Access the Linux kernel using the /proc filesystem

This virtual filesystem opens a window of communication between the kernel and user space

The /proc filesystem is a virtual filesystem that permits a novel approach for communication between the Linux® kernel and user space. In the /proc filesystem, virtual files can be read from or written to as a means of communicating with entities in the kernel, but unlike regular files, the content of these virtual files is dynamically created. This article introduces you to the /proc virtual filesystem and demonstrates its use.

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M. Tim Jones (mtj@mtjones.com), Consultant Engineer, Emulex

M. Tim JonesM. Tim Jones is an embedded software engineer and the author of GNU/Linux Application Programming, AI Application Programming, and BSD Sockets Programming from a Multilanguage Perspective. His engineering background ranges from the development of kernels for geosynchronous spacecraft to embedded systems architecture and networking protocols development. Tim is a senior principal engineer at Emulex Corp.



14 March 2006

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The /proc filesystem was originally developed to provide information on the processes in a system. But given the filesystem's usefulness, many elements of the kernel use it both to report information and to enable dynamic runtime configuration.

The /proc filesystem contains directories (as a way of organizing information) and virtual files. A virtual file can present information from the kernel to the user and also serve as a means of sending information from the user to the kernel. It's not actually required to do both, but this article show you how to configure the filesystem for input and output.

A short article like this can't detail all the uses of /proc, but it does demonstrate a couple of uses to give you an idea of how powerful /proc can be. Listing 1 is an interactive tour of some of the /proc elements. It shows the root level of the /proc filesystem. Note the series of numbered files on the left. Each of these is a directory representing a process in the system. Because the first process created in GNU/Linux is the init process, it has a process-id of 1. Next, performing an ls on the directory shows a list of files. Each file provides details on the particular process. For example, to see the command-line entry for init, simply cat the cmdline file.

Some of the other interesting files in /proc are cpuinfo, which identifies the type of processor and its speed; pci, which shows the devices found on the PCI buses; and modules, which identifies the modules that are currently loaded into the kernel.

Listing 1. Interactive tour of /proc
[root@plato]# ls /proc

1     2040  2347  2874  474          fb           mdstat      sys

104   2061  2356  2930  9            filesystems  meminfo     sysrq-trigger

113   2073  2375  2933  acpi         fs           misc        sysvipc

1375  21    2409  2934  buddyinfo    ide          modules     tty

1395  2189  2445  2935  bus          interrupts   mounts      uptime

1706  2201  2514  2938  cmdline      iomem        mtrr        version

179   2211  2515  2947  cpuinfo      ioports      net         vmstat

180   2223  2607  3     crypto       irq          partitions

181   2278  2608  3004  devices      kallsyms     pci

182   2291  2609  3008  diskstats    kcore        self

2     2301  263   3056  dma          kmsg         slabinfo

2015  2311  2805  394   driver       loadavg      stat

2019  2337  2821  4     execdomains  locks        swaps

[root@plato 1]# ls /proc/1

auxv     cwd      exe  loginuid  mem     oom_adj    root  statm   task

cmdline  environ  fd   maps      mounts  oom_score  stat  status  wchan

[root@plato]# cat /proc/1/cmdline

init [5]

[root@plato]#

Listing 2 illustrates reading from and then writing to a virtual file in /proc. This example checks and then enables IP forwarding within the kernel's TCP/IP stack.

Listing 2. Reading from and writing to /proc (configuring the kernel)
[root@plato]# cat /proc/sys/net/ipv4/ip_forward

0

[root@plato]# echo "1" > /proc/sys/net/ipv4/ip_forward

[root@plato]# cat /proc/sys/net/ipv4/ip_forward

1

[root@plato]#

Alternatively, you could use sysctl to configure these kernel items. See the Resources section for more information on that.

By the way, the /proc filesystem isn't the only virtual filesystem in GNU/Linux. One such system, sysfs, is similar to /proc but a bit more organized (having learned lessons from /proc). However, /proc is entrenched and therefore, even though sysfs has some advantages over it, /proc is here to stay. There's also the debugfs filesystem, but it tends to be (as the name implies) more of a debugging interface. An advantage to debugfs is that it's extremely simple to export a single value to user space (in fact, it's a single call).

Introducing kernel modules

Loadable Kernel Modules (LKM) are an easy way to demonstrate the /proc filesystem, because they're a novel way to dynamically add or remove code from the Linux kernel. LKMs are also a popular mechanism for device drivers and filesystems in the Linux kernel.

If you've ever recompiled the Linux kernel, you probably found that in the kernel configuration process, many device drivers and other kernel elements are compiled as modules. If a driver is compiled directly into the kernel, its code and static data occupy space even if they're not used. But if the driver is compiled as a module, it requires memory only if memory is needed and subsequently loaded, into the kernel. Interestingly, you won't notice a performance hit for LKMs, so they're a powerful means of creating a lean kernel that adapts to its environment based upon the available hardware and attached devices.

Here's a simple LKM to help you understand how it differs from standard (non-dynamically loadable) code that you'll find in the Linux kernel. Listing 3 presents the simplest LKM. (You can download the sample code for this article from the Downloads section, below.)

Listing 3 includes the necessary module header (which defines the module APIs, types, and macros). It then defines the license for the module using MODULE_LICENSE. Here, it specifies GPL to avoid tainting the kernel.

Listing 3 then defines the module init and cleanup functions. The my_module_init function is called when the module is loaded and the function can be used for initialization purposes. The my_module_cleanup function is called when the module is being unloaded and is used to free memory and generally remove traces of the module. Note the use of printk here: this is the kernel printf function. The KERN_INFO symbol is a string that you can use to filter information from entering the kernel ring buffer (much like syslog).

Finally, Listing 3 declares the entry and exit functions using the module_init and module_exit macros. This allows you to name the module init and cleanup functions the way you want but then tell the kernel which functions are the maintenance functions.

Listing 3. A simple but functional LKM (simple-lkm.c)
#include <linux/module.h>



/* Defines the license for this LKM */

MODULE_LICENSE("GPL");



/* Init function called on module entry */

int my_module_init( void )

{

  printk(KERN_INFO "my_module_init called.  Module is now loaded.\n");



  return 0;

}



/* Cleanup function called on module exit */

void my_module_cleanup( void )

{

  printk(KERN_INFO "my_module_cleanup called.  Module is now unloaded.\n");



  return;

}



/* Declare entry and exit functions */

module_init( my_module_init );

module_exit( my_module_cleanup );

Listing 3 is a real LKM, albeit a simple one. Now, let's build it and test it out on a 2.6 kernel. The 2.6 kernel introduces a new method for kernel module building that I find simpler than the older methods. With the file simple-lkm.c, create a makefile whose sole content is:

obj-m += simple-lkm.o

To build the LKM, use the make command as shown in Listing 4.

Listing 4. Building an LKM
[root@plato]# make -C /usr/src/linux-`uname -r` SUBDIRS=$PWD modules

make: Entering directory `/usr/src/linux-2.6.11'

  CC [M]  /root/projects/misc/module2.6/simple/simple-lkm.o

  Building modules, stage 2.

  MODPOST

  CC      /root/projects/misc/module2.6/simple/simple-lkm.mod.o

  LD [M]  /root/projects/misc/module2.6/simple/simple-lkm.ko

make: Leaving directory `/usr/src/linux-2.6.11'

[root@plato]#

The result is simple-lkm.ko. The new naming convention helps to distinguish kernel objects (LKMs) from standard objects. You can now load and unload the module and then view its output. To load the module, use the insmod command; conversely, to unload the module, use the rmmod command. lsmod shows the currently loaded LKMs (see Listing 5).

Listing 5. Inserting, checking, and removing an LKM
[root@plato]# insmod simple-lkm.ko

[root@plato]# lsmod

Module                  Size  Used by

simple_lkm              1536  0

autofs4                26244  0

video                  13956  0

button                  5264  0

battery                 7684  0

ac                      3716  0

yenta_socket           18952  3

rsrc_nonstatic          9472  1 yenta_socket

uhci_hcd               32144  0

i2c_piix4               7824  0

dm_mod                 56468  3

[root@plato]# rmmod simple-lkm

[root@plato]#

Note that kernel output goes to the kernel ring buffer and not to stdout, because stdout is process specific. To inspect messages on the kernel ring buffer, you can use the dmesg utility (or work through /proc itself with the command cat /proc/kmsg). Listing 6 shows the output of the last few messages from dmesg.

Listing 6. Reviewing the kernel output from the LKM
[root@plato]# dmesg | tail -5

cs: IO port probe 0xa00-0xaff: clean.

eth0: Link is down

eth0: Link is up, running at 100Mbit half-duplex

my_module_init called.  Module is now loaded.

my_module_cleanup called.  Module is now unloaded.

[root@plato]#

You can see the module's messages in the kernel output. Now let's move beyond this simple example and look at some of the kernel APIs that allow you to develop useful LKMs.


Integrating into the /proc filesystem

The standard APIs that are available to kernel programmers are also available to LKM programmers. It's even possible for an LKM to export new variables and functions that the kernel can use. A complete treatment of the APIs is beyond the scope of this article, so I simply present some of the elements that I use later to demonstrate a more useful LKM.

Creating and removing a /proc entry

To create a virtual file in the /proc filesystem, use the create_proc_entry function. This function accepts a file name, a set of permissions, and a location in the /proc filesystem in which the file is to reside. The return value of create_proc_entry is a proc_dir_entry pointer (or NULL, indicating an error in create). You can then use the return pointer to configure other aspects of the virtual file, such as the function to call when a read is performed on the file. The prototype for create_proc_entry and a portion of the proc_dir_entry structure are shown in Listing 7.

Listing 7. Elements for managing a /proc filesystem entry
struct proc_dir_entry *create_proc_entry( const char *name, mode_t mode,

                                             struct proc_dir_entry *parent );



struct proc_dir_entry {

	const char *name;			// virtual file name

	mode_t mode;				// mode permissions

	uid_t uid;				// File's user id

	gid_t gid;				// File's group id

	struct inode_operations *proc_iops;	// Inode operations functions

	struct file_operations *proc_fops;	// File operations functions

	struct proc_dir_entry *parent;		// Parent directory

	...

	read_proc_t *read_proc;			// /proc read function

	write_proc_t *write_proc;		// /proc write function

	void *data;				// Pointer to private data

	atomic_t count;				// use count

	...

};



void remove_proc_entry( const char *name, struct proc_dir_entry *parent );

Later you see how to use the read_proc and write_proc commands to plug in functions for reading and writing the virtual file.

To remove a file from /proc, use the remove_proc_entry function. To use this function, provide the file name string as well as the location of the file in the /proc filesystem (its parent). The function prototype is also shown in Listing 7.

The parent argument can be NULL for the /proc root or a number of other values, depending upon where you want the file to be placed. Table 1 lists some of the other parent proc_dir_entrys that you can use, along with their location in the filesystem.

Table 1. Shortcut proc_dir_entry variables
proc_dir_entryFilesystem location
proc_root_fs/proc
proc_net/proc/net
proc_bus/proc/bus
proc_root_driver/proc/driver

The Write Callback function

You can write to a /proc entry (from the user to the kernel) by using a write_proc function. This function has this prototype:

int mod_write( struct file *filp, const char __user *buff,

               unsigned long len, void *data );

The filp argument is essentially an open file structure (we'll ignore this). The buff argument is the string data being passed to you. The buffer address is actually a user-space buffer, so you won't be able to read it directly. The len argument defines how much data in buff is being written. The data argument is a pointer to the private data (see Listing 7). In the module, I declare a function of this type to deal with the incoming data.

Linux provides a set of APIs to move data between user space and kernel space. For the write_proc case, I use the copy_from_user functions to manipulate the user-space data.

The Read Callback function

You can read data from a /proc entry (from the kernel to the user) by using the read_proc function. This function has the following prototype:

int mod_read( char *page, char **start, off_t off,

              int count, int *eof, void *data );

The page argument is the location into which you write the data intended for the user, where count defines the maximum number of characters that can be written. Use the start and off arguments when returning more than a page of data (typically 4KB). When all the data have been written, set the eof (end-of-file) argument. As with write, data represents private data. The page buffer provided here is in kernel space. Therefore, you can write to it without having to invoke copy_to_user.

Other useful functions

You can also create directories within the /proc filesystem using proc_mkdir as well as symlinks with proc_symlink. For simple /proc entries that require only a read function, use create_proc_read_entry, which creates the /proc entry and initializes the read_proc function in one call. The prototypes for these functions are shown in Listing 8.

Listing 8. Other useful /proc functions
/* Create a directory in the proc filesystem */

struct proc_dir_entry *proc_mkdir( const char *name,

                                     struct proc_dir_entry *parent );



/* Create a symlink in the proc filesystem */

struct proc_dir_entry *proc_symlink( const char *name,

                                       struct proc_dir_entry *parent,

                                       const char *dest );



/* Create a proc_dir_entry with a read_proc_t in one call */

struct proc_dir_entry *create_proc_read_entry( const char *name,

                                                  mode_t mode,

                                                  struct proc_dir_entry *base,

                                                  read_proc_t *read_proc,

                                                  void *data );



/* Copy buffer to user-space from kernel-space */

unsigned long copy_to_user( void __user *to,

                              const void *from,

                              unsigned long n );



/* Copy buffer to kernel-space from user-space */

unsigned long copy_from_user( void *to,

                                const void __user *from,

                                unsigned long n );



/* Allocate a 'virtually' contiguous block of memory */

void *vmalloc( unsigned long size );



/* Free a vmalloc'd block of memory */

void vfree( void *addr );



/* Export a symbol to the kernel (make it visible to the kernel) */

EXPORT_SYMBOL( symbol );



/* Export all symbols in a file to the kernel (declare before module.h) */

EXPORT_SYMTAB

Fortune cookies through the /proc filesystem

Here's an LKM that supports both reading and writing. This simple application provides a fortune cookie dispenser. After the module is loaded, the user can load text fortunes into it using the echo command and then read them back out individually using the cat command.

Listing 9 presents the basic module functions and variables. The init function (init_fortune_module) allocates space for the cookie pot with vmalloc and then clears it out with memset. With the cookie_pot allocated and empty, I create my proc_dir_entry next in the /proc root called fortune. With proc_entry successfully created, I initialize my local variables and the proc_entry structure. I load my /proc read and write functions (shown in Listings 9 and 10) and identify the owner of the module. The cleanup function simply removes the entry from the /proc filesystem and then frees the memory that cookie_pot occupies.

The cookie_pot is a page in length (4KB) and is managed by two indexes. The first, cookie_index, identifies where the next cookie will be written. The variable next_fortune identifies where the next cookie will be read for output. I simply wrap next_fortune to the beginning when all fortunes have been read.

Listing 9. Module init/cleanup and variables
#include <linux/module.h>

#include <linux/kernel.h>

#include <linux/proc_fs.h>

#include <linux/string.h>

#include <linux/vmalloc.h>

#include <asm/uaccess.h>



MODULE_LICENSE("GPL");

MODULE_DESCRIPTION("Fortune Cookie Kernel Module");

MODULE_AUTHOR("M. Tim Jones");



#define MAX_COOKIE_LENGTH       PAGE_SIZE

static struct proc_dir_entry *proc_entry;



static char *cookie_pot;  // Space for fortune strings

static int cookie_index;  // Index to write next fortune

static int next_fortune;  // Index to read next fortune





int init_fortune_module( void )

{

  int ret = 0;



  cookie_pot = (char *)vmalloc( MAX_COOKIE_LENGTH );



  if (!cookie_pot) {

    ret = -ENOMEM;

  } else {



    memset( cookie_pot, 0, MAX_COOKIE_LENGTH );



    proc_entry = create_proc_entry( "fortune", 0644, NULL );



    if (proc_entry == NULL) {



      ret = -ENOMEM;

      vfree(cookie_pot);

      printk(KERN_INFO "fortune: Couldn't create proc entry\n");



    } else {



      cookie_index = 0;

      next_fortune = 0;



      proc_entry->read_proc = fortune_read;

      proc_entry->write_proc = fortune_write;

      proc_entry->owner = THIS_MODULE;

      printk(KERN_INFO "fortune: Module loaded.\n");



    }



  }



  return ret;

}





void cleanup_fortune_module( void )

{

  remove_proc_entry("fortune", &proc_root);

  vfree(cookie_pot);

  printk(KERN_INFO "fortune: Module unloaded.\n");

}





module_init( init_fortune_module );

module_exit( cleanup_fortune_module );

Writing a new cookie to the pot is a simple process (shown in Listing 10). With the length of the cookie being written, I check to see that space is available for it. If not, I return -ENOSPC, which is communicated to the user process. Otherwise, the space exists, and I use copy_from_user to copy the user buffer directly into the cookie_pot. I then increment the cookie_index (based upon the length of the user buffer) and NULL terminate the string. Finally, I return the number of characters actually written into the cookie_pot that is propagated to the user process.

Listing 10. Function to write a fortune
ssize_t fortune_write( struct file *filp, const char __user *buff,

                        unsigned long len, void *data )

{

  int space_available = (MAX_COOKIE_LENGTH-cookie_index)+1;



  if (len > space_available) {



    printk(KERN_INFO "fortune: cookie pot is full!\n");

    return -ENOSPC;



  }



  if (copy_from_user( &cookie_pot[cookie_index], buff, len )) {

    return -EFAULT;

  }



  cookie_index += len;

  cookie_pot[cookie_index-1] = 0;



  return len;

}

Reading a fortune is just as simple, as shown in Listing 11. Because the buffer that I'll write to (page) is already in kernel space, I can manipulate it directly and use sprintf to write the next fortune. If the next_fortune index is greater than the cookie_index (next position to write), I wrap next_fortune back to zero, which is the index of the first fortune. After the fortune is written to the user buffer, I increment the next_fortune index by the length of the last fortune written. This places me at the index of the next available fortune. The length of the fortune is returned and propagated to the user.

Listing 11. Function to read a fortune
int fortune_read( char *page, char **start, off_t off,

                   int count, int *eof, void *data )

{

  int len;



  if (off > 0) {

    *eof = 1;

    return 0;

  }



  /* Wrap-around */

  if (next_fortune >= cookie_index) next_fortune = 0;



  len = sprintf(page, "%s\n", &cookie_pot[next_fortune]);



  next_fortune += len;



  return len;

}

You can see from this simple example that communicating with the kernel through the /proc filesystem is a trivial effort. Now take a look at the fortune module in action (Listing 12).

Listing 12. Demonstrating the fortune cookie LKM
[root@plato]# insmod fortune.ko

[root@plato]# echo "Success is an individual proposition.  Thomas Watson" > /proc/fortune

[root@plato]# echo "If a man does his best, what else is there?  Gen. Patton" > /proc/fortune

[root@plato]# echo "Cats: All your base are belong to us.  Zero Wing" > /proc/fortune

[root@plato]# cat /proc/fortune

Success is an individual proposition.  Thomas Watson

[root@plato]# cat /proc/fortune

If a man does his best, what else is there?  Gen. Patton

[root@plato]#

The /proc virtual filesystem is widely used to report kernel information and also for dynamic configuration. You'll find it integral to both driver and module programming. You can learn more about it in the Resources below.

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