Socket System Call

How to trace the system call ?

• Compile with debug symbols and with static option  'gcc -g -static client.c'
•  Lookup socket symbol in the executable nm a.out | grep 'socke*'
  0804e780 T __socket
  0804e780 W socket
• W socket  indicates that it is a glibc function.
• Debug a.out using gdb

• Call 0x66 = decimal 102
• In /usr/src/linux/arch/i386/kernel/entry.S the 102th entry is -

Entrypoint for system call :  arch/i386/kernel/entry.S

include/linux/net.h

net/socket.c/sys_socketcall

net/socket.c/sys_socket()

List of Linux Kernel Networking Functions

destroy_sock - net/ipv4/af_inet.c
  deletes any timers
  calls any protocols specific destroy functions
  frees the socket's queues
  frees the socket structure itself

fib_lookup() - include/net/ip_fib.h
  calls tb_lookup() [= fn_hash_lookup()] on local and main tables
  returns route or unreachable error

fn_hash_lookup() - net/ipv4/fib_hash.c
  looks up and returns route to an address

inet_create() - net/ipv4/af_inet.c
  calls sk_alloc() to get memory for sock
  initializes sock structure:
    sets proto structure to appropriate values for TCP or UDP
    calls sock_init_data()
    sets family,protocol,etc. variables
  calls the protocol init function (if any)

inet_release() - net/ipv4/af_inet.c
  changes socket state to disconnecting
  calls ip_mc_drop_socket to leave multicast group (if necessary)
  sets owning socket's data member to NULL
  calls sk->prot->close() [=TCP/UDP_close()]

ip_route_connect() - include/net/route.h
  calls ip_route_output() to get a destination address
  returns if the call works or generates an error
  otherwise clears the route pointer and try again

ip_route_output() - net/ipv4/route.c
  calculates hash value for address
  runs through table (starting at hash) to match addresses and TOS
  if there is a match, updates stats and return route entry
  else calls ip_route_output_slow()

ip_route_output_slow() - net/ipv4/route.c
  if source address is known, looks up output device
  if destination address is unknown, sets up loopback
  calls fib_lookup() to find route in FIB
  allocates memory new routing table entry
  initializes table entry with source, destination, TOS, output device,
      flags
  calls rt_set_nexthop() to find next destination
  returns rt_intern_hash(), which installs route in routing table

rt_intern_hash() - net/ipv4/route.c
  loops through rt_hash_table (starting at hash value)
  if keys match, put rtable entry in front bucket
  else put rtable entry into hash table at hash

sock_close() - net/socket.c
  checks if socket exists (could be null)
  calls sock_fasync() to remove socket from async list
  calls sock_release()

sock_create() - net/socket.c
  checks parameters
  calls sock_alloc() to get an available inode for the socket and
      initialize it
  sets socket->type (to SOCK_STREAM, SOCK_DGRAM...)
  calls net_family->create() [= inet_create()] to build sock structure
  returns established socket

sock_init_data() - net/core/sock.c
  initializes all generic sock values

  sock_read() - net/socket.c
  sets up message headers
  returns sock_recvmsg() with result of read

sock_recvmsg() - net/socket.c
  reads socket management packet (scm) or packet by calling sock->ops[inet]->recvmsg()

sock_release() - net/socket.c
  changes state to disconnecting
  calls sock->ops->release() [= inet_release()]
  calls iput() to remove socket from inode list

sys_socket() - net/socket.c
  calls sock_create() to get and initialize socket
  calls get_fd() to assign an fd to the socket
  sets socket->file to fcheck() (pointer to file)
  calls sock_release() if anything fails

tcp_close() - net/ipv4/tcp.c
  check for errors
  pops and discards all packets off incoming queue
  sends messages to destination to close connection (if required)

tcp_connect() - net/ipv4/tcp_output.c
  completes connection packet with appropriate bits and window sizes set
  puts packet on socket output queue
  calls tcp_transmit_skb() to send packet, initiating TCP connection

tcp_v4_connect() - net/ipv4/tcp_ipv4.c
  checks for errors
  calls ip_route_connect() to find route to destination
  creates connection packet  
  calls tcp_connect() to send packet

udp_close() - net/ipv4/udp.c
  calls udp_v4_unhash() to remove socket from socket list
  calls destroy_sock()

udp_connect() - net/ipv4/udp.c
  calls ip_route_connect() to find route to destination
  updates socket with source and destination addresses and ports
  changes socket state to established
  saves the destination route in sock->dst_cache