Understanding Linux Network Internals

Module Options

Kernel modules define their parameters by means of macros such as module_param; see include/linux/moduleparam.h for a list. module_param requires three input parameters, as shown in the following example from drivers/net/sis900.c:

...
module_param(multicast_filter_limit, int 0444);
module_param(max_interrupt_work, int, 0444);
module_param(debug, int, 0444);
...

The first input parameter is the name of the parameter to be offered to the user. The second is the type of the parameter (e.g., integer), and the third represents the permissions assigned to the file in /sys to which the parameter will be exported.

This is what you would get when listing the module's directory in /sys:

[root@localhost src]# ls -la /sys/module/sis900/parameters/
total 0
drwxr-xr-x  2 root root    0 Apr  9 18:31 .
drwxr-xr-x  4 root root    0 Apr  9 18:31 ..
-r--r--r--  1 root root    0 Apr  9 18:31 debug
-r--r--r--  1 root root 4096 Apr  9 18:31 max_interrupt_work
-r--r--r--  1 root root 4096 Apr  9 18:31 multicast_filter_limit
[root@localhost src]#

Each module is assigned a directory in /sys/modules. The subdirectory /sys/modules/ module / parameters holds a file for each parameter exported by module. The previous snapshot from drivers/net/sis900.c shows three options that are readable by anyone, but not writable (they cannot be changed).

Permissions on /sys files (and on /proc files, incidentally) are defined using the same syntax as common files, so you can specify read, write, and execute permissions for the owner, ...