Accessing allocated space in a debugfs file

[Greg KH]

On Wed, Sep 30, 2020 at 10:32:05PM +0530, ymdatta wrote:
> Hello everyone,
> 
> I am trying to create a debugfs file for exposing some information to the
> userspace. (I am exploring on how to use debugfs)
> 
> >From the documentation (Documentation/filesystems/debugfs.txt), i came
> across a function called
> 
>        struct dentry *debugfs_create_file_size(...,loff_t file_size);
> 
> This essentially creates a file with an initial size.
> 
> I want to write in this file, how should i be accessing the space created
> from previous function call.

That's not what "size" means here.  "size" just sets the value that you
see if you look at the directory for that debugfs file (or stat() it).

If you don't set a specific "size", it will just show up as 4k
(PAGE_SIZE).

> I have looked through the source code, but i have found very few places
> where this function is used. In one of the use cases [0], the memory
> equal to the size is allocated in the open file operation, and the
> private_data member is pointed to this. (Looking at [0] helps in
> understanding this). But from #kernelnewbies channel, i found that
> private_data is a pointer for tty/char drivers.

debugfs is a virtual filesystem, there is no "backing store" or place to
put your data in it.  It is there so that you can write code that can
handle open/read/write/close to happen on a file, and your code will
provide the data to userspace directly.

The simplest way to create a debugfs file is to just point it at a
variable, and then you can change the variable value in the kernel, and
userspace reading from the file will see whatever the value is at that
point in time.

Does that help?

thanks,

greg k-h