In the last post
we saw how to create linux kernel module with a parameter. During that we had
passed an integer 0660 for the access rights argument to macro module_param.
Let’s see what that is and how does it work?
Whenever we load any kernel module, a directory structure under /sys/module/<module-name>
is created.
This module tree consist of following structure.
(source: kernel.org doc)
/sys/module/MODULENAME
The name of the module that is in the kernel. This
module name will always show up if the module is loaded as a
dynamic module. If it is built directly into the kernel, it
will only show up if it has a version or at least one
parameter.
Note: The conditions of creation in the built-in case are not
by design and may be removed in the future.
/sys/module/MODULENAME/parameters
This directory contains individual files that are each
individual parameters of the module that are able to be
changed at runtime. See the individual module
documentation as to the contents of these parameters and
what they accomplish.
Note: The individual parameter names and values are not
considered stable, only the fact that they will be
placed in this location within sysfs. See the
individual driver documentation for details as to the
stability of the different parameters.
/sys/module/MODULENAME/refcnt
If the module is able to be unloaded from the kernel, this file
will contain the current reference count of the module.
Note: If the module is built into the kernel, or if the
CONFIG_MODULE_UNLOAD kernel configuration value is not enabled,
this file will not be present.So when we do,
$ sudo insmod mod-args.ko dest_port=8080a directory structure under /sys/module/mod_args is created that looks like.
[05:09 PM][girish@fedo-vm]:module $ ls -1F /sys/module/mod_args/ coresize holders/ initsize initstate notes/ parameters/ refcnt sections/ taint uevent
Under /sys/module/mod_args/parameters you will find a file called dest_port
[05:09 PM][girish@fedo-vm]:module $ ll /sys/module/mod_args/parameters/ total 0 -rw-rw----. 1 root root 4096 Aug 22 17:21 dest_port
It has the permission to read and write for the owner and group.
If you check the contents in dest_port.
[05:09 PM][girish@fedo-vm]:module $ sudo cat /sys/module/mod_args/parameters/dest_port 8080
It is the value that we had set while loading the module. We can change this value even after the module is loaded by changing data in this file.
[05:10 PM][root@fedo-vm]:module $ echo 8081 > /sys/module/mod_args/parameters/dest_port [05:10 PM][root@fedo-vm]:module $ cat /sys/module/mod_args/parameters/dest_port 8081 [05:10 PM][root@fedo-vm]:module $ exit [05:10 PM][girish@fedo-vm]:module $ sudo rmmod mod_args [05:10 PM][girish@fedo-vm]:module $ dmesg ..... ..... ..... ..... [36340.288695] hello.... dest_port=8080 [36394.368996] bye dest_port=8081
Things to be noted:
if the value of access rights/permission argument of
module_paramis0then the parameter won’t show in the/sysdirector at all.While defining module parameters using
module_parammacro; you can use<linux/stat.h>or<uapi/linux/stat.h>for fairly intutive macros instead of access right value.If the parameter is changed on the fly, the kernel has no mechanism to know that it has been changed. So if it needs to be reflected immidiately and based on that some processesing needs to be done, this is not the right way to do it. In other words there are no call-backs when the pearmeter is updated on the go.
References: