logcat 和 liblog 这两篇文章,讲到了android系统中如何读log和写log. 那么,log存放的位置在哪里? 本文就介绍一下android 系统中存放log的地方: logger device.
Android 在 kernel 层提供了四个虚拟的device 设备,用于存放log. 可以通过输入 adb shell ls /dev/log/ 来查看系统的虚拟logger 设备. 这些设备是在系统启动的时候以内核模块的方式初始化.
device_initcall(logger_init);static int __init logger_init(void){
int ret;
ret = create_log(LOGGER_LOG_MAIN, 256*1024);
if (unlikely(ret))
goto out;
ret = create_log(LOGGER_LOG_EVENTS, 256*1024);
if (unlikely(ret))
goto out;
ret = create_log(LOGGER_LOG_RADIO, 256*1024);
if (unlikely(ret))
goto out;
ret = create_log(LOGGER_LOG_SYSTEM, 256*1024);
if (unlikely(ret))
goto out;out:
return ret;}模块初始话函数通过create_log()生成四个device,并指定了每个device的大小.
static int __init create_log(char *log_name, int size){
int ret = 0;
struct logger_log *log;
unsigned char *buffer;
buffer = vmalloc(size);
if (buffer == NULL)
return -ENOMEM;
log = kzalloc(sizeof(struct logger_log), GFP_KERNEL);
if (log == NULL) {
ret = -ENOMEM;
goto out_free_buffer;
}
log->buffer = buffer;
log->misc.minor = MISC_DYNAMIC_MINOR;
log->misc.name = kstrdup(log_name, GFP_KERNEL);
if (log->misc.name == NULL) {
ret = -ENOMEM;
goto out_free_log;
}
log->misc.fops = &logger_fops;
log->misc.parent = NULL;
init_waitqueue_head(&log->wq);
INIT_LIST_HEAD(&log->readers);
mutex_init(&log->mutex);
log->w_off = 0;
log->head = 0;
log->size = size;
INIT_LIST_HEAD(&log->logs);
list_add_tail(&log->logs, &log_list);
/* finally, initialize the misc device for this log */
ret = misc_register(&log->misc);
if (unlikely(ret)) {
pr_err("failed to register misc device for log '%s'!\n",
log->misc.name);
goto out_free_log;
}
pr_info("created %luK log '%s'\n",
(unsigned long) log->size >> 10, log->misc.name);
return 0;out_free_log:
kfree(log);out_free_buffer:
vfree(buffer);
return ret;}对于每一个logger device,都对应一个核心的结构体: struct logger_log. create_log()函数的作用就是分配一个logger_log,初始化其变量,并通过misc_register()注册为misc设备.
对于之前介绍的 logcat 和 liblog, 讲到都是通过read()/write()函数来读写log, read/write的实现则对应到driver层注册到file system的 fops.
log->misc.fops = &logger_fops;static const struct file_operations logger_fops = {
.owner = THIS_MODULE,
.read = logger_read,
.aio_write = logger_aio_write,
.poll = logger_poll,
.unlocked_ioctl = logger_ioctl,
.compat_ioctl = logger_ioctl,
.open = logger_open,
.release = logger_release,};打开Logger设备
在应用层通过调用open(“/dev/log/main”,O_RDWR)的方式可以打开一个logger设备,对应的kernel 层的实现是logger_open.
/* logger_open() */log = get_log_from_minor(MINOR(inode->i_rdev));if (!log)
return -ENODEV;if (file->f_mode & FMODE_READ) {
struct logger_reader *reader;
reader = kmalloc(sizeof(struct logger_reader), GFP_KERNEL);
if (!reader)
return -ENOMEM;
reader->log = log;
reader->r_ver = 1;
reader->r_all = in_egroup_p(inode->i_gid) ||
capable(CAP_SYSLOG);
INIT_LIST_HEAD(&reader->list);
mutex_lock(&log->mutex);
reader->r_off = log->head;
list_add_tail(&reader->list, &log->readers);
mutex_unlock(&log->mutex);
file->private_data = reader;} else
file->private_data = log;通过传入的inode节点的次设备号从log_list链表中找到对应的logger device的结构体. 接着会判断打开方式,如果打开方式中包含”read”(例如logcat)的话,会分配一个logger_read结构体被赋值给file的private_data变量,同时会把reader的读开始位置设为logger buffer的head位置(也就是从头开始读),然后把reader加入到logger的reader链表中.否则file的private_data变量直接指向logger.
读logger
read()函数对应logger_read.
.read = logger_read,static ssize_t logger_read(struct file *file, char __user *buf,
size_t count, loff_t *pos){
struct logger_reader *reader = file->private_data;
struct logger_log *log = reader->log;
ssize_t ret;
DEFINE_WAIT(wait);start:
while (1) {
mutex_lock(&log->mutex);
prepare_to_wait(&log->wq, &wait, TASK_INTERRUPTIBLE);
ret = (log->w_off == reader->r_off);
mutex_unlock(&log->mutex);
if (!ret)
break;
if (file->f_flags & O_NONBLOCK) {
ret = -EAGAIN;
break;
}
if (signal_pending(current)) {
ret = -EINTR;
break;
}
schedule();
}
finish_wait(&log->wq, &wait);
if (ret)
return ret;首先程序会在一个while循环中做一些判断:如果w_off不等于r_off,表明目前logger中有log可读,跳出循环.否则,如果设备以非阻塞的方式打开,直接返回 -EAGAIN 的错误. 如果程序被信号打断,则返回 -EINTR. 如果这些条件都不满足,表示目前没有log可读,调用schedule()让出cpu.
/*logger_read()*/
mutex_lock(&log->mutex);
if (!reader->r_all)
reader->r_off = get_next_entry_by_uid(log,
reader->r_off, current_euid());
/* is there still something to read or did we race? */
if (unlikely(log->w_off == reader->r_off)) {
mutex_unlock(&log->mutex);
goto start;
}r_all部分目前还不太理解,以后再补充…..(从代码来看,这个变量应该是与reader的权限有关,通过这个变量可以控制该reader是否有权限去读所有的log, 如果为0,表明reader没有该权限,只能读自己进程euid相等的log)
/*logger_read()*/
ret = get_user_hdr_len(reader->r_ver) +
get_entry_msg_len(log, reader->r_off);
if (count < ret) {
ret = -EINVAL;
goto out;
}通过get_user_hdr_len()及get_entry_msg_len()获取entry的header长度和entry长度,加起来就是一条log的长度.
static size_t get_user_hdr_len(int ver){
if (ver < 2)
return sizeof(struct user_logger_entry_compat);
else
return sizeof(struct logger_entry);}该函数会根据传入的reader成员r_ver的值来决定返回哪个长度的entry header值,因为在logger_open中该值被设定为1, 故该函数的返回值为 user_logger_entry_compat 的长度. 接着读取log entry的长度.
static __u32 get_entry_msg_len(struct logger_log *log, size_t off){
struct logger_entry scratch;
struct logger_entry *entry;
entry = get_entry_header(log, off, &scratch);
return entry->len;}static struct logger_entry *get_entry_header(struct logger_log *log,
size_t off, struct logger_entry *scratch){
size_t len = min(sizeof(struct logger_entry), log->size - off);
if (len != sizeof(struct logger_entry)) {
memcpy(((void *) scratch), log->buffer + off, len);
memcpy(((void *) scratch) + len, log->buffer,
sizeof(struct logger_entry) - len);
return scratch;
}
return (struct logger_entry *) (log->buffer + off);}因为每个logger device的size都是固定大小,而系统中的log量要远远大于该size,故logger device都是采用 ring buffer的方式存放log. 这样就可能出现这个的情况,一条log的一部分在buffer尾部,而另一部分在buffer头部,所以每次从buffer读log都要考虑这种情况. 获得entry之后,通过entry的变量len就可以知道msg的长度. 调用 do_read_log_to_user()将entry+msg写到user的buf中.
ret = do_read_log_to_user(log, reader, buf, ret);
Log write
之前有讲,user space在写log的流程最后调用到了write()函数,对应到driver层的实现为 logger_aio_write(). 让我们一段一段的分析这个函数的实现.
static ssize_t logger_aio_write(struct kiocb *iocb, const struct iovec *iov,
unsigned long nr_segs, loff_t ppos){
struct logger_log *log = file_get_log(iocb->ki_filp);
size_t orig = log->w_off;
struct logger_entry header;
struct timespec now;
ssize_t ret = 0;首先是调用file_get_log()函数获得这个文件结构体对应的logger设备. 在打开设备的代码中有讲,file结构体的private_data变量会存放两个值之一:logger或reader,所以这里会判断文件是否以FMODE_READ的方式打开,如果是,则private_data为reader,需要去reader中找logger,否则直接返回private_data.
static inline struct logger_log *file_get_log(struct file *file){
if (file->f_mode & FMODE_READ) {
struct logger_reader *reader = file->private_data;
return reader->log;
} else
return file->private_data;}下面的代码通过系统参数初始化log entry的header. now = current_kernel_time();
header.pid = current->tgid; header.tid = current->pid; header.sec = now.tv_sec; header.nsec = now.tv_nsec; header.euid = current_euid(); header.len = min_t(size_t, iocb->ki_left, LOGGER_ENTRY_MAX_PAYLOAD); header.hdr_size = sizeof(struct logger_entry); /* null writes succeed, return zero */ if (unlikely(!header.len)) return 0; mutex_lock(&log->mutex);
接下来调用fix_up_readers()函数,通过传入本次log的长度对该logger设备的readers进行修正. /* * Fix up any readers, pulling them forward to the first readable * entry after (what will be) the new write offset. We do this now * because if we partially fail, we can end up with clobbered log * entries that encroach on readable buffer. */ fix_up_readers(log, sizeof(struct logger_entry) + header.len);
static void fix_up_readers(struct logger_log *log, size_t len){
size_t old = log->w_off;
size_t new = logger_offset(log, old + len);
struct logger_reader *reader;
if (is_between(old, new, log->head))
log->head = get_next_entry(log, log->head, len);
list_for_each_entry(reader, &log->readers, list)
if (is_between(old, new, reader->r_off))
reader->r_off = get_next_entry(log, reader->r_off, len);}static size_t get_next_entry(struct logger_log *log, size_t off, size_t len){
size_t count = 0;
do {
size_t nr = sizeof(struct logger_entry) +
get_entry_msg_len(log, off);
off = logger_offset(log, off + nr);
count += nr;
} while (count < len);
return off;} 为什么要对reader进行修正?前面有讲过,logger buffer的size是固定的,系统采用ring buffer的方式写log,那么就会出现这样的情况,最新的logger会有机会覆盖前面的一条log,那么在这种情况下,对于reader来说,r_off这个参数就是无效的,因为下一条log(或者后面几条log)已经不存在了.get_next_entry()的实现不难理解,因为新加入的log长度为len,即寻找从r_off+len位置之后的第一条有效log.
接下来就是真正把log的内容写入buffer
do_write_log(log, &header, sizeof(struct logger_entry));
while (nr_segs-- > 0) {
size_t len;
ssize_t nr;
/* figure out how much of this vector we can keep */
len = min_t(size_t, iov->iov_len, header.len - ret);
/* write out this segment's payload */
nr = do_write_log_from_user(log, iov->iov_base, len);
if (unlikely(nr < 0)) {
log->w_off = orig;
mutex_unlock(&log->mutex);
return nr;
}
iov++;
ret += nr;
}
mutex_unlock(&log->mutex);
/* wake up any blocked readers */
wake_up_interruptible(&log->wq);
return ret;}首先会调用do_write_log()把header先写入buffer,这里直接调用memcpy(),header有可能被写到buffer的尾部和首部(ring buffer). 然后就是把user space传入的iovec数组的内容依次写入buffer. 如果写失败,会直接把logger的w_off位置roll back会之前的值.
logger_poll
在logcat的实现中曾讲到,logcat在打开logger设备后,会调用select()函数监控该logger设备,如果函数返回,表明有log可读,接下来就会调用read()读log.这里select对应的driver层函数就是logger_poll()
static unsigned int logger_poll(struct file *file, poll_table *wait){
struct logger_reader *reader;
struct logger_log *log;
unsigned int ret = POLLOUT | POLLWRNORM;
if (!(file->f_mode & FMODE_READ))
return ret;
reader = file->private_data;
log = reader->log;
poll_wait(file, &log->wq, wait);
mutex_lock(&log->mutex);
if (!reader->r_all)
reader->r_off = get_next_entry_by_uid(log,
reader->r_off, current_euid());
if (log->w_off != reader->r_off)
ret |= POLLIN | POLLRDNORM;
mutex_unlock(&log->mutex);
return ret;}函数首先会判断是否以read的方式打开设备,如果不是,直接返回.(因为select()一般对应读操作,如果不读那么select()就没什么意义了).判断log是否可读的唯一条件就是w_off是否等于r_off.
OK,logger设备暂时就写到这里,以后有新的理解会继续补充.