2009-01-22

DRBL Shutdown Agent

• [源起] 由於 DRBL Client 在 NFS 斷線後會因為 ROOTFS 消失而找不到關機程序所需的檔案。因此需要有一個背景程序持續等在記憶體中來協助 DRBL Client 關閉電源。
• 準備 sysvinit 套件環境

  # apt-get install dpkg-dev build-essential
  # apt-get source sysvinit
  # apt-get build-dep sysvinit
  # cd sysvinit-2.86.ds1/src
  ~/sysvinit-2.86.ds1/src# vi Makefile
  

  Makefile

  @@ -9 @@
  -CFLAGS = -Wall -O2 -fomit-frame-pointer -D_GNU_SOURCE
  -LDFLAGS= -s
  +CFLAGS = -g -Wall -O2 -fomit-frame-pointer -D_GNU_SOURCE # 加入 -g ，讓 gcc 加入 DEBUG Symbol
  +LDFLAGS=                                                 # 拿掉 -s ，讓 LD 進行 Linking 時不進行 strip 

  ~/sysvinit-2.86.ds1/src# make
  ~/sysvinit-2.86.ds1/src# nm init                          # 用 nm 可以看到是否有保留 Symbol
  ~/sysvinit-2.86.ds1/src# file init                        # 顯示 not stripped 才會保留 Symbol 哦!
  init: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), for GNU/Linux 2.4.1, dynamically linked (uses shared libs), for GNU/Linux 2.4.1, not stripped
  

• 先前有追蹤過 shutdown.c 跟 halt.c 的原始碼，發現最後都還是呼叫 init 來執行關機程序。因此這次改追蹤 init.c
• 用 gdb 追蹤 init 0 的程序

  ~/sysvinit-2.86.ds1/src# gdb init
  (gdb) set args 0
  (gdb) show args
  Argument list to give program being debugged when it is started is "0".
  (gdb) br 2598
  Breakpoint 1 at 0x804e140: file init.c, line 2598.
  (gdb) br 2555
  Breakpoint 2 at 0x8049bfd: file init.c, line 2555.
  (gdb) run
  Starting program: /root/sysvinit-2.86.ds1/src/init 0
  Failed to read a valid object file image from memory.
  
  Breakpoint 1, main (argc=1282785413, argv=0x54a165) at init.c:2598
  2598    {
  (gdb) c
  Continuing.
  
  Breakpoint 2, telinit (progname=0xbfff5c53 "init", argc=2, argv=0xbfff5b34)
      at init.c:2555
  2555            SETSIG(sa, SIGALRM, signal_handler, 0);
  (gdb) handle SIGALRM nopass
  Signal        Stop      Print   Pass to program Description
  SIGALRM       No        No      No              Alarm clock
  (gdb) s
  

• 用 strace init 0 觀察關機程序

  root@debian:~/sysvinit-2.86.ds1/src# strace init 0
  execve("/sbin/init", ["init", "0"], [/* 14 vars */]) = 0
  rt_sigaction(SIGALRM, {0x8049ad0, [], 0}, NULL, 8) = 0
  alarm(3)                                = 0
  open("/dev/initctl", O_WRONLY)          = 3
  write(3, "i\31\t\3\1\0\0\0000\0\0\0\5\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 384) = 384
  close(3)                                = 0
  alarm(0)                                = 3
  exit_group(0)                           = ?