C：在内核模块中读取文件指针的filp-> private_data

Question

I'am wondering if it is possible to read the private_data of a filepointer? Or is it really "private"?!

I am in a kernel module!

struct file *intercepted_fp;

intercepted_fp = filp_open("/dev/snd/pcmC0D0p_bak", O_RDWR, (S_IRWXU | S_IRWXG | S_IRWXO));

if (intercepted_fp == NULL) {
    LOGI("Cannot open intercepted device!");
    return -ENODEV;
}

mm_segment_t old_fs = get_fs();
set_fs(get_ds());

LOGI("private data: %p", intercepted_fp->private_data); // -> prints "private data: ffffffff"
LOGI("buffer: %d", ((struct snd_pcm_file*) intercepted_fp->private_data)->substream->runtime->buffer_size); // ->  Unable to handle kernel paging request at virtual address ffffffff
set_fs(old_fs);

Answer 1

Of course you can. It is just a field in struct file .

It is intended to pass around data known only to a particular device driver from a file system interface.

Answer 2

The private_data pointer is a useful resource for preserving state information by the module across system calls. The open system call sets this pointer to NULL before calling the open method for the driver. The kernel module code needs to allocate the memory and free it. So the memory is allocated in kernel space - not so accessible from user space, but easily accessible from kernel space. You can access it, but changing it can disturb functioning of the driver. You can find more about private_data pointer here (archive) in a great book Linux Device Drivers, 3rd Edition (archive) .