The net_device structure

1
The net_device structure

• A look at the main kernel-object concerned with a
  Linux systems network interface controller

2
data consolidation

• Experienced software developers usually employ a
  locality of reference strategy to simplify
  design and maintenance of code
• This principle says Things which logically are
  related should be kept together
• Accordingly, all the information that the system
  needs to keep track of about a network interface
  is put into a single struct

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struct net_device
dev_base_head
next
prev
struct net_device
struct net_device
struct net_device
lo
eth0
eth1
About 90 separate fields belong to each
net_device member of this doubly-linked list
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A few examples
struct net_device char name IFNAMSIZ
int ifindex \ / inteface
index / unsigned int mtu /
maximum transmission unit / unsigned
char dev_addr MAX_ADDR_LEN / hardware
address / void ip_pointer / points to IFs
IPv4 specific data / unsigned
int flags unsigned long trans_start /
time (in jiffies) of last transmission / struct
net_device_stats stats / a default set of
device statistics / // some
function-pointers (i.e., these are virtual
functions) int (open)( struct net_device
) int (stop)( struct net_device
) int (hard_start_xmit)( struct sk_buff ,
struct net_device ) int (get_stats)( struct
net_device )
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An analogy?

• The struct net_device kernel-objects play a
  similar role for network device-drivers as is
  played by struct file_operations objects with
  regard to character device-drivers
• This analogy isnt perfect (i.e., key differences)

struct file_operations
struct net_device
open() release() write() read() llseek()
ioctl()
open() stop() hard_start_xmit() get_stats()
set_mac_address() do_ioctl()
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struct net_device_stats

• Notice that the net_device object contains a
  sub-structure for storing device statistics

struct net_device_stats unsigned
long rx_packets / total packets received
/ unsigned long tx_packets / total packets
transmitted / unsigned long rx_bytes / total
bytes received / unsigned long tx_bytes /
total bytes transmitted / unsigned
long rx_errors / bad packets received
/ unsigned long tx_errors / packet transmit
problems /
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struct sk_buff

• Notice that the net_device object also has a
  member-field which can hold a pointer to to a
  kernel-object of type struct sk_buff

struct sk_buff
skb
packet data
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Kernels header-files

• The kernel is written in C (with occasional uses
  of some inline assembly language)
• All the source-code for our Linux kernel is in
  this system directory lt/usr/src/linuxgt
• Most header-files are in lt/include/linuxgt
• Those header-files that are CPU-specific are in
  lt/include/asmgt

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Our directory tree
/
sbin
bin
usr
home
src
include
web
ifconfig lsmod insmod rmmod
linux
ping cat echo ls vi dmesg
include
cruse
linux
asm
cs686
netdevice.h if.h module.h pci.h
io.h uaccess.h unistd.h
our course demos
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ltlinux/netdevice.hgt

• This is the header-file where you will find the
  struct net_device definition (and the struct
  net_device_stats definition as well)
• And if you want to see how struct sk_buff is
  defined, then look in ltlinux/skbuff.hgt
• NOTE The kernel developers often move
  structure-definitions to different headers in new
  versions of the kernel source-code

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Our netdevs.c module

• We can create a Loadable Kernel Module that lets
  us see current information stored in the kernels
  data-structures
• Our example-module netdevs.c does this
• It needs a special command-sequence to be
  compiled, then a special command to be installed
  as a live add-on our running Linux operating
  systems kernel

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mmake.cpp

• You can download this utility-program from our
  cs686 website and compile it with g, like this
• cp /home/web/cruse/cs686/mmake.cpp .
• g mmake.cpp o mmake
• Then you can use it to automate the steps
  required for compiling an LKM, like this
• ./mmake netdevs

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/sbin/insmod

• When you have compiled our netdevs.c
  source-file, you will have a kernel-object file
  named netdevs.ko and you can use the insmod
  command to install it
• /sbin/insmod netdevs.ko
• When it is installed, this LKM creates a
  pseudo-file (in the /proc directory) that you
  can send to your screen with cat
• cat /proc/netdevs

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/sbin/rmmod

• If you decide you would like to modify the output
  from your /proc/netdevs file, you can remove
  netdevs.ko from the kernel, edit the modules
  source-code, recompile the netdevs.c file with
  mmake, and then install the new version of
  netdevs.ko
• /sbin/rmmod netdevs.ko

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In-class exercise 1

• Modify the output shown by /proc/netdevs so
  that the current state of each interface (i.e.,
  UP or DOWN) will get displayed
• HINT The kernel uses a status-bit in the flags
  field of a struct net_device object to keep
  track of whether the device is now up or down
  (compare /sbin/ifconfig a)
• Look in the header-file ltlinux/if.hgt to find out
  how bits of the flags field are used

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Two coding approaches

• One way to write your solution for in-class
  exercise 1 is to add a line like this
• But a different solution which produces the same
  effect avoids the if-else construct

if ( dev-gtflags IFF_UP ) len sprintf(
buflen, UP ) else len sprintf( buflen,
DOWN )
char state 2 DOWN, UP // an array
of string-pointers len
sprintf( buflen, s, state dev-gtflags
IFF_UP )
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In-class exercise 2

• Look at the output produced when you execute the
  Linux ifconfig program
• Choose some added item of information about our
  stations network interfaces (from the ifconfig
  output) and see if you can enhance our
  netdevs.c demo so its pseudo-file will display
  that extra item of information (e.g.,
  dev-gtstats.tx_bytes)