Android(安卓)系统服务管家servicemanager启动过程详解
16lz
2021-12-04
Android考虑到移动设备耗电以及跨进程通信效率等因素，基于OpenBinder专门为进程通信开发了一套框架：binder。例如，客户端程序需要获取WindowManager,TelephonyManager等系统服务时，就需要通过binder这个中介来负责数据与指令的传输。那么，Android提供了如此多的系统服务（可达上百个，可通过 adb shell service list查看），又是谁在负责管理这些服务了？Android为此专门提供了一个系统管家servicemanager。servicemanager主要负责添加服务，获取服务，查找服务以及当某个服务意外终止时，对该服务的资源进行回收。ServiceManager跨越了两个框架层:Java层、C++层。这篇文章先看一看native(C)层的具体实现原理。
源码路径：
- C++： /android/frameworks/native/cmds/servicemanager/
- Java： /android/frameworks/base/core/java/android/os/
servicemanager的启动
Android在init进程启动以后，通过脚本init.rc,启动ServiceManager:
service servicemanager /system/bin/servicemanager class core user system group system critical onrestart restart healthd onrestart restart zygote onrestart restart media onrestart restart surfaceflinger onrestart restart keystore在service_manager.c中找到对应的main()函数，这是servicemanager启动的入口：
int main() { struct binder_state *bs; // 打开binder bs = binder_open(128*1024); if (!bs) { ALOGE("failed to open binder driver\n"); return -1; } // 成为系统服务的管家 if (binder_become_context_manager(bs)) { ALOGE("cannot become context manager (%s)\n", strerror(errno)); return -1; } .... // 进入binder循环 binder_loop(bs, svcmgr_handler); return 0; }main函数里主要做了三件事:一是打开binder文件节点，并分配IPC所需的内存；二让ServiceManager成为系统服务的管家(context manager);三开启相应的工作线程，并循环处理IPC请求。
binder_open
函数binder_open()主要作用：（1）打开binder设备文件，获取到对应的文件描述符，（2）从Binder驱动获取binder版本号，（3）接着创建一个binder内存区域，用于跨进程数据的交换（这里大小为128K)。
struct binder_state *binder_open(size_t mapsize) { struct binder_state *bs; struct binder_version vers; // 为binder_state分配内存空间 bs = malloc(sizeof(*bs)); if (!bs) { errno = ENOMEM; return NULL; } // 打开Binder驱动对应的文件节点，最终调用驱动函数binder_open bs->fd = open("/dev/binder", O_RDWR | O_CLOEXEC); // 获取当前Binder版本号 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; } //分配一块只读内存区域，用于IPC交换数据，最终调用binder驱动接口binder_mmap 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; fail_map: close(bs->fd); fail_open: free(bs); return NULL; }binder相关的
ioctl指令定义在/android/kernel/drivers/staging/android/uapi/binder.h
#define BINDER_WRITE_READ _IOWR('b', 1, struct binder_write_read) #define BINDER_SET_IDLE_TIMEOUT _IOW('b', 3, __s64) #define BINDER_SET_MAX_THREADS _IOW('b', 5, __u32) #define BINDER_SET_IDLE_PRIORITY _IOW('b', 6, __s32) #define BINDER_SET_CONTEXT_MGR _IOW('b', 7, __s32) #define BINDER_THREAD_EXIT _IOW('b', 8, __s32) #define BINDER_VERSION _IOWR('b', 9, struct binder_version)binder驱动提供了诸如ioctl,mmap,open,flush等操作，在驱动初始化时会将其注册到系统中
static const struct file_operations binder_fops = { .owner = THIS_MODULE, .poll = binder_poll, .unlocked_ioctl = binder_ioctl, .compat_ioctl = binder_ioctl, .mmap = binder_mmap, .open = binder_open, .flush = binder_flush, .release = binder_release, }; static struct miscdevice binder_miscdev = { .minor = MISC_DYNAMIC_MINOR, .name = "binder", .fops = &binder_fops };binder_become_context_manager
int binder_become_context_manager(struct binder_state *bs) { // 系统调用，告知binder驱动，ServiceManager将成为系统服务管家，对应的handle为0 return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0); }系统调用ioctl最终会调用binder驱动对应的操作函数。查看binder驱动的代码，在/android/kernel/drivers/staging/android/binder.c中有一个函数binder_ioctl，这就是用于处理kernel系统调用ioctl对应的指令，找到请求BINDER_SET_CONTEXT_MGR对应的代码，在设置系统服务管家完成后，返回对应的结果。
static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg) { int ret; struct binder_proc *proc = filp->private_data; struct binder_thread *thread; unsigned int size = _IOC_SIZE(cmd); void __user *ubuf = (void __user *)arg; ret = wait_event_interruptible(binder_user_error_wait, binder_stop_on_user_error < 2); // 加锁 binder_lock(__func__); thread = binder_get_thread(proc); switch (cmd) { .... // 设置服务管家 case BINDER_SET_CONTEXT_MGR: ret = binder_ioctl_set_ctx_mgr(filp); if (ret) goto err; break; .... // 查询Binder版本 case BINDER_VERSION: { struct binder_version __user *ver = ubuf; if (put_user_preempt_disabled(BINDER_CURRENT_PROTOCOL_VERSION, &ver->protocol_version)) { ret = -EINVAL; goto err; } break; } default: ret = -EINVAL; goto err; } ret = 0; .... }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; // 让ServiceManager线程进入循环 binder_write(bs, readbuf, sizeof(uint32_t)); for (;;) { bwr.read_size = sizeof(readbuf); bwr.read_consumed = 0; bwr.read_buffer = (uintptr_t) readbuf; // 读取 driver中的数据 res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr); // 解析读取到的数据，并通过svcmgr_handler进行处理 res = binder_parse(bs, 0, (uintptr_t) readbuf, bwr.read_consumed, func); .... } }读取到binder驱动缓冲区域里的数据后，解析之：
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); switch(cmd) { case BR_NOOP: break; case BR_TRANSACTION_COMPLETE: break; case BR_INCREFS: case BR_ACQUIRE: case BR_RELEASE: case BR_DECREFS: ptr += sizeof(struct binder_ptr_cookie); break; case BR_TRANSACTION: { struct binder_transaction_data *txn = (struct binder_transaction_data *) ptr; .... if (func) { unsigned rdata[256/4]; struct binder_io msg; struct binder_io reply; int res; bio_init(&reply, rdata, sizeof(rdata), 4); bio_init_from_txn(&msg, txn); // 处理客户端请求，func就是函数svcmgr_handler res = func(bs, txn, &msg, &reply); if (txn->flags & TF_ONE_WAY) { binder_free_buffer(bs, txn->data.ptr.buffer); } else { // 发送回复给客户端 binder_send_reply(bs, &reply, txn->data.ptr.buffer, res); } } 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; .... default: ALOGE("parse: OOPS %d\n", cmd); return -1; } } return r; }svcmgr_handler处理客户端的请求，如添加服务，查找服务以及获取服务等。
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; if (txn->target.ptr != BINDER_SERVICE_MANAGER) return -1; if (txn->code == PING_TRANSACTION) return 0; .... switch(txn->code) { 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(s, len, txn->sender_euid, txn->sender_pid); 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, 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, 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; }这里总结下ServiceManager进程启动的过程，其流程简图如下：
- binder_open(): 打开驱动设备节点，并为进程分配一个128K大小的内存区域用于IPC数据交换；
- binder_become_context_manager(): 使servicemanager称为系统服务的管家;
- binder_loop(): 进入线程执行状态，等待客户端程序的请求
参考文献
- http://gityuan.com/android/
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