Binder机制情景分析之C服务应用

一. 概述

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_servertest_client的代码,这些都是用c写的.其中led_control_servertest_client
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;

1.1 Binder通信模型

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。

1.2 运行环境

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);

1.3 文章代码

文章所有代码已上传

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

二. ServiceManager

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;

2.1 main

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\n");
        return -1;
    }

    if (binder_become_context_manager(bs)) {                                   ②
        ALOGE("cannot become context manager (%s)\n", 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clientserver的启动流程和manager的启动类似,后面再详细分析;

2.2 binder_open

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)\n",
                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\n");
        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)\n",
                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;

2.2 binder_become_context_manager

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;

2.3 binder_loop

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)\n", strerror(errno));
            break;
        }

        res = binder_parse(bs, 0, (uintptr_t) readbuf, bwr.read_consumed, func);       ③
        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;
        }
    }
}
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:

clipboard.png

2.4 binder_parse

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:\n", 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\n", (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!\n");
                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!\n");
                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\n", 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;

2.5 svcmgr_handler

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\n",
    //  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\n", 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\n",
                    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\n", 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);

2.6 do_find_service

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\n",
             str8(s, len), uid);
        return 0;
    }
    si = find_svc(s, len);                                                         ②
    //ALOGI("check_service('%s') handle = %x\n", 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

2.7 do_add_service

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;

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

    ALOGI("add_service('%s'), handle = %d\n", 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;

2.8 调用时序图

从上面分析,可以知道ServiceManager的主要工作流程如下:

clipboard.png

三. led_control_server

3.1 main

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\n");
        return -1;
    }

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

    if (ret) {
        ALOGE("failed to publish %s service\n", 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);

3.2 svcmgr_publish

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;

3.2.1 bio_init

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:

3.2.2 bio_put_uint32

void bio_put_uint32(struct binder_io *bio, uint32_t n)
{
    uint32_t *ptr = bio_alloc(bio, sizeof(n));
    if (ptr)
        *ptr = n;
}

这个函数往buf里面填充一个uint32的数据,这个数据的最小单位为4个字节;
前面svcmgr_publish调用bio_put_uint32(&msg, 0);,实质buf中的数据是00 00 00 00 ;

3.2.3 bio_alloc

static void *bio_alloc(struct binder_io *bio, size_t size)
{
    size = (size + 3) & (~3);
    if (size > bio->data_avail) {
        bio->flags |= BIO_F_OVERFLOW;
        return NULL;
    } else {
        void *ptr = bio->data;
        bio->data += size;
        bio->data_avail -= size;
        return ptr;
    }
}

这个函数分配的数据宽度为4的倍数,先判断当前可使用的数据宽度是否小于待分配的宽度;
如果小于则置标志BIO_F_OVERFLOW否则分配数据,并对data往后偏移size个字节,可使用数据宽度data_avail减去size个字节;

3.2.4 bio_put_string16_x

void bio_put_string16_x(struct binder_io *bio, const char *_str)
{
    unsigned char *str = (unsigned char*) _str;
    size_t len;
    uint16_t *ptr;

    if (!str) {                                                            ①
        bio_put_uint32(bio, 0xffffffff);
        return;
    }

    len = strlen(_str);

    if (len >= (MAX_BIO_SIZE / sizeof(uint16_t))) {
        bio_put_uint32(bio, 0xffffffff);
        return;
    }

    /* Note: The payload will carry 32bit size instead of size_t */
    bio_put_uint32(bio, len);                                             ②
    ptr = bio_alloc(bio, (len + 1) * sizeof(uint16_t));
    if (!ptr)
        return;

    while (*str)                                                          ③
        *ptr++ = *str++;
    *ptr++ = 0;
}
①: 这里到bio_alloc前都是为了计算和判断自己串的长度再填充到buf中;
②: 填充字符串前会填充字符串的长度;
③: 填充字符串到buf中,一个字符占两个字节,注意 uint16_t *ptr;;

3.2.5 bio_put_obj

void bio_put_obj(struct binder_io *bio, void *ptr)
{
    struct flat_binder_object *obj;

    obj = bio_alloc_obj(bio);                                             ①
    if (!obj)
        return;

    obj->flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;
    obj->type = BINDER_TYPE_BINDER;                                       ②
    obj->binder = (uintptr_t)ptr;                                         ③
    obj->cookie = 0;
}
struct flat_binder_object {
/* WARNING: DO NOT EDIT, AUTO-GENERATED CODE - SEE TOP FOR INSTRUCTIONS */
 __u32 type;
 __u32 flags;
 union {
 binder_uintptr_t binder;
/* WARNING: DO NOT EDIT, AUTO-GENERATED CODE - SEE TOP FOR INSTRUCTIONS */
 __u32 handle;
 };
 binder_uintptr_t cookie;
};
①: 分配一个flat_binder_object大小的空间(详见3.2.6);
②: type的类型为BINDER_TYPE_BINDER时则type传入的是binder实体,一般是服务端注册服务时传入;
type的类型为BINDER_TYPE_HANDLE时则type传入的为handle,一般由客户端请求服务时;
③: obj->binder值,跟随type改变;

3.2.6 bio_alloc_obj

static struct flat_binder_object *bio_alloc_obj(struct binder_io *bio)
{
    struct flat_binder_object *obj;

    obj = bio_alloc(bio, sizeof(*obj));                                    ①

    if (obj && bio->offs_avail) {
        bio->offs_avail--;
        *bio->offs++ = ((char*) obj) - ((char*) bio->data0);               ②
        return obj;
    }

    bio->flags |= BIO_F_OVERFLOW;
    return NULL;
}
①: 在data后分配struct flat_binder_object长度的空间;
②: bio->offs空间记下此时插入obj,相对于data0的偏移值;

看到这终于知道offs是干嘛的了,原来是用来记录数据中是否有obj类型的数据;

3.2.7 完整数据格式图

综上分析,传输一次完整的数据的格式如下:

clipboard.png

3.3 binder_call

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\n");
        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)\n", 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;

3.4 binder_done

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;

3.5 binder_set_maxthreads

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;

3.6 调用时序图

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

clipboard.png

四. test_client

4.1 main

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:\n");
        ALOGE("%s led <on|off>\n", argv[0]);
        return -1;
    }

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

    ALOGI("Handle for led control service = %d\n", 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\n");
            }
        } else if (!strcmp(argv[2], "off")) {
            if (interface_led_off(bs, g_led_control_handle, 2) == 0) {
                ALOGI("led was off\n");
            }
        }
    }

    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;

4.2 svcmgr_lookup

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);

4.2.1 bio_get_ref

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;

4.2.2 _bio_get_obj

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:

4.3 interface_led_on

int interface_led_on(struct binder_state *bs, unsigned int handle, unsigned char led_enum)
{
    unsigned iodata[512/4];
    struct binder_io msg, reply;
    int ret = -1;
    int exception;

    bio_init(&msg, iodata, sizeof(iodata), 4);
    bio_put_uint32(&msg, 0);  // strict mode header
    bio_put_uint32(&msg, led_enum);

    if (binder_call(bs, &msg, &reply, handle, LED_CONTROL_ON))
        return ret;

    exception = bio_get_uint32(&reply);
    if (exception == 0)
        ret = bio_get_uint32(&reply);

    binder_done(bs, &msg, &reply);

    return ret;
}

这个流程和前面svcmgr_lookup的请求服务差不多,只是最后是获取led_control_server的返回值.
注意这里为什么获取了两次uint32类型的数据,这是因为服务方在回复数据的时候添加了头帧,这个是可以调节的,非规则;

4.4 binder_release

void binder_release(struct binder_state *bs, uint32_t target)
{
    uint32_t cmd[2];
    cmd[0] = BC_RELEASE;
    cmd[1] = target;
    binder_write(bs, cmd, sizeof(cmd));
}

通知驱动层减小对target进程的引用,结合驱动讲解就更能明白了;

4.5 调用时序图

test_client的调用时序如下,过程和led_control_server的调用过程相识:

clipboard.png

A: 表BR_含义

BR个人理解是缩写为binder reply

消息 含义 参数
BR_ERROR 发生内部错误(如内存分配失败) ---
BR_OK
BR_NOOP
操作完成 ---
BR_SPAWN_LOOPER 该消息用于接收方线程池管理。当驱动发现接收方所有
线程都处于忙碌状态且线程池里的线程总数没有超过
BINDER_SET_MAX_THREADS设置的最大线程数时,
向接收方发送该命令要求创建更多线程以备接收数据。
---
BR_TRANSACTION 对应发送方的BC_TRANSACTION binder_transaction_data
BR_REPLY 对应发送方BC_REPLY的回复 binder_transaction_data
BR_ACQUIRE_RESULT
BR_FINISHED
未使用 ---
BR_DEAD_REPLY 交互时向驱动发送binder调用,如果对方已经死亡,则
驱动回应此命令
---
BR_TRANSACTION_COMPLETE 发送方通过BC_TRANSACTION或BC_REPLY发送
完一个数据包后,都能收到该消息做为成功发送的反馈。
这和BR_REPLY不一样,是驱动告知发送方已经发送成
功,而不是Server端返回请求数据。所以不管
同步还是异步交互接收方都能获得本消息。
---
BR_INCREFS
BR_ACQUIRE
BR_RELEASE
BR_DECREFS
这一组消息用于管理强/弱指针的引用计数。只有
提供Binder实体的进程才能收到这组消息。
binder_uintptr_t binder:Binder实体在用户空间中的指针
binder_uintptr_t cookie:与该实体相关的附加数据
BR_DEAD_BINDER
向获得Binder引用的进程发送Binder实体
死亡通知书;收到死亡通知书的进程接下
来会返回BC_DEAD_BINDER_DONE做确认。
---
BR_CLEAR_DEATH_NOTIFICATION_DONE 回应命令BC_REQUEST_DEATH_NOTIFICATION ---
BR_FAILED_REPLY 如果发送非法引用号则返回该消息 ---

B: 表BC_含义

BC个人理解是缩写为binder call or cmd

消息 含义 参数
BC_TRANSACTION
BC_REPLY
BC_TRANSACTION用于Client向Server发送请求数据;
BC_REPLY用于Server向Client发送回复(应答)数据。
其后面紧接着一个binder_transaction_data结构体表明要写
入的数据。
struct binder_transaction_data
BC_ACQUIRE_RESULT
BC_ATTEMPT_ACQUIRE
未使用 ---
BC_FREE_BUFFER 请求驱动释放调刚在内核空间创建用来保存用户空间数据的内存块 ---
BC_INCREFS
BC_ACQUIRE
BC_RELEASE
BC_DECREFS
这组命令增加或减少Binder的引用计数,用以实现强指针或
弱指针的功能。
---
BC_INCREFS_DONE
BC_ACQUIRE_DONE
第一次增加Binder实体引用计数时,驱动向Binder
实体所在的进程发送BR_INCREFS, BR_ACQUIRE消息;
Binder实体所在的进程处理完毕回馈BC_INCREFS_DONE,
BC_ACQUIRE_DONE
---
BC_REGISTER_LOOPER
BC_ENTER_LOOPER
BC_EXIT_LOOPER
这组命令同BINDER_SET_MAX_THREADS一道实现Binder驱
动对接收方线程池管理。BC_REGISTER_LOOPER通知驱动线程
池中一个线程已经创建了;BC_ENTER_LOOPER通知驱动该线程
已经进入主循环,可以接收数据;BC_EXIT_LOOPER通知驱动
该线程退出主循环,不再接收数据。
---
BC_REQUEST_DEATH_NOTIFICATION 获得Binder引用的进程通过该命令要求驱动在Binder实体销毁得到
通知。虽说强指针可以确保只要有引用就不会销毁实体,但这毕竟
是个跨进程的引用,谁也无法保证实体由于所在的Server关闭Binder
驱动或异常退出而消失,引用者能做的是要求Server在此刻给出通知。
---
BC_DEAD_BINDER_DONE 收到实体死亡通知书的进程在删除引用后用本命令告知驱动。 ---

参考

表格参考博客:

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