Driving Me Nuts - Things You Never Should Do in the Kernel

How do you read and write files from a kernel module? Wait, make that “how would you read and write files from a kernel module if that weren't a bad thing to do?”
I Never Told You about This

In conclusion, reading and writing a file from within the kernel is a bad, bad thing to do. Never do it. Ever. Both modules from this article, along with a Makefile for compiling them, are available from the Linux Journal FTP site, but we expect to see no downloads in the logs. And, I never told you how to do it either. You picked it up from someone else, who learned it from his sister's best friend, who heard about how to do it from her coworker.

Resources for this article: /article/8130.

Greg Kroah-Hartman is one of the authors of Linux Device Drivers, 3rd edition and is the kernel maintainer for more driver subsystems than he likes to admit. He works for SuSE Labs, doing various kernel-specific things and can be reached at greg@kroah.com for issues unrelated to this article.



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I think there is a typo on the write code...

Anonymous's picture

I think there's a typo on the final write code. The first call after checking for the file descriptor should be vfs_write and no the sys_write call.

fd = sys_open(filename, O_WRONLY|O_CREAT, 0644);
if (fd >= 0) {
vfs_write(fd, data, strlen(data));

about usb drivers

Anonymous's picture

I am writing usb device driver and i want some help.
OS that i am using is suse10.2. If i write down complete usb driver that is mentioned in your documents, what are the extra things that i need to do apart from inserting that module in to kernel (insmod).If i insert my usb driver module in to kernel how can we stop kernel from using previous usb driver and now kernel should use the driver(module) that i have inserted.
Similarly i have also written mouse device driver for ps2 mouse.But i faced the simiar problem.How we should tell to kernel that dont use previous driver for mouse use that i have inserted.
Please Help Me.If i could get way for this i will be able to run both the drivers.

Yeah, the kernel has

Anonymous's picture

Yeah, the kernel has internal API to open files: filp_open(), filp_close(), vfs_read(), ...
Consult for example with sound/sound_firmware.c.

In proprietary spec I have

Anonymous's picture

In proprietary spec I have

int ioctl(int fd, int req=DMX_SET_SOURCE, int *frontend_fd);
where frontend_fd is the pointer to file descriptor of another driver opened previousely.

The logic here - the user is responsible for which driver to connect for internal communication

Anyway I need sys_ioctl to call one driver from another. I can not parse user file descriptor to determine which driver I need.

Don't use sys_*()

Anonymous's picture

If you do read/write files from the kernel, then please do it without too much hacks, meaning you use filp_open, vfs_read and vfs_write instead of sys_open, sys_read and sys_write, respectively. Thanks!

Hey you said that you can

Saurabh Chokshi's picture

Hey you said that you can use filp_open , vfs_read and vfs_write. I tried that but I could not write and read into the file.

static void write_file(char *filename, char *data)
struct file *file;
file = filp_open(filename, O_WRONLY|O_CREAT, 0777);
ssize_t wc;
wc = vfs_write(file, data, strlen(data),0);
if(wc < strlen(data))
printk(KERN_INFO "Problem in Writing Data\n");


This is my code.

Please let me know where I am wrong.



Anonymous's picture

Wow... Can someone translate this into English for me?

How very clever

C. J. Stiller's picture

Greg: thanks for this summary. And how clever of you to obscure your advice concerning file writes from kernel space by including the sys_write() call in your sample, misdirecting readers into ignoring the vfs_write() and thinking that they would still need that syscall.


Anonymous's picture

Your final example still uses sys_write and the snippet above that has a syntax error in it.


Vanessa Kay's picture

God, I hate re-writing kernels. I used to use suse linux. Everytime I had to get my wireless card to work, I would have to re-write the kernel and that shit takes forever.

Rides and Whips

Clearly, you know a lot

Anonymous's picture

Clearly, you know a lot about linux. You don't re-write a kernel. You recompile it with drivers or code for your device. If you were rewriting a kernel that shit would take forever. Recompiling a kernel takes me only a few minutes on my machine, granted, it's faster than most. Keep anti-linux comments on topic and try to back them up with fact. The simple fact is, you don't need to make any thing up to criticize any operating system, there is plenty wrong with any given one to be criticized.

You had to re-write the

Anonymous's picture

You had to re-write the kernel everything you wanted your wifi card to work?



Anonymous's picture


Not using sys_write

Anonymous's picture

"how this can be done by not using the sys_write call"

But you've used it right there!

Stacked Drivers Concept

Ilya. I's picture

I know one case when this is the cleanest practical solution.

Linux kernel is supposed to be build on "stacked drivers" concept. In reality, kernel supports it in very limited number of specially designed core drivers. Try to write a translation driver for a device which already has a generic driver. Good example is having a USB-to-serial chip talking to a custom hardware. Now, anyone trying to create a driver for that custom hardware is facing a huge uphill battle with the kernel, unless you can just open device file created by generic driver and talk to it in vfs. However, vfs_read/vfs_write are user-space restricted. (Unacceptable alternatives: user-space daemon, full-blown rewrite of USB-to-serial driver, etc.)

Using technique in this article proves to be the only way for given situation.

You've failed to explain why

Anonymous's picture

You've failed to explain why you can't solve the problem in userspace in your example.

What's wrong with a libmycustomhardware.so?

Problem with sys_write and sys_open

Sarang's picture

information given in this article is very useful.
i have tried above codes on kernel 2.6.9, but getting error with sys_open and sys_write.
it gives error in system log file:
fileops: Unknown symbol sys_open
fileops: Unknown symbol sys_write
Please suggest me how can i overcome this problem.
I welcome your suggestions.
Thank you

dont use sys_read or

Anonymous's picture

dont use sys_read or sys_write or sys_open or sys_open calls.. instead u can use vfs_read, vfs_write, filp_open, filp_close calls..

don't do that!

Anonymous's picture

don't do that!


I think using sys_open and

Anonymous's picture

I think using sys_open and sys_read is more general than vfs_open vfs_read. In my case, I need to create to module to read a file from SD card (fat), vfs_read always give me many errors.

In thread

iPAS's picture

Could this function be called by Thread ?

Used this to interface a stack with the DVBS drivers of Linux

GeorgeJ's picture

Used this to interface a stack with the DVBS drivers of Linux.
Good Job!!
Hope to see you in person some day!!!

i guess filpopen should work

Anonymous's picture

i guess filpopen should work ...

Did u look into copyfromuser() and copytouser()

These are useful for getting data back and forth between kernel and user space.

netlink sockets are the very

Anonymous's picture

netlink sockets are the very efficient way to communicate between user, kernel modules and vice-versa.. Its very easy to use also..