CODEWITHSUNDEEP
filepermissionslinux-kernel
Zephyr
Zephyr

Reputation: 377

Can I change file permissions from within a program?

When writing a device driver, I use the function device_create(), which creates a file in /dev linked to the functions registered through fops.

The problem is, once I insmod this module, I can't fprintf to write to the /dev file. A page domain fault occurs. I can still write to a normal file, so I imagine that I don't have permission to write to the file in /dev. Is there anything I can do to set the file as writable within the kernel module while calling device_create() so I wouldn't need to externally set it?

Upvotes: 0

Views: 264

Answers (2)

user4822941
user4822941

Reputation:

If I read this right, you have a userspace program doing fopen + fprintf on a device file backed by your custom driver. On use, the kernel crashes.

First of all the use of FILE abstraction (given with fopen and fprintf) is extremely sketchy when applied to device drivers. Since it does internal buffering, you never know for sure what data actually hits the driver and in what chunks. Use the standard file descriptors directly instead (open + write).

Now, the suspicion that there is a permission problem cannot be right. If the open routine of your driver is reached, the kernel already determined you have necessary privileges. Similarly, if the write routine is reached the file was already opened, so we know you have permissions to use it. But even if there was a permission problem of some kind, a page domain fault is definitely not valid for the kernel to encounter in response.

Given the quality of the question I would argue you are too new to programming to play with this stuff and would recommend sticking to userspace for the time being.

Upvotes: 1

user69874
user69874

Reputation: 155

Take a look at init/initramfs.c where there are sample uses of syscalls by the kernel. Include linux/syscalls.h and just use sys_chmod. It works like the userspace variant. This can be applied to pretty much any system call.(Not that it's a good idea to use socket in the kernel)

Upvotes: 0

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