15.1. Debugging a kernel crash dump with kgdb

Here are some instructions for getting kernel debugging working on a crash dump, it assumes that you have enough swap space for a crash dump. If you have multiple swap partitions and the first one is too small to hold the dump, you can configure your kernel to use an alternate dump device (in the config kernel line), or you can specify an alternate using the dumpon(8) command. Dumps to non-swap devices, tapes for example, are currently not supported. Config your kernel using config -g. See Kernel Configuration for details on configuring the FreeBSD kernel.

Use the dumpon(8) command to tell the kernel where to dump to (note that this will have to be done after configuring the partition in question as swap space via swapon(8)). This is normally arranged via /etc/sysconfig and /etc/rc. Alternatively, you can hard-code the dump device via the `dump' clause in the `config' line of your kernel config file. This is deprecated, use only if you want a crash dump from a kernel that crashes during booting.

Note: In the following, the term `kgdb' refers to gdb run in `kernel debug mode'. This can be accomplished by either starting the gdb with the option -k, or by linking and starting it under the name kgdb. This is not being done by default, however.

When the kernel has been built make a copy of it, say kernel.debug, and then run strip -x on the original. Install the original as normal. You may also install the unstripped kernel, but symbol table lookup time for some programs will drastically increase, and since the whole kernel is loaded entirely at boot time and cannot be swapped out later, several megabytes of physical memory will be wasted.

If you are testing a new kernel, for example by typing the new kernel's name at the boot prompt, but need to boot a different one in order to get your system up and running again, boot it only into single user state using the -s flag at the boot prompt, and then perform the following steps:

  fsck -p
  mount -a -t ufs       # so your file system for /var/crash is writable
  savecore -N /kernel.panicked /var/crash
  exit                  # ...to multi-user

Now, after a crash dump, go to /sys/compile/WHATEVER and run kgdb. From kgdb do:

  symbol-file kernel.debug
  exec-file /var/crash/system.0
  core-file /var/crash/ram.0

Here's a script log of a kgdb session illustrating the procedure. Long lines have been folded to improve readability, and the lines are numbered for reference. Despite of this, it's a real-world error trace taken during the development of the pcvt console driver.

   1:Script started on Fri Dec 30 23:15:22 1994
   2:uriah # cd /sys/compile/URIAH
   3:uriah # kgdb kernel /var/crash/vmcore.1 
   4:Reading symbol data from /usr/src/sys/compile/URIAH/kernel...done.
   5:IdlePTD 1f3000
   6:panic: because you said to!
   7:current pcb at 1e3f70
   8:Reading in symbols for ../../i386/i386/machdep.c...done.
   9:(kgdb) where
  10:#0  boot (arghowto=256) (../../i386/i386/machdep.c line 767)
  11:#1  0xf0115159 in panic ()
  12:#2  0xf01955bd in diediedie () (../../i386/i386/machdep.c line 698)
  13:#3  0xf010185e in db_fncall ()
  14:#4  0xf0101586 in db_command (-266509132, -266509516, -267381073)
  15:#5  0xf0101711 in db_command_loop ()
  16:#6  0xf01040a0 in db_trap ()
  17:#7  0xf0192976 in kdb_trap (12, 0, -272630436, -266743723)
  18:#8  0xf019d2eb in trap_fatal (...)
  19:#9  0xf019ce60 in trap_pfault (...)
  20:#10 0xf019cb2f in trap (...)
  21:#11 0xf01932a1 in exception:calltrap ()
  22:#12 0xf0191503 in cnopen (...)
  23:#13 0xf0132c34 in spec_open ()
  24:#14 0xf012d014 in vn_open ()
  25:#15 0xf012a183 in open ()
  26:#16 0xf019d4eb in syscall (...)
  27:(kgdb) up 10
  28:Reading in symbols for ../../i386/i386/trap.c...done.
  29:#10 0xf019cb2f in trap (frame={tf_es = -260440048, tf_ds = 16, tf_\
  30:edi = 3072, tf_esi = -266445372, tf_ebp = -272630356, tf_isp = -27\
  31:2630396, tf_ebx = -266427884, tf_edx = 12, tf_ecx = -266427884, tf\
  32:_eax = 64772224, tf_trapno = 12, tf_err = -272695296, tf_eip = -26\
  33:6672343, tf_cs = -266469368, tf_eflags = 66066, tf_esp = 3072, tf_\
  34:ss = -266427884}) (../../i386/i386/trap.c line 283)
  35:283                             (void) trap_pfault(&frame, FALSE);
  36:(kgdb) frame frame->tf_ebp frame->tf_eip
  37:Reading in symbols for ../../i386/isa/pcvt/pcvt_drv.c...done.
  38:#0  0xf01ae729 in pcopen (dev=3072, flag=3, mode=8192, p=(struct p\
  39:roc *) 0xf07c0c00) (../../i386/isa/pcvt/pcvt_drv.c line 403)
  40:403             return ((*linesw[tp->t_line].l_open)(dev, tp));
  41:(kgdb) list
  42:398        
  43:399             tp->t_state |= TS_CARR_ON;
  44:400             tp->t_cflag |= CLOCAL;  /* cannot be a modem (:-) */
  45:401     
  46:402     #if PCVT_NETBSD || (PCVT_FREEBSD >= 200)
  47:403             return ((*linesw[tp->t_line].l_open)(dev, tp));
  48:404     #else
  49:405             return ((*linesw[tp->t_line].l_open)(dev, tp, flag));
  50:406     #endif /* PCVT_NETBSD || (PCVT_FREEBSD >= 200) */
  51:407     }
  52:(kgdb) print tp
  53:Reading in symbols for ../../i386/i386/cons.c...done.
  54:$1 = (struct tty *) 0x1bae
  55:(kgdb) print tp->t_line
  56:$2 = 1767990816
  57:(kgdb) up
  58:#1  0xf0191503 in cnopen (dev=0x00000000, flag=3, mode=8192, p=(st\
  59:ruct proc *) 0xf07c0c00) (../../i386/i386/cons.c line 126)
  60:       return ((*cdevsw[major(dev)].d_open)(dev, flag, mode, p));
  61:(kgdb) up
  62:#2  0xf0132c34 in spec_open ()
  63:(kgdb) up
  64:#3  0xf012d014 in vn_open ()
  65:(kgdb) up
  66:#4  0xf012a183 in open ()
  67:(kgdb) up
  68:#5  0xf019d4eb in syscall (frame={tf_es = 39, tf_ds = 39, tf_edi =\
  69: 2158592, tf_esi = 0, tf_ebp = -272638436, tf_isp = -272629788, tf\
  70:_ebx = 7086, tf_edx = 1, tf_ecx = 0, tf_eax = 5, tf_trapno = 582, \
  71:tf_err = 582, tf_eip = 75749, tf_cs = 31, tf_eflags = 582, tf_esp \
  72:= -272638456, tf_ss = 39}) (../../i386/i386/trap.c line 673)
  73:673             error = (*callp->sy_call)(p, args, rval);
  74:(kgdb) up
  75:Initial frame selected; you cannot go up.
  76:(kgdb) quit
  77:uriah # exit
  78:exit
  79:
  80:Script done on Fri Dec 30 23:18:04 1994

line 6:

This is a dump taken from within DDB (see below), hence the panic comment ``because you said to!'', and a rather long stack trace; the initial reason for going into DDB has been a page fault trap though.

line 20:

This is the location of function trap() in the stack trace.

line 36:

Force usage of a new stack frame; this is no longer necessary now. The stack frames are supposed to point to the right locations now, even in case of a trap. (I don't have a new core dump handy <g>, my kernel didn't panic for rather long.) From looking at the code in source line 403, there's a high probability that either the pointer access for ``tp'' was messed up, or the array access was out of bounds.

line 52:

The pointer looks suspicious, but happens to be a valid address.

line 56:

However, it obviously points to garbage, so we have found our error! (For those unfamiliar with that particular piece of code: tp->t_line refers to the line discipline of the console device here, which must be a rather small integer number.)