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);
}

I think that you should just go ahead and use `udev`. Device permissions are typically the pervue of the local administrator, who may have different ideas about access than the device driver author. You could drop default settings in `/etc/udev/rules.d` or something. — larsks, Aug 07 '12 at 13:07
Understood, but we (my group) are the admin of the system, so if I wanted everything to have 777, I could do so. Also, I have an issue with using rules.d (updated in the original description), since the names of the device drivers are dynamically created... unless I misunderstand how to use udev. — Mike, Aug 07 '12 at 13:32
@Mike: But udev rules can use wildcars to match the name of the devices (`KERNEL=="foo*"`) and they can also match the name of the driver (`DRIVERS=="bar"`). — rodrigo, Aug 07 '12 at 14:26
@Mike have you solved this problem? i need to do the same for android kernel? — Jeegar Patel, Jul 22 '13 at 12:25

score 14 · Answer 1 · edited May 16 '16 at 13:35

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.

What do we need to be careful? If the device gets destroyed, we don't care about it anymore, right? — Alexis, Mar 13 '20 at 05:57

score 4 · Answer 2 · answered Aug 07 '12 at 14:15

4

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

answered Aug 07 '12 at 14:15

askmish

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score 1 · Answer 3 · answered Aug 07 '12 at 14:17

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

How can I programmatically set permissions on my char device

3 Answers3

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