binder学习笔记（十）—— 穿越到驱动层
Binder驱动层的代码在kernel/goldfish/drivers/staging/android下的binder.c和binder.h。Android源码是不带Linux内核的，驱动正是在这个内核里，需要单独下载，出门左转参见《Anrdoid源码、内核编译》。驱动的相关知识先不在这里展开了，那又是一个庞大的体系，以后再啃。直奔我们的主题——客户端为test()组织的请求数据是：
驱动程序是如何处理这个数据包的呢？
从应用层登陆，顺流直下
为此，还需要先从应用层往下看，frameworks/native/libs/binder/IPCThreadState.cpp:548，就从这里登陆吧。客户端组织test()请求数据时，调用到IPCThreadState::transact(...)
status_t IPCThreadState::transact(int32_t handle,
uint32_t code, const Parcel& data,
Parcel* reply, uint32_t flags)
{ // code=TEST, flag=0
flags |= TF_ACCEPT_FDS;
......
err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);
......
if (reply) {
err = waitForResponse(reply); // 这次重点看这里
} else {
Parcel fakeReply;
err = waitForResponse(&fakeReply);
}
......
return err;
}
函数使用writeTransactionData(…)打包好数据后，接下来调用waitForResponse(…)把数据发出去。
frameworks/native/libs/binder/IPCThreadState.cpp:712
status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
{
uint32_t cmd;
int32_t err;
while (1) {
if ((err=talkWithDriver()) < NO_ERROR) break;
......
}
......
return err;
}
继续调用talkWithDriver()和驱动对话,frameworks/native/libs/binder/IPCThreadState.cpp:803
status_t IPCThreadState::talkWithDriver(bool doReceive)
{ // doReceive=true
......
binder_write_read bwr;
......
const bool needRead = mIn.dataPosition() >= mIn.dataSize();// mIn有上一轮IO读出尚未解析的数据，因此needRead=true
......
const size_t outAvail = (!doReceive || needRead) ? mOut.dataSize() : 0; // outAvail=mOut.dataSize()
bwr.write_size = outAvail;
bwr.write_buffer = (uintptr_t)mOut.data();
......
if (doReceive && needRead) {
bwr.read_size = mIn.dataCapacity();
bwr.read_buffer = (uintptr_t)mIn.data();
}
......
bwr.write_consumed = 0;
bwr.read_consumed = 0;
status_t err;
do {
......
if (ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) >= 0) // 重点在这
err = NO_ERROR;
......
} while (err == -EINTR);
......
return err;
}
doReceive取默认值为true，在通过test()调用到talkWithDriver(...)之前，和驱动的对话已经做了好几轮了，比如defaultServiceManager()和ServiceManager的对话，getService(...)和Service的对话，此时mIn中应该是有之前读出尚未解析的数据，因此needRead=true，outAvail=mOut.dataSize()。可以组织一个gdb确认mIn此时的内容。
组织一个gdb确认此时mIn的内容
需要开启三个终端完成调试：
- Target1 在模拟器上启动server
$ adb shell /data/local/tmp/testservice/TestServer
- Target2 在模拟器上通过gdbserver启动客户端
$ adb shell gdbserver :1234 /data/local/tmp/testservice/TestClient
Process /data/local/tmp/testservice/TestClient created; pid = 1254
Listening on port 1234
Remote debugging from host 127.0.0.1
- Host1 在宿主端启动gdb
$ ./prebuilts/gcc/darwin-x86/arm/arm-linux-androideabi-4.9/bin/arm-linux-androideabi-gdb out/debug/target/product/generic/obj/EXECUTABLES/TestClient_intermediates/LINKED/TestClient
......
(gdb) b main
Breakpoint 1 at 0xb6f571fc: file external/testservice/TestClient.cpp, line 14.
(gdb) c
Continuing.
......
(gdb) set solib-absolute-prefix out/debug/target/product/generic/symbols/
Reading symbols from ...... linker...done.
......
Loaded symbols for ......
......
(gdb) b IPCThreadState.cpp:846 # 在talkWithDriver(...)内下断点
Breakpoint 2 at 0xb6eaf884: file frameworks/native/libs/binder/IPCThreadState.cpp, line 846.
(gdb) c
......
然后就是若干轮的continue和backtrace，直到停在由test()调用触发的talkWithDriver(...)
......
Breakpoint 2, android::IPCThreadState::talkWithDriver (this=this@entry=0xb6c24000, doReceive=doReceive@entry=true) at frameworks/native/libs/binder/IPCThreadState.cpp:846
846 if ((bwr.write_size == 0) && (bwr.read_size == 0)) return NO_ERROR;
(gdb) bt
0 android::IPCThreadState::talkWithDriver (this=this@entry=0xb6c24000, doReceive=doReceive@entry=true) at frameworks/native/libs/binder/IPCThreadState.cpp:846
1 0xb6eafed2 in android::IPCThreadState::waitForResponse (this=0xb6c24000, reply=0xbeaa1ad4, acquireResult=0x0) at frameworks/native/libs/binder/IPCThreadState.cpp:718
2 0xb6eb0088 in android::IPCThreadState::transact (this=0xb6c24000, handle=1, code=code@entry=1, data=..., reply=reply@entry=0xbeaa1ad4, flags=16, flags@entry=0) at frameworks/native/libs/binder/IPCThreadState.cpp:604
3 0xb6eab08e in android::BpBinder::transact (this=0xb6c090c0, code=1, data=..., reply=0xbeaa1ad4, flags=0) at frameworks/native/libs/binder/BpBinder.cpp:165
4 0xb6f3e42e in android::BpTestService::test (this=<optimized out>) at external/testservice/TestClient.cpp:10
5 0xb6f3e23c in main () at external/testservice/TestClient.cpp:18
(gdb) p mIn
$1 = {mError = 0, mData = 0xb6c27000 "\fr", mDataSize = 48, mDataCapacity = 256, mDataPos = 48, mObjects = 0x0, mObjectsSize = 0, mObjectsCapacity = 0, mNextObjectHint = 0, mFdsKnown = true, mHasFds = false, mAllowFds = true, mOwner = 0x0, mOwnerCookie = 0x0,
mOpenAshmemSize = 0}
(gdb) p needRead
$2 = true
结果和我猜测的一致。
综上所述，在IPCThreadState::talkWithDriver(...)中调用
ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &brw);
传入的数据bwr即为：
进入驱动层
终于可以有此穿越到驱动层了！binder驱动层对接ioctl的函数是binder_ioctl(...)。kernel/goldfish/drivers/staging/android/binder.c:2716
static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{ // cmd=BINDER_WRITE_READ
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;
......
thread = binder_get_thread(proc);
......
switch (cmd) {
case BINDER_WRITE_READ: {
struct binder_write_read bwr;
......
if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {// 把上图总用户空间的bwr复制到内核
......
}
......
if (bwr.write_size > 0) {
ret = binder_thread_write(proc, thread, (void __user *)bwr.write_buffer, bwr.write_size, &bwr.write_consumed);
trace_binder_write_done(ret);
......
}
// bwr.read_size来自IPCThreadState::talkWithDriver，当读缓冲区为空，会将
// mIn缓冲区交给它，bwr.read_size=mIn.dataCapacity()
if (bwr.read_size > 0) {
ret = binder_thread_read(proc, thread, (void __user *)bwr.read_buffer, bwr.read_size, &bwr.read_consumed, filp->f_flags & O_NONBLOCK);
trace_binder_read_done(ret);
if (!list_empty(&proc->todo))
wake_up_interruptible(&proc->wait);
... ...
}
......
}
......
}
ret = 0;
......
return ret;
}
先尽可能地剪掉细枝末节，来看重点调用
binder_thread_write(proc, thread, (void __user *)bwr.write_buffer,
bwr.write_size, &bwr.write_consumed);
kernel/goldfish/drivers/staging/android/binder.c:1837
int binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,
void __user *buffer, int size, signed long *consumed)
{
uint32_t cmd;
void __user *ptr = buffer + *consumed;
void __user *end = buffer + size;
while (ptr < end && thread->return_error == BR_OK) {
if (get_user(cmd, (uint32_t __user *)ptr))
return -EFAULT;
ptr += sizeof(uint32_t);
trace_binder_command(cmd);
if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) {
binder_stats.bc[_IOC_NR(cmd)]++;
proc->stats.bc[_IOC_NR(cmd)]++;
thread->stats.bc[_IOC_NR(cmd)]++;
}
switch (cmd) {
......
case BC_TRANSACTION:
case BC_REPLY: {
struct binder_transaction_data tr;
if (copy_from_user(&tr, ptr, sizeof(tr)))
return -EFAULT;
ptr += sizeof(tr); // 对照前面的图逆向拆解
binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
break;
}
......
}
*consumed = ptr - buffer;
}
return 0;
}
这个cmd的值是BC_TRANSACTION，因此应该继续binder_transaction(proc, thread, &tr, false)。这个函数实在太长了，后面再花一节的篇幅深入该函数。
不过很清晰的一点：该函数仅在出错的时候才返回小于零的整数，如果一切正常就返回0。函数binder_ioctl(...)在case BINDER_WRITE_READ这一枝上，如果没有发生错误，则返回binder_thread_write(...)。也就是说：如果一切正常，binder_ioctl(...)会返回0，不管io的数据有多大。
