前言
之前分析的两篇文章
Android Adb 源码分析(一)
嵌入式Linux:Android root破解原理(二)
写完之后,都没有写到相关的实现代码,这篇文章写下ADB的通信流程的一些细节
看这篇文章之前,请先阅读
Linux的SOCKET编程详解 - 江召伟 - 博客园
对socket通信有简单的了解
1、ADB基本通信
理解:
(1)adb的本质,就是socket的通信,通过secket传送数据及文件
(2)adb传送是以每个固定格式的包发送的数据,包的格式如下:
#define A_SYNC 0x434e5953
#define A_CNXN 0x4e584e43
#define A_OPEN 0x4e45504f
#define A_OKAY 0x59414b4f
#define A_CLSE 0x45534c43
#define A_WRTE 0x45545257
#define A_AUTH 0x48545541struct amessage {unsigned command; /* command identifier constant */unsigned arg0; /* first argument */unsigned arg1; /* second argument */unsigned data_length; /* length of payload (0 is allowed) */unsigned data_check; /* checksum of data payload */unsigned magic; /* command ^ 0xffffffff */
};struct apacket
{apacket *next;unsigned len;unsigned char *ptr;amessage msg;unsigned char data[MAX_PAYLOAD];
};
发送的包格式为apacket格式,其中msg为消息部分,data为数据部分。msg的消息类型有很多种,包括A_SYNC, A_CNXN, A_OPEN, A_OKAY等等。
(3)adb给我们预留了调试的信息,我们只需要在adb.h中定义指定的宏,即可看到每次数据的传输过程:
#define DEBUG_PACKETS 1
打开调试信息后,我们可以看到传输过程中的细节,在串口打印里面。
(4)我们使用adb push命令,来跟踪分析下这个apacket数据是怎样传输的:
我们以adb push profile /命令为例,在串口我们可以看见如下详细的传输信息:
status command arg0 arg1 len data
recv: OPEN 00141028 00000000 0006 "sync:."
send: OKAY 0000003e 00141028 0000 ""
recv: WRTE 00141028 0000003e 0009 "STAT..../"
send: OKAY 0000003e 00141028 0000 ""
send: WRTE 0000003e 00141028 0010 "STAT.A......[oHZ"
recv: OKAY 00141028 0000003e 0000 ""
recv: WRTE 00141028 0000003e 0027 "SEND..../profile,33206DATA....2D
send: OKAY 0000003e 00141028 0000 ""
send: WRTE 0000003e 00141028 0008 "OKAY...."
recv: OKAY 00141028 0000003e 0000 ""
recv: WRTE 00141028 0000003e 0008 "QUIT...."
send: OKAY 0000003e 00141028 0000 ""
send: CLSE 00000000 00141028 0000 ""
recv: CLSE 00141028 0000003e 0000 ""
以上recv表示接收的数据包,send表示回传的数据包。后面五个分别为数据包的数据字段值(command arg0 arg1 len data),这样数据我们还是不够直观,我们翻译成更加直接的数据辅以文字解释
这样是不是容易理解多了呢,经过这样的数据发送,我们就通过adb push命令把本地的profile文件推送到远程设备的根目录了。哇..... 原来这么简单,一个profile文件就传输了。流程理解了,我们再来看代码,现在结果你知道了,流程你也懂了,再来看源码,是不是容易理解了呢。
2、代码分析
我们看代码也是逆向的看,这样利于我们理解,不会被源码看到晕乎乎,上面流程懂了,知道了每次是以apacket的格式发送的,我们先来研究这个apacket的接收与发送函数。
接收函数handle_packet
void handle_packet(apacket *p, atransport *t)
{asocket *s;D("handle_packet() %c%c%c%c\n", ((char*) (&(p->msg.command)))[0],((char*) (&(p->msg.command)))[1],((char*) (&(p->msg.command)))[2],((char*) (&(p->msg.command)))[3]);print_packet("recv", p);switch(p->msg.command){case A_SYNC:if(p->msg.arg0){send_packet(p, t);if(HOST) send_connect(t);} else {t->connection_state = CS_OFFLINE;handle_offline(t);send_packet(p, t);}return;case A_CNXN: /* CONNECT(version, maxdata, "system-id-string") *//* XXX verify version, etc */if(t->connection_state != CS_OFFLINE) {t->connection_state = CS_OFFLINE;handle_offline(t);}parse_banner((char*) p->data, t);if (HOST || !auth_enabled) {handle_online(t);if(!HOST) send_connect(t);} else {send_auth_request(t);}break;case A_AUTH:if (p->msg.arg0 == ADB_AUTH_TOKEN) {t->key = adb_auth_nextkey(t->key);if (t->key) {send_auth_response(p->data, p->msg.data_length, t);} else {/* No more private keys to try, send the public key */send_auth_publickey(t);}} else if (p->msg.arg0 == ADB_AUTH_SIGNATURE) {if (adb_auth_verify(t->token, p->data, p->msg.data_length)) {adb_auth_verified(t);t->failed_auth_attempts = 0;} else {if (t->failed_auth_attempts++ > 10)adb_sleep_ms(1000);send_auth_request(t);}} else if (p->msg.arg0 == ADB_AUTH_RSAPUBLICKEY) {adb_auth_confirm_key(p->data, p->msg.data_length, t);}break;case A_OPEN: /* OPEN(local-id, 0, "destination") */if (t->online) {char *name = (char*) p->data;name[p->msg.data_length > 0 ? p->msg.data_length - 1 : 0] = 0;s = create_local_service_socket(name);if(s == 0) {send_close(0, p->msg.arg0, t);} else {s->peer = create_remote_socket(p->msg.arg0, t);s->peer->peer = s;send_ready(s->id, s->peer->id, t);s->ready(s);}}break;case A_OKAY: /* READY(local-id, remote-id, "") */if (t->online) {if((s = find_local_socket(p->msg.arg1))) {if(s->peer == 0) {s->peer = create_remote_socket(p->msg.arg0, t);s->peer->peer = s;}s->ready(s);}}break;case A_CLSE: /* CLOSE(local-id, remote-id, "") */if (t->online) {if((s = find_local_socket(p->msg.arg1))) {s->close(s);}}break;case A_WRTE:if (t->online) {if((s = find_local_socket(p->msg.arg1))) {unsigned rid = p->msg.arg0;p->len = p->msg.data_length;if(s->enqueue(s, p) == 0) {D("Enqueue the socket\n");send_ready(s->id, rid, t);}return;}}break;default:printf("handle_packet: what is %08x?!\n", p->msg.command);}put_apacket(p);
}
哇,这个函数好像不复杂
一个函数,然后解析apacket *p数据,根据msg.command的命令值, 然后对应不同的case,有着不同的响应。事实上也就是这样,这个函数主要就是根据不同的消息类型,来处理这个apacket的数据。
下面解析下上面push命令的过程
1、OPEN响应
recv: OPEN 00141028 00000000 0006 "sync:."
send: OKAY 0000003e 00141028 0000 ""
接收到了OPEN的消息,然后附带了一个sync的数据,我们看看是如何响应的。
case A_OPEN: /* OPEN(local-id, 0, "destination") */if (t->online) {char *name = (char*) p->data;name[p->msg.data_length > 0 ? p->msg.data_length - 1 : 0] = 0;s = create_local_service_socket(name);if(s == 0) {send_close(0, p->msg.arg0, t);} else {s->peer = create_remote_socket(p->msg.arg0, t);s->peer->peer = s;send_ready(s->id, s->peer->id, t);s->ready(s);}}break;
调用create_local_service_socket(“sync”);
fd = service_to_fd(name);
//创建本地socket,并为这个socket创建数据处理线程file_sync_service
ret = create_service_thread(file_sync_service, NULL);
//把这个本地socket关联到结构asocket *s
s = create_local_socket(fd);
调用create_remote_socket(p->msg.arg0, t); //把远程的socket也与这个结构体asocket 关联。
如上两个函数调用,主要是初始化本地的socket对,本地socket用来跟后台服务线程之间的通信,以及跟对应命令的后台服务线程通信。初始化adb通信的环境。其中asocket *s为本地socket与远程socket的一个关联结构体,其中s保存的是本地socket的信息,s->peer保存的是远程socket相关的信息。
send_ready(s->id, s->peer->id, t); 然后发送OKAY给PC端。
static void send_ready(unsigned local, unsigned remote, atransport *t)
{D("Calling send_ready \n");apacket *p = get_apacket();p->msg.command = A_OKAY;p->msg.arg0 = local;p->msg.arg1 = remote;send_packet(p, t);
}
这个与我们看到的流程相符合。接收到OPEN的消息,初始化一些状态,然后返回一个OKAY的状
2、WRITE响应
recv: WRTE 00141028 0000003e 0009 "STAT..../"
send: OKAY 0000003e 00141028 0000 ""
send: WRTE 0000003e 00141028 0010 "STAT.A......[oHZ"
接收到了WRITE的消息,顺带了一个查询STAT的数据,我们看看是如何响应的:
case A_WRTE:if (t->online) {if((s = find_local_socket(p->msg.arg1))) {unsigned rid = p->msg.arg0;p->len = p->msg.data_length;if(s->enqueue(s, p) == 0) {D("Enqueue the socket\n");send_ready(s->id, rid, t);}return;}}break;
先通过参数p->msg.arg1找到我们在OPEN的时候建立的结构体信息asocket *s, 然后处理本地socket队列中的数据(s为本地,s->peer为远程)
s->enqueue(s, p)即为之前 关联的函数local_socket_enqueue,其在create_local_socket(fd); 的时候设置。
static int local_socket_enqueue(asocket *s, apacket *p)
{D("LS(%d): enqueue %d\n", s->id, p->len);p->ptr = p->data;/* if there is already data queue'd, we will receive** events when it's time to write. just add this to** the tail*/if(s->pkt_first) {goto enqueue;}/* write as much as we can, until we** would block or there is an error/eof*/while(p->len > 0) {int r = adb_write(s->fd, p->ptr, p->len);if(r > 0) {p->len -= r;p->ptr += r;continue;}if((r == 0) || (errno != EAGAIN)) {D( "LS(%d): not ready, errno=%d: %s\n", s->id, errno, strerror(errno) );s->close(s);return 1; /* not ready (error) */} else {break;}}if(p->len == 0) {put_apacket(p);return 0; /* ready for more data */}enqueue:p->next = 0;if(s->pkt_first) {s->pkt_last->next = p;} else {s->pkt_first = p;}s->pkt_last = p;/* make sure we are notified when we can drain the queue */fdevent_add(&s->fde, FDE_WRITE);return 1; /* not ready (backlog) */
}
我们通过adb_write(s->fd, p->ptr, p->len)把要处理的数据,写入到本地socket对应的fd中,等待处理。
然后调用send_ready(s->id, rid, t);返回一个OKAY的状态
我们把待处理的数据adb_write之后,又是在哪里处理的呢,我们之前在创建本地socket的时候,就创建了一个线程,对应的处理socket数据的函数file_sync_service。
我们来看看file_sync_service函数是如何处理的
void file_sync_service(int fd, void *cookie)
{syncmsg msg;char name[1025];unsigned namelen;char *buffer = malloc(SYNC_DATA_MAX);if(buffer == 0) goto fail;for(;;) {D("sync: waiting for command\n");if(readx(fd, &msg.req, sizeof(msg.req))) {fail_message(fd, "command read failure");break;}namelen = ltohl(msg.req.namelen);if(namelen > 1024) {fail_message(fd, "invalid namelen");break;}if(readx(fd, name, namelen)) {fail_message(fd, "filename read failure");break;}name[namelen] = 0;msg.req.namelen = 0;D("sync: '%s' '%s'\n", (char*) &msg.req, name);switch(msg.req.id) {case ID_STAT:if(do_stat(fd, name)) goto fail;break;case ID_LIST:if(do_list(fd, name)) goto fail;break;case ID_SEND:if(do_send(fd, name, buffer)) goto fail;break;case ID_RECV:if(do_recv(fd, name, buffer)) goto fail;break;case ID_QUIT:goto fail;default:fail_message(fd, "unknown command");goto fail;}}fail:if(buffer != 0) free(buffer);D("sync: done\n");adb_close(fd);
}
原来在这里处理的数据,终于找到你, 我们收到的消息是查看路径是否存在,这里对应的就是ID_STAT,还有其他的消息处理,比如ID_SEND,ID_RECV,ID_QUIT,望文生义,我们就不具体解释了。我们还是看看ID_STAT对应的处理吧do_stat(fd, name)。
static int do_stat(int s, const char *path)
{syncmsg msg;struct stat st;msg.stat.id = ID_STAT;if(lstat(path, &st)) {msg.stat.mode = 0;msg.stat.size = 0;msg.stat.time = 0;} else {msg.stat.mode = htoll(st.st_mode);msg.stat.size = htoll(st.st_size);msg.stat.time = htoll(st.st_mtime);}return writex(s, &msg.stat, sizeof(msg.stat));
}
这里就是判断路径是否存在的逻辑了,这个就是我们想要的,我们把判断的结果存储在msg.stat, 然后把对应的结果写回去writex。
我们把检测的状态writex之后,但是这个数据还没有发送回PC端啊,是在哪里发送回去的呢,我们继续跟踪
我们在create_local_socket创建本地socket的时候,顺便还注册了一个回调函数local_socket_event_func
static void local_socket_event_func(int fd, unsigned ev, void *_s)
{asocket *s = _s;D("LS(%d): event_func(fd=%d(==%d), ev=%04x)\n", s->id, s->fd, fd, ev);/* put the FDE_WRITE processing before the FDE_READ** in order to simplify the code.*/if(ev & FDE_WRITE){apacket *p;while((p = s->pkt_first) != 0) {while(p->len > 0) {int r = adb_write(fd, p->ptr, p->len);if(r > 0) {p->ptr += r;p->len -= r;continue;}if(r < 0) {/* returning here is ok because FDE_READ will** be processed in the next iteration loop*/if(errno == EAGAIN) return;if(errno == EINTR) continue;}D(" closing after write because r=%d and errno is %d\n", r, errno);s->close(s);return;}if(p->len == 0) {s->pkt_first = p->next;if(s->pkt_first == 0) s->pkt_last = 0;put_apacket(p);}}/* if we sent the last packet of a closing socket,** we can now destroy it.*/if (s->closing) {D(" closing because 'closing' is set after write\n");s->close(s);return;}/* no more packets queued, so we can ignore** writable events again and tell our peer** to resume writing*/fdevent_del(&s->fde, FDE_WRITE);s->peer->ready(s->peer);}if(ev & FDE_READ){apacket *p = get_apacket();unsigned char *x = p->data;size_t avail = MAX_PAYLOAD;int r;int is_eof = 0;while(avail > 0) {r = adb_read(fd, x, avail);D("LS(%d): post adb_read(fd=%d,...) r=%d (errno=%d) avail=%d\n", s->id, s->fd, r, r<0?errno:0, avail);if(r > 0) {avail -= r;x += r;continue;}if(r < 0) {if(errno == EAGAIN) break;if(errno == EINTR) continue;}/* r = 0 or unhandled error */is_eof = 1;break;}D("LS(%d): fd=%d post avail loop. r=%d is_eof=%d forced_eof=%d\n",s->id, s->fd, r, is_eof, s->fde.force_eof);if((avail == MAX_PAYLOAD) || (s->peer == 0)) {put_apacket(p);} else {p->len = MAX_PAYLOAD - avail;r = s->peer->enqueue(s->peer, p);D("LS(%d): fd=%d post peer->enqueue(). r=%d\n", s->id, s->fd, r);if(r < 0) {/* error return means they closed us as a side-effect** and we must return immediately.**** note that if we still have buffered packets, the** socket will be placed on the closing socket list.** this handler function will be called again** to process FDE_WRITE events.*/return;}if(r > 0) {/* if the remote cannot accept further events,** we disable notification of READs. They'll** be enabled again when we get a call to ready()*/fdevent_del(&s->fde, FDE_READ);}}/* Don't allow a forced eof if data is still there */if((s->fde.force_eof && !r) || is_eof) {D(" closing because is_eof=%d r=%d s->fde.force_eof=%d\n", is_eof, r, s->fde.force_eof);s->close(s);}}if(ev & FDE_ERROR){/* this should be caught be the next read or write** catching it here means we may skip the last few** bytes of readable data.*/
// s->close(s);D("LS(%d): FDE_ERROR (fd=%d)\n", s->id, s->fd);return;}
}
我们看后面if(ev & FDE_READ)部分:
adb_read(fd, x, avail);把数据读出来,然后调用r = s->peer->enqueue(s->peer, p);,即把数据发送给远程socket的队列处理。(s->speer即远程端,之前已经说明)
s->peer->enqueue函数即remote_socket_enqueue
static int remote_socket_enqueue(asocket *s, apacket *p)
{D("entered remote_socket_enqueue RS(%d) WRITE fd=%d peer.fd=%d\n",s->id, s->fd, s->peer->fd);p->msg.command = A_WRTE;p->msg.arg0 = s->peer->id;p->msg.arg1 = s->id;p->msg.data_length = p->len;send_packet(p, s->transport);return 1;
}
这样我们就把STAT的结果,通过WRITE返回给了PC端
这个与我们看到的流程也是相符的,接收到WRITE(STAT)的消息,先返回一个OKAY的状态,在返回WRITE(STAT)的结果。
我们可以观察之前的数据接收及发送流程,可以发现每次一个WRITE消息,后面都是返回一个OKAY WRITE消息。
贴了这么多的代码,是不是有点晕了,再贴就真的看不下去了,我们下面重新来理一理思路。
1. adb其实就是个socket通信,数据发过来发过去。
2. adb每次都是发送的一个数据包,数据结构是struct apacket,其中包含msg消息部分,及data数据部分。
3. 从PC跟device通信的过程,有一条协议流程,通过不断的数据交互发送,实现数据文件传递。
4. 我们可以定义 #define DEBUG_PACKETS 1 这样可以看到socket通信的数据发送过程。
5. socket数据建立传输过程,会创建socket,创建事件监听线程,注册回调响应函数,乱七八糟的....
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