Access the Linux kernel using the /proc filesystem

http://www.ibm.com/developerworks/linux/library/l-proc/index.html

Access the Linux kernel using the /proc filesystem

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

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

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.

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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_entry

s that you can use, along with their location in the filesystem.

Table 1. Shortcut proc_dir_entry variables

proc_dir_entry	Filesystem 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

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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.

Resources

Learn

"Administer Linux on the fly" (developerWorks, May 2003) gives you a thorough grounding in /proc, including how you can administer many details of the operating system without ever
having to shut down and reboot the machine.

Explore the
files and subdirectories in the /proc filesystem.

This article on driver porting to the 2.6 Linux kernel discusses kernel modules in detail.

LinuxHQ is a great site for information on the Linux kernel.

The debugfs filesystem is a debugging alternative to /proc.

"Kernel comparison: Improvements in kernel development from 2.4 to 2.6" (developerWorks, February 2004) takes a look behind the scenes at the tools, tests, and techniques that make up
kernel 2.6.

"Kernel debugging with Kprobes" (developerWorks, August 2004) shows how in combination with 2.6 kernels, Kprobes provides a lightweight, non-disruptive, and powerful mechanism
to insert the

printk

function dynamically.

The

printk

function and

dmesg

methods are common means for kernel debugging. Allessando Rubini's book
Linux Device Drivers provides an
online chapter about kernel debugging techniques.

The sysctl command is another option for dynamic kernel configuration.

In the developerWorks Linux zone, find more resources for Linux developers.

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