How can I programmatically set permissions on my char device

Question

I've recently inherited some code at work, this is old 2.4.X linux kernel drivers and I've been tasked with getting them working on a newer kernel 2.6 or greater. I'm running on OpenSUSE 12.1 with a 3.1.10 kernel.

I've updated the original code from register_chrdev() to use class_create()/device_create() calls and I can see my devices show up in /dev correctly. My current issue is that the permissions for my device are being set to r/w for user only:

crw-------  1 root root    244,   0 Aug  7 07:57 gcanain

I know I can "chmod" the file via command line, and or I can set up udev permissions... but is there anyway to do this programmatically, such that when I issue the insmod command, the dev will be mounted with the correct rules in place already?

Are there any APIs that might exist that I can call to do this, any options I'm missing in one of these creation APIs?

Just to clarify, part of the reason I don't want to use udev rules is that I don't know the names of the device drivers ahead of time. The device drivers are spawned in a loop and so the names are appended with a digit, nNumDevs can be pretty much anything:

for (i = 0; i < nNumDevs; i++) {
  strcpy(Modname,GC_ANAIN_MODULE_NAME);
  strcat(Modname,"%d");
  device_create(c1, NULL, MKDEV(nMajor, GC_ANAIN_MINOR_VERSION+i), NULL, Modname, i);
}

Answer 1

This is the method used by the TTY driver to set permission to 0666 on creation:

static char *tty_devnode(struct device *dev, umode_t *mode)
{
        if (!mode)
                return NULL;
        if (dev->devt == MKDEV(TTYAUX_MAJOR, 0) ||
            dev->devt == MKDEV(TTYAUX_MAJOR, 2))
                *mode = 0666;
        return NULL;
}

static int __init tty_class_init(void)
{
        tty_class = class_create(THIS_MODULE, "tty");
        if (IS_ERR(tty_class))
                return PTR_ERR(tty_class);
        tty_class->devnode = tty_devnode;
        return 0;
}

The devnode attribute in struct class has a parameter pointer mode that will allow you to set permissions.

Beware, mode may be NULL when the device gets destroyed.

Answer 2

Try this: #include <sys/stat.h>

int chmod(const char *path, mode_t mode); OR int fchmod(int fd, mode_t mode);

Source: man -s 2 chmod

Answer 3

udev has rules for permissions, you need to create them under /etc/udev/rules.d

First try this:

In the file /etc/udev/udev.conf, add this line:

# My default permissions
default_mode="0660"

If this doesn't work add a rule in /etc/udev/rules.d, more on that here : http://www.reactivated.net/writing_udev_rules.html