How to get address of a struct object in python?

3
votes

I am doing kernel module programming using ioctl calls to communicate between userspace and kernel space. I plan on making the userspace api in python.

To pass data between kernel and userspace ioctl calls utilizes address, and data is copied using copy_to_user, or copy_from_user. The address here is given by unsigned long arg.

int ioctl(struct inode *inode,struct file *filp,unsigned int cmd,unsigned long arg)

In C userspace programming it is trivial to pass address. What I want to do, is to use struct of python to create a structure which is compatible with the structure which i have defined in the kernel module, and pass the populated python struct, using ioctl from fcntl module of python.

Is it possible? If it is possible, how do i pass the address of python struct in the ioctl call?

I do not want to use ctypes, or extend python with c. Pure python code, is what i would like.

2
pythonkernel
(+1) for using [python] and [kernel-programming] in the same question :) - NPE
Not sure why you "do not want to use ctypes", as this is exactly the task for which it was created! - Francis Avila
Have you read the basics of fcntl ? Because it already does what you are asking for: ".. When the argument is a string it represents a binary structure, e.g. created by struct.pack(). The binary data is copied to a buffer whose address is passed to the C fcntl() call. ..". Anything else you wanna do ? - mmgp
@Francis Avila: The source code of the module which i am working on, is very big, and I'm not the author. So its pretty hard for me to implement ctypes, redefining all the structures, etc. So basically, I just want to make structures on the fly, and pass it in the ioctl calls. - Raunak Mukhia

2 Answers

0
votes

Maybe the ctypes-module will help you here, especially this section

0
votes

The fcntl docs show you examples how to do this. They use struct and array, but you can also use plain strings. If you pass a mutable buffer and mutate_flag is True, the actual backing buffer will be passed to the OS call. If it is unmutable, a copy will be passed and the resulting change in buffer will be returned by the call. See the documentation for fcntl.ioctl().

So you don't have to care about the address of the buffer backing the struct. Just pass it as a struct or string.