Binder机制情景分析之C服务应用

摘要：接下来会举例说明从服务名在链表中查找对应的服务，并返回详见添加服务，一般都是发起的请求。通知驱动线程池中一个线程已经创建了通知驱动该线程已经进入主循环，可以接收数据通知驱动该线程退出主循环，不再接收数据。

这里只讲下binder的实现原理，不牵扯到android的java层是如何调用;
涉及到的会有ServiceManager,led_control_server和test_client的代码,这些都是用c写的.其中led_control_server和test_client是
仿照bctest.c写的; 在linux平台下运行binder更容易分析binder机制实现的原理(可以增加大量的log,进行分析);
在Linux运行时.先运行ServiceManager,再运行led_control_server最后运行test_client;

Binder通信采用C/S架构，从组件视角来说，包含Client、Server、ServiceManager以及binder驱动，其中ServiceManager用于管理系统中的各种服务。

本文中的代码运行环境是在imx6ul上跑的，运行的是linux系统，内核版本4.10(非android环境分析);

文章所有代码已上传

https://github.com/SourceLink...

涉及到的源码地址:

frameworks/native/cmds/servicemanager/sevice_manager.c  
frameworks/native/cmds/servicemanager/binder.c  
frameworks/native/cmds/servicemanager/bctest.c

ServiceManager相当于binder通信过程中的守护进程，本身也是个binder服务、好比一个root管理员一样;
主要功能是查询和注册服务;接下来结合代码从main开始分析下serviceManager的服务过程;

源码中的sevice_manager.c中主函数中使用了selinux，为了在我板子的linux环境中运行，把这些代码屏蔽，删减后如下：

int main(int argc, char **argv)
{
    struct binder_state *bs;

    bs = binder_open(128*1024);                                                ①
    if (!bs) {
        ALOGE("failed to open binder driver
");
        return -1;
    }

    if (binder_become_context_manager(bs)) {                                   ②
        ALOGE("cannot become context manager (%s)
", strerror(errno));
        return -1;
    }


    svcmgr_handle = BINDER_SERVICE_MANAGER;
    binder_loop(bs, svcmgr_handler);                                           ③

    return 0;
}

①: 打开binder驱动(详见2.2.1)  
②: 注册为管理员(详见2.2.2)  
③: 进入循环，处理消息(详见2.2.3)

从主函数的启动流程就能看出sevice_manager的工作流程并不是特别复杂;
其实client和server的启动流程和manager的启动类似，后面再详细分析;

struct binder_state *binder_open(size_t mapsize)
{
    struct binder_state *bs;
    struct binder_version vers;

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

    bs->fd = open("/dev/binder", O_RDWR);                                     ①
    if (bs->fd < 0) {
        fprintf(stderr,"binder: cannot open device (%s)
",
                strerror(errno));
        goto fail_open;
    }

    if ((ioctl(bs->fd, BINDER_VERSION, &vers) == -1) ||                       ②
        (vers.protocol_version != BINDER_CURRENT_PROTOCOL_VERSION)) {
        fprintf(stderr, "binder: driver version differs from user space
");
        goto fail_open;
    }

    bs->mapsize = mapsize;
    bs->mapped = mmap(NULL, mapsize, PROT_READ, MAP_PRIVATE, bs->fd, 0);      ③
    if (bs->mapped == MAP_FAILED) {
        fprintf(stderr,"binder: cannot map device (%s)
",
                strerror(errno));
        goto fail_map;
    }

    return bs;

fail_map:
    close(bs->fd);
fail_open:
    free(bs);
    return NULL;
}

①: 打开binder设备  
②: 通过ioctl获取binder版本号  
③: mmp内存映射

这里说明下为什么binder驱动是用ioctl来操作，是因为ioctl可以同时进行读和写操作；

int binder_become_context_manager(struct binder_state *bs)
{
    return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);
}

还是通过ioctl请求类型BINDER_SET_CONTEXT_MGR注册成manager;

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));                                       ①

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

        res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);                                  ②

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

        res = binder_parse(bs, 0, (uintptr_t) readbuf, bwr.read_consumed, func);       ③
        if (res == 0) {
            ALOGE("binder_loop: unexpected reply?!
");
            break;
        }
        if (res < 0) {
            ALOGE("binder_loop: io error %d %s
", res, strerror(errno));
            break;
        }
    }
}

①: 写入命令BC_ENTER_LOOPER通知驱动该线程已经进入主循环，可以接收数据;  
②: 先读一次数据，因为刚才写过一次;  
③: 然后解析读出来的数据(详见2.2.4);

binder_loop函数的主要流程如下:

int binder_parse(struct binder_state *bs, struct binder_io *bio,
                 uintptr_t ptr, size_t size, binder_handler func)
{
    int r = 1;
    uintptr_t end = ptr + (uintptr_t) size;

    while (ptr < end) {
        uint32_t cmd = *(uint32_t *) ptr;
        ptr += sizeof(uint32_t);
#if TRACE
        fprintf(stderr,"%s:
", cmd_name(cmd));
#endif
        switch(cmd) {
        case BR_NOOP:
            break;
        case BR_TRANSACTION_COMPLETE:
            /* check服务 */
            break;
        case BR_INCREFS:
        case BR_ACQUIRE:
        case BR_RELEASE:
        case BR_DECREFS:
#if TRACE
            fprintf(stderr,"  %p, %p
", (void *)ptr, (void *)(ptr + sizeof(void *)));
#endif
            ptr += sizeof(struct binder_ptr_cookie);
            break;
        case BR_SPAWN_LOOPER: {
            /* create new thread */
            //if (fork() == 0) {
            //}
            pthread_t thread;
            struct binder_thread_desc btd;

            btd.bs = bs;
            btd.func = func;
            
            pthread_create(&thread, NULL, binder_thread_routine, &btd);

            /* in new thread: ioctl(BC_ENTER_LOOPER), enter binder_looper */

            break;
        }
        case BR_TRANSACTION: {
            struct binder_transaction_data *txn = (struct binder_transaction_data *) ptr;
            if ((end - ptr) < sizeof(*txn)) {
                ALOGE("parse: txn too small!
");
                return -1;
            }
            if (func) {
                unsigned rdata[256/4];
                struct binder_io msg;
                struct binder_io reply;
                int res;

                bio_init(&reply, rdata, sizeof(rdata), 4);                               ①
                bio_init_from_txn(&msg, txn);
                res = func(bs, txn, &msg, &reply);                                       ②
                binder_send_reply(bs, &reply, txn->data.ptr.buffer, res);                ③
            }
            ptr += sizeof(*txn);
            break;
        }
        case BR_REPLY: {
            struct binder_transaction_data *txn = (struct binder_transaction_data *) ptr;
            if ((end - ptr) < sizeof(*txn)) {
                ALOGE("parse: reply too small!
");
                return -1;
            }
            binder_dump_txn(txn);
            if (bio) {
                bio_init_from_txn(bio, txn);
                bio = 0;
            } else {
                /* todo FREE BUFFER */
            }
            ptr += sizeof(*txn);
            r = 0;
            break;
        }
        case BR_DEAD_BINDER: {
            struct binder_death *death = (struct binder_death *)(uintptr_t) *(binder_uintptr_t *)ptr;
            ptr += sizeof(binder_uintptr_t);
            death->func(bs, death->ptr);
            break;
        }
        case BR_FAILED_REPLY:
            r = -1;
            break;
        case BR_DEAD_REPLY:
            r = -1;
            break;
        default:
            ALOGE("parse: OOPS %d
", cmd);
            return -1;
        }
    }

    return r;
}

①: 按照一定的格式初始化rdata数据,请注意这里rdata是在用户空间创建的buf;  
②: 调用设置进来的处理函数svcmgr_handler(详见2.2.5);  
③: 发送回复信息;

这个函数我们只重点关注下BR_TRANSACTION其他的命令含义可以参考表格A;

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=%x code=%d pid=%d uid=%d
",
    //  txn->target.handle, txn->code, txn->sender_pid, txn->sender_euid);

    if (txn->target.handle != svcmgr_handle)
        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);                                           ①
    s = bio_get_string16(msg, &len);
    if (s == NULL) {
        return -1;
    }

    if ((len != (sizeof(svcmgr_id) / 2)) ||                                        ②
        memcmp(svcmgr_id, s, sizeof(svcmgr_id))) {
        fprintf(stderr,"invalid id %s
", str8(s, len));
        return -1;
    }


    switch(txn->code) {                                                            ③
    case SVC_MGR_GET_SERVICE:
    case SVC_MGR_CHECK_SERVICE:
        s = bio_get_string16(msg, &len);
        if (s == NULL) {
            return -1;
        }
        handle = do_find_service(bs, s, len, txn->sender_euid, txn->sender_pid);   ④
        if (!handle)
            break;
        bio_put_ref(reply, handle);
        return 0;

    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,                   ⑤
            allow_isolated, txn->sender_pid))
            return -1;
        break;

    case SVC_MGR_LIST_SERVICES: {
        uint32_t n = bio_get_uint32(msg);

        if (!svc_can_list(txn->sender_pid)) {
            ALOGE("list_service() uid=%d - PERMISSION DENIED
",
                    txn->sender_euid);
            return -1;
        }
        si = svclist;
        while ((n-- > 0) && si)                                                    ⑥
            si = si->next;
        if (si) {
            bio_put_string16(reply, si->name);
            return 0;
        }
        return -1;
    }
    default:
        ALOGE("unknown code %d
", txn->code);
        return -1;
    }

    bio_put_uint32(reply, 0);
    return 0;
}

①: 获取帧头数据,一般为0，因为发送方发送数据时都会在数据最前方填充4个字节0数据(分配数据空间的最小单位4字节);  
②: 对比svcmgr_id是否和我们原来定义相同#define SVC_MGR_NAME "linux.os.ServiceManager"(我改写了);  
③: 根据code 做对应的事情,就想到与根据编码去执行对应的fun(client请求服务后去执行服务，service也是根据不同的code来执行。接下来会举例说明);、
④: 从服务名在server链表中查找对应的服务，并返回handle(详见2.2.6);  
⑤: 添加服务，一般都是service发起的请求。将handle和服务名添加到服务链表中(这里的handle是由binder驱动分配);  
⑥: 查找server_manager中链表中第n个服务的名字(该数值由查询端决定);

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

    if (!svc_can_find(s, len, spid)) {                                             ①
        ALOGE("find_service("%s") uid=%d - PERMISSION DENIED
",
             str8(s, len), uid);
        return 0;
    }
    si = find_svc(s, len);                                                         ②
    //ALOGI("check_service("%s") handle = %x
", str8(s, len), si ? si->handle : 0);
    if (si && si->handle) {
        if (!si->allow_isolated) {                                                 ③
            // 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;
            }
        }
        return si->handle;                                                         ④
    } else {
        return 0;
    }
}

①: 检测调用进程是否有权限请求服务(这里用selinux管理权限,为了让代码可以方便允许,这里面的代码有做删减);  
②: 遍历server_manager服务链表;  
③: 如果binder服务不允许服务从沙箱中访问,则执行下面检查;  
④: 返回查询到handle;

do_find_service函数主要工作是搜索服务链表,返回查找到的服务

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
", str8(s, len), handle,
    //        allow_isolated ? "allow_isolated" : "!allow_isolated", uid);

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

    if (!svc_can_register(s, len, spid)) {                                     ①
        ALOGE("add_service("%s",%x) uid=%d - PERMISSION DENIED
",
             str8(s, len), handle, uid);
        return -1;
    }

    si = find_svc(s, len);                                                     ②
    if (si) {
        if (si->handle) {
            ALOGE("add_service("%s",%x) uid=%d - ALREADY REGISTERED, OVERRIDE
",
                 str8(s, len), handle, uid);
            svcinfo_death(bs, si);
        }
        si->handle = handle;
    } else {                                                                   ③
        si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));
        if (!si) {
            ALOGE("add_service("%s",%x) uid=%d - OUT OF MEMORY
",
                 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] = "";
        si->death.func = (void*) svcinfo_death;
        si->death.ptr = si;
        si->allow_isolated = allow_isolated;
        si->next = svclist;
        svclist = si;
    }

    ALOGI("add_service("%s"), handle = %d
", str8(s, len), handle);

    binder_acquire(bs, handle);                                               ④
    binder_link_to_death(bs, handle, &si->death);                             ⑤
    return 0;
}

①: 判断请求进程是否有权限注册服务;  
②: 查找ServiceManager的服务链表中是否已经注册了该服务,如果有则通知驱动杀死原先的binder服务,然后更新最新的binder服务;  
③: 如果原来没有创建该binder服务,则进行一系列的赋值,再插入到服务链表的表头;  
④: 增加binder服务的引用计数;  
⑤: 告诉驱动接收服务的死亡通知;

从上面分析,可以知道ServiceManager的主要工作流程如下:

int main(int argc, char **argv) 
{
    int fd;
    struct binder_state *bs;
    uint32_t svcmgr = BINDER_SERVICE_MANAGER;
    uint32_t handle;
      int ret;

    struct register_server  led_control[3] = {                                ①
        [0] = {
            .code = 1,
            .fun = led_on
        } , 
        [1] = {
            .code = 2,
            .fun = led_off
        }
    };

    
    bs = binder_open(128*1024);                                               ②
    if (!bs) {
        ALOGE("failed to open binder driver
");
        return -1;
    }

    
    ret = svcmgr_publish(bs, svcmgr, LED_CONTROL_SERVER_NAME, led_control);   ③

    if (ret) {
        ALOGE("failed to publish %s service
", LED_CONTROL_SERVER_NAME);
        return -1;
    }

    binder_set_maxthreads(bs, 10);                                            ④

    binder_loop(bs, led_control_server_handler);                              ⑤

    return 0;
}

①: led_control_server提供的服务函数;  
②:   初始化binder组件( 详见2.2);  
③:   注册服务,svcmgr是发送的目标, LED_CONTROL_SERVER_NAME注册的服务名, led_control注册的binder实体;  
④:  设置创建线程最大数(详见3.5);  
⑤:  进入线程循环(详见2.3);

int svcmgr_publish(struct binder_state *bs, uint32_t target, const char *name, void *ptr)
{
    int status;
    unsigned iodata[512/4];
    struct binder_io msg, reply;

    bio_init(&msg, iodata, sizeof(iodata), 4);                                ①
    bio_put_uint32(&msg, 0);  // strict mode header
    bio_put_string16_x(&msg, SVC_MGR_NAME);
    bio_put_string16_x(&msg, name);
    bio_put_obj(&msg, ptr);

    if (binder_call(bs, &msg, &reply, target, SVC_MGR_ADD_SERVICE))           ②
        return -1;

    status = bio_get_uint32(&reply);                                          ③

    binder_done(bs, &msg, &reply);                                            ④

    return status;
}

①: 初始化用户空间的数据iodata,设置了四个字节的offs,接着按一定格式往buf里面填充数据;  
②: 调用ServiceManager服务的SVC_MGR_ADD_SERVICE功能;  
③: 获取ServiceManager回复数据,成功返回0;  
④: 结束注册过程,释放内核中刚才交互分配的buf;

void bio_init(struct binder_io *bio, void *data,
              size_t maxdata, size_t maxoffs)
{
    size_t n = maxoffs * sizeof(size_t);

    if (n > maxdata) {
        bio->flags = BIO_F_OVERFLOW;
        bio->data_avail = 0;
        bio->offs_avail = 0;
        return;
    }

    bio->data = bio->data0 = (char *) data + n;                               ①
    bio->offs = bio->offs0 = data;                                            ②
    bio->data_avail = maxdata - n;                                            ③
    bio->offs_avail = maxoffs;                                                ④
    bio->flags = 0;                                                           ⑤
}

①: 根据传进来的参数,留下一定长度的offs数据空间, data指针则从 data + n开始;  
②: offs指针则从 data开始,则offs可使用的数据空间只有n个字节;  
③: 可使用的data空间计数;  
④: 可使用的offs空间计数;  
⑤: 清除buf的flag;

init后此时buf空间的分配情况如下图:

int binder_call(struct binder_state *bs,
                struct binder_io *msg, struct binder_io *reply,
                uint32_t target, uint32_t code)
{
    int res;
    struct binder_write_read bwr;
    struct {
        uint32_t cmd;
        struct binder_transaction_data txn;
    } __attribute__((packed)) writebuf;
    unsigned readbuf[32];

    if (msg->flags & BIO_F_OVERFLOW) {
        fprintf(stderr,"binder: txn buffer overflow
");
        goto fail;
    }

    writebuf.cmd = BC_TRANSACTION;   // binder call transaction 
    writebuf.txn.target.handle = target;                                               ①
    writebuf.txn.code = code;                                                          ②
    writebuf.txn.flags = 0;
    writebuf.txn.data_size = msg->data - msg->data0;                                   ③
    writebuf.txn.offsets_size = ((char*) msg->offs) - ((char*) msg->offs0);
    writebuf.txn.data.ptr.buffer = (uintptr_t)msg->data0;
    writebuf.txn.data.ptr.offsets = (uintptr_t)msg->offs0;

    bwr.write_size = sizeof(writebuf);                                                 ④
    bwr.write_consumed = 0;
    bwr.write_buffer = (uintptr_t) &writebuf;

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

        res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);                                  ⑤

        if (res < 0) {
            fprintf(stderr,"binder: ioctl failed (%s)
", strerror(errno));
            goto fail;
        }

        res = binder_parse(bs, reply, (uintptr_t) readbuf, bwr.read_consumed, 0);      ⑥
        if (res == 0) return 0;
        if (res < 0) goto fail;
    }

fail:
    memset(reply, 0, sizeof(*reply));
    reply->flags |= BIO_F_IOERROR;
    return -1;
}

①: 这个target就是我们这次请求服务的目标,即ServiceManager;  
②: code是我们请求服务的功能码,由服务端提供;  
③: 把binder_io数据转化成binder_transaction_data数据;   
④: 驱动进行读写是根据这个size来的,分析驱动的时候再详细分析;  
⑤: 进行一次读写;  
⑥: 解析发送的后返回的数据,判断是否注册成功;

void binder_done(struct binder_state *bs,
                 struct binder_io *msg,
                 struct binder_io *reply)
{
    struct {
        uint32_t cmd;
        uintptr_t buffer;
    } __attribute__((packed)) data;

    if (reply->flags & BIO_F_SHARED) {
        data.cmd = BC_FREE_BUFFER;
        data.buffer = (uintptr_t) reply->data0;
        binder_write(bs, &data, sizeof(data));
        reply->flags = 0;
    }
}

这个函数比较简单发送BC_FREE_BUFFER命令给驱动,让驱动释放内核态由刚才交互分配的buf;

void binder_set_maxthreads(struct binder_state *bs, int threads)
{
    ioctl(bs->fd, BINDER_SET_MAX_THREADS, &threads);
}

这里主要调用ioctl函数写入命令BINDER_SET_MAX_THREADS进行设置最大线程数;

led_control_server主要提供led的控制服务,具体的流程如下:

int main(int argc, char **argv)
{
    struct binder_state *bs;
    uint32_t svcmgr = BINDER_SERVICE_MANAGER;
    unsigned int g_led_control_handle;

    if (argc < 3) {
        ALOGE("Usage:
");
        ALOGE("%s led 
", argv[0]);
        return -1;
    }

    bs = binder_open(128*1024);                                                        ①
    if (!bs) {
        ALOGE("failed to open binder driver
");
        return -1;
    }

    g_led_control_handle = svcmgr_lookup(bs, svcmgr, LED_CONTROL_SERVER_NAME);         ②
    if (!g_led_control_handle) {
        ALOGE( "failed to get led control service
");
        return -1;
    }

    ALOGI("Handle for led control service = %d
", g_led_control_handle);

    if (!strcmp(argv[1], "led")) {
        if (!strcmp(argv[2], "on")) {
            if (interface_led_on(bs, g_led_control_handle, 2) == 0) {                  ③
                ALOGI("led was on
");
            }
        } else if (!strcmp(argv[2], "off")) {
            if (interface_led_off(bs, g_led_control_handle, 2) == 0) {
                ALOGI("led was off
");
            }
        }
    }

    binder_release(bs, g_led_control_handle);                                          ④

    return 0;
}

①: 打开binder设备(详见2.2);  
②: 根据名字获取led控制服务;  
③: 根据获取到的handle,调用led控制服务(详见4.3);  
④: 释放服务;

client的流程也很简单,按步骤1.2.3.4读下来就是了;

uint32_t svcmgr_lookup(struct binder_state *bs, uint32_t target, const char *name)
{
    uint32_t handle;
    unsigned iodata[512/4];
    struct binder_io msg, reply;

    bio_init(&msg, iodata, sizeof(iodata), 4);                                         ①
    bio_put_uint32(&msg, 0);  // strict mode header
    bio_put_string16_x(&msg, SVC_MGR_NAME);
    bio_put_string16_x(&msg, name);

    if (binder_call(bs, &msg, &reply, target, SVC_MGR_GET_SERVICE))                    ②
        return 0;

    handle = bio_get_ref(&reply);                                                      ③

    if (handle)
        binder_acquire(bs, handle);                                                    ④

    binder_done(bs, &msg, &reply);                                                     ⑤

    return handle;
}

①: 因为是请求服务,所以这里不用添加binder实体数据,具体的参考3.2,这里就不重复解释了;  
②: 向target进程(ServiceManager)请求获取led_control服务(详细参考3.3);  
③: 从ServiceManager返回的数据buf中获取led_control服务的handle;  
④: 增加该handle的引用计数;  
⑤: 释放内核空间buf(详3.4);

uint32_t bio_get_ref(struct binder_io *bio)
{
    struct flat_binder_object *obj;

    obj = _bio_get_obj(bio);                                                          ①
    if (!obj)
        return 0;

    if (obj->type == BINDER_TYPE_HANDLE)                                              ②
        return obj->handle;

    return 0;
}

①: 把bio的数据转化成flat_binder_object格式;  
②: 判断binder数据类型是否为引用,是则返回获取到的handle;

static struct flat_binder_object *_bio_get_obj(struct binder_io *bio)
{
    size_t n;
    size_t off = bio->data - bio->data0;                                              ①
    /* TODO: be smarter about this? */
    for (n = 0; n < bio->offs_avail; n++) {
        if (bio->offs[n] == off)
            return bio_get(bio, sizeof(struct flat_binder_object));                   ②
    }

    bio->data_avail = 0;
    bio->flags |= BIO_F_OVERFLOW;
    return NULL;
}

①: 一般情况下该值都为0,因为在reply时获取ServiceManager传来的数据,bio->data和bio->data都指向同一个地址;  
②: 获取到struct flat_binder_object数据的头指针;

从ServiceManager传来的数据是struct flat_binder_object的数据,格式如下:

BR个人理解是缩写为binder reply

BC个人理解是缩写为binder call or cmd

表格参考博客:

https://blog.csdn.net/univers...