In Linux there is an additional mechanism for the kernel and kernel modules to send information to processes -- the /proc file system. Originally designed to allow easy access to information about processes (hence the name), it is now used by every bit of the kernel which has something interesting to report, such as /proc/modules which has the list of modules and /proc/meminfo which has memory usage statistics.
The method to use the proc file system is very similar to the one used with device drivers -- you create a structure with all the information needed for the /proc file, including pointers to any handler functions (in our case there is only one, the one called when somebody attempts to read from the /proc file). Then, init_module registers the structure with the kernel and cleanup_module unregisters it.
The reason we use proc_register_dynamic is because we don't want to determine the inode number used for our file in advance, but to allow the kernel to determine it to prevent clashes. Normal file systems are located on a disk, rather than just in memory (which is where /proc is), and in that case the inode number is a pointer to a disk location where the file's index-node (inode for short) is located. The inode contains information about the file, for example the file's permissions, together with a pointer to the disk location or locations where the file's data can be found.
Because we don't get called when the file is opened or closed, there's no where for us to put MOD_INC_USE_COUNT and MOD_DEC_USE_COUNT in this module, and if the file is opened and then the module is removed, there's no way to avoid the consequences. In the next chapter we'll see a harder to implement, but more flexible, way of dealing with /proc files which will allow us to protect against this problem as well.
/* procfs.c - create a "file" in /proc
* Copyright (C) 1998 by Ori Pomerantz
*/
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
/* Necessary because we use the proc fs */
#include <linux/proc_fs.h>
/* Put data into the proc fs file.
Arguments
=========
1. The buffer where the data is to be inserted, if you decide to use
it.
2. A pointer to a pointer to characters. This is useful if you don't
want to use the buffer allocated by the kernel.
3. The current position in the file.
4. The size of the buffer in the first argument.
5. Zero (for future use?).
Usage and Return Value
======================
If you use your own buffer, like I do, put its location in the
second argument and return the number of bytes used in the buffer.
A return value of zero means you have no further information at this
time (end of file). A negative return value is an error condition.
For More Information
====================
The way I discovered what to do with this function wasn't by reading
documentation, but by reading the code which used it. I just looked to
see what uses the get_info field of proc_dir_entry struct (I used a
combination of find and grep, if you're interested), and I saw that
it is used in <kernel source directory>/fs/proc/array.c.
If something is unknown about the kernel, this is usually the way
to go. In Linux we have the great advantage of having the kernel source
code for free - use it.
*/
int procfile_read(char *buffer, char **buffer_location, off_t offset,
int buffer_length, int zero)
{
int len; /* The number of bytes actually used */
/* This is static so it will still be in memory when we leave this
* function */
static char my_buffer[80];
static int count = 1;
/* We give all of our information in one go, so if the user asks us
* if we have more information the answer should always be no.
*
* This is important because the standard read function from the library
* would continue to issue the read system call until the kernel replies
* that it has no more information, or until its buffer is filled.
*/
if (offset > 0)
return 0;
/* Fill the buffer and get its length */
len = sprintf(my_buffer, "For the %d%s time, go away!\n", count,
(count % 100 > 10 && count % 100 < 14) ? "th" :
(count % 10 == 1) ? "st" :
(count % 10 == 2) ? "nd" :
(count % 10 == 3) ? "rd" : "th" );
count++;
/* Tell the function which called us where the buffer is */
*buffer_location = my_buffer;
/* Return the length */
return len;
}
struct proc_dir_entry Our_Proc_File =
{
0, /* Inode number - ignore, it will be filled by
* proc_register_dynamic */
4, /* Length of the file name */
"test", /* The file name */
S_IFREG | S_IRUGO, /* File mode - this is a regular file which can
* be read by its owner, its group, and everybody
* else */
1, /* Number of links (directories where the file is referenced) */
0, 0, /* The uid and gid for the file - we give it to root */
80, /* The size of the file reported by ls. */
NULL, /* functions which can be done on the inode (linking, removing,
* etc.) - we don't support any. */
procfile_read, /* The read function for this file, the function called
* when somebody tries to read something from it. */
NULL /* We could have here a function to fill the file's inode, to
* enable us to play with permissions, ownership, etc. */
};
/* Initialize the module - register the proc file */
int init_module()
{
/* Success if proc_register_dynamic is a success, failure otherwise */
return proc_register_dynamic(&proc_root, &Our_Proc_File);
/* proc_root is the root directory for the proc fs (/proc). This is
* where we want our file to be located.
*/
}
/* Cleanup - unregister our file from /proc */
void cleanup_module()
{
proc_unregister(&proc_root, Our_Proc_File.low_ino);
}