTER_LOOPER;
    binder_write(bs, readbuf, sizeof(uint32_t));

    for (;;) {
        bwr.read_size = sizeof(readbuf);
        bwr.read_consumed = 0;
        bwr.read_buffer = (uintptr_t) readbuf;

        res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);

        if (res < 0) {
            ALOGE("binder_loop: ioctl failed (%s)\n", strerror(errno));
            break;
        }

        //解析消息
        res = binder_parse(bs, 0, (uintptr_t) readbuf, bwr.read_consumed, func);
        if (res == 0) {
            ALOGE("binder_loop: unexpected reply?!\n");
            break;
        }
        if (res < 0) {
            ALOGE("binder_loop: io error %d %s\n", res, strerror(errno));
            break;
        }
    }
}

binder_parse 解析接受到的消息 

binder_send_reply 接收到消息，並解析完消息后，binder將解析后的消息返回給ServiceManager

void binder_send_reply(struct binder_state *bs,
                       struct binder_io *reply,
                       binder_uintptr_t buffer_to_free,
                       int status)
{
    struct {
        uint32_t cmd_free;
        binder_uintptr_t buffer;
        uint32_t cmd_reply;
        struct binder_transaction_data txn;
    } __attribute__((packed)) data;

    data.cmd_free = BC_FREE_BUFFER;
    data.buffer = buffer_to_free;
    data.cmd_reply = BC_REPLY;
    data.txn.target.ptr = 0;
    data.txn.cookie = 0;
    data.txn.code = 0;
    if (status) {
        data.txn.flags = TF_STATUS_CODE;
        data.txn.data_size = sizeof(int);
        data.txn.offsets_size = 0;
        data.txn.data.ptr.buffer = (uintptr_t)&status;
        data.txn.data.ptr.offsets = 0;
    } else {
        data.txn.flags = 0;
        data.txn.data_size = reply->data - reply->data0;
        data.txn.offsets_size = ((char*) reply->offs) - ((char*) reply->offs0);
        data.txn.data.ptr.buffer = (uintptr_t)reply->data0;
        data.txn.data.ptr.offsets = (uintptr_t)reply->offs0;
    }
    binder_write(bs, &data, sizeof(data));
}

繼續回到serviceManager里分析binder解析返回回來的服務消息

frameworks\native\cmds\servicemanager\servicemanager.rc

int svcmgr_handler(struct binder_state *bs,
                   struct binder_transaction_data *txn,
                   struct binder_io *msg,
                   struct binder_io *reply)
{
    struct svcinfo *si;
    uint16_t *s;
    size_t len;
    uint32_t handle;
    uint32_t strict_policy;
    int allow_isolated;

    //ALOGI("target=%p code=%d pid=%d uid=%d\n",
    //      (void*) txn->target.ptr, txn->code, txn->sender_pid, txn->sender_euid);

    if (txn->target.ptr != BINDER_SERVICE_MANAGER)
        return -1;

    if (txn->code == PING_TRANSACTION)
        return 0;

    // Equivalent to Parcel::enforceInterface(), reading the RPC
    // header with the strict mode policy mask and the interface name.
    // Note that we ignore the strict_policy and don't propagate it
    // further (since we do no outbound RPCs anyway).
    strict_policy = bio_get_uint32(msg);
    s = bio_get_string16(msg, &len);
    if (s == NULL) {
        return -1;
    }

    //檢查是否是servicemanager服務
    if ((len != (sizeof(svcmgr_id) / 2)) ||
        memcmp(svcmgr_id, s, sizeof(svcmgr_id))) {
        fprintf(stderr,"invalid id %s\n", str8(s, len));
        return -1;
    }

    if (sehandle && selinux_status_updated() > 0) {
        struct selabel_handle *tmp_sehandle = selinux_android_service_context_handle();
        if (tmp_sehandle) {
            selabel_close(sehandle);
            sehandle = tmp_sehandle;
        }
    }

    switch(txn->code) {
    case SVC_MGR_GET_SERVICE:
    //檢查服務，do_find_service 查找服務。
    case SVC_MGR_CHECK_SERVICE:
        s = bio_get_string16(msg, &len);
        if (s == NULL) {
            return -1;
        }
        handle = do_find_service(s, len, txn->sender_euid, txn->sender_pid);
        if (!handle)
            break;
        bio_put_ref(reply, handle);
        return 0;

    //添加服務 do_add_service 獲取服務 在do_add_service會檢查是否具有權限
    case SVC_MGR_ADD_SERVICE:
        s = bio_get_string16(msg, &len);
        if (s == NULL) {
            return -1;
        }
        handle = bio_get_ref(msg);
        allow_isolated = bio_get_uint32(msg) ? 1 : 0;
        if (do_add_service(bs, s, len, handle, txn->sender_euid,
            allow_isolated, txn->sender_pid))
            return -1;
        break;
    //查詢服務
    case SVC_MGR_LIST_SERVICES: {
        uint32_t n = bio_get_uint32(msg);

        if (!svc_can_list(txn->sender_pid, txn->sender_euid)) {
            ALOGE("list_service() uid=%d - PERMISSION DENIED\n",
                    txn->sender_euid);
            return -1;
        }
        si = svclist;
        while ((n-- > 0) && si)
            si = si->next;
        if (si) {
            bio_put_string16(reply, si->name);
            return 0;
        }
        return -1;
    }
    default:
        ALOGE("unknown code %d\n", txn->code);
        return -1;
    }

    bio_put_uint32(reply, 0);
    return 0;
}

進入 do_find_service  do_add_service了解下做了什么事。

uint32_t do_find_service(const uint16_t *s, size_t len, uid_t uid, pid_t spid)
{
    struct svcinfo *si = find_svc(s, len);

    if (!si || !si->handle) {
        return 0;
    }

    if (!si->allow_isolated) {
        // If this service doesn't allow access from isolated processes,
        // then check the uid to see if it is isolated.
        uid_t appid = uid % AID_USER;
        if (appid >= AID_ISOLATED_START && appid <= AID_ISOLATED_END) {
            return 0;
        }
    }

    if (!svc_can_find(s, len, spid, uid)) {
        return 0;
    }

    return si->handle;
}

int do_add_service(struct binder_state *bs,
                   const uint16_t *s, size_t len,
                   uint32_t handle, uid_t uid, int allow_isolated,
                   pid_t spid)
{
    struct svcinfo *si;

    //ALOGI("add_service('%s',%x,%s) uid=%d\n", str8(s, len), handle,
    //        allow_isolated ? "allow_isolated" : "!allow_isolated", uid);

    if (!handle || (len == 0) || (len > 127))
        return -1;

    //檢查是否有權限
    if (!svc_can_register(s, len, spid, uid)) {
        ALOGE("add_service('%s',%x) uid=%d - PERMISSION DENIED\n",
             str8(s, len), handle, uid);
        return -1;
    }

    si = find_svc(s, len);
    if (si) {
        if (si->handle) {
            ALOGE("add_service('%s',%x) uid=%d - ALREADY REGISTERED, OVERRIDE\n",
                 str8(s, len), handle, uid);
            svcinfo_death(bs, si);
        }
        si->handle = handle;
    } else {
        si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));
        if (!si) {
            ALOGE("add_service('%s',%x) uid=%d - OUT OF MEMORY\n",
                 str8(s, len), handle, uid);
            return -1;
        }
        si->handle = handle;
        si->len = len;
        memcpy(si->name, s, (len + 1) * sizeof(uint16_t));
        si->name[len] = '\0';
        si->death.func = (void*) svcinfo_death;
        si->death.ptr = si;
        si->allow_isolated = allow_isolated;
        si->next = svclist;
        svclist = si;
    }

    binder_acquire(bs, handle);
    binder_link_to_death(bs, handle, &si->death);
    return 0;
}

二. ServiceManager如何管理服務

 從上面源碼分析我們知道了ServiceManager利用binder通信機制來管理一系列服務。
 上面源碼的執行路徑可以用下圖所示：（注意，圖中do_find_service7 應該是do_find_service.操作手誤。）

                                        

 

　　代碼分析抽象成邏輯分析，如下圖所示：