How do I make my kernel module's sysfs entry be owned by a non-root user?

Question

I have the following code:

static struct kobj_attribute my_attribute =__ATTR(my_node, 0444, my_show_fn, my_store_fn);
...
        kobject_init(&my_module->kobj, &ktype_my_module);
        ret = kobject_add(&my_module->kobj, kernel_kobj, "%s", "my_module");
        ...
        ret = sysfs_create_file(&my_module->kobj, &my_attribute.attr);

This creates a /sys/kernel/my_module/my_node node, owned by root with 0444 permissions.

How can I make a non-root user own this node, from within the kernel code?

Have you tried to change ownership of the node after sysfs_create_file ? — Philippe, Dec 09 '21 at 18:11
@Philippe Yes this works, but for my use case only it's better to set it in the kernel. — fadedbee, Dec 10 '21 at 10:47

score 3 · Accepted Answer · answered Dec 09 '21 at 20:01

Owner's and group's identifiers of a sysfs attribute are determined by the the kobject for which this attribute is created.

Specifically, it is get_ownership field of the struct kobj_type which may contain a callback for set owner's and group's identifiers. The field has following signature:

struct kobj_type {
    ...
    void (*get_ownership)(struct kobject *kobj, kuid_t *uid, kgid_t *gid);
};

When the callback is called, both *uid and *gid contain 0 (which corresponds to the root user). The callback may set these output parameters to other values.

How do I make my kernel module's sysfs entry be owned by a non-root user?

1 Answers1