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424 lines
16 KiB
Plaintext
424 lines
16 KiB
Plaintext
<!-- $Id:$ -->
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<!-- The FreeBSD Documentation Project -->
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<!--
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<!DOCTYPE linuxdoc PUBLIC "-//FreeBSD//DTD linuxdoc//EN" [
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<!ENTITY % authors SYSTEM "authors.sgml">
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%authors;
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]>
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-->
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<chapt><heading>Kernel Debugging</heading>
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<p><em>Contributed by &a.paul; and &a.joerg;</em>
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<sect><heading>Debugging a kernel crash dump with kgdb</heading>
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<p>Here are some instructions for getting kernel debugging
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working on a crash dump, it assumes that you have enough swap
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space for a crash dump. If you happen to have multiple swap
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partitions with the first one being too small to keep the dump,
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you can configure your kernel to use an alternate dump device
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(in the <tt>kernel</tt> line). Dumps to non-swap devices,
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tapes for example, are currently not supported. Config your
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kernel using <tt>config -g</tt>.
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<!-- XXX obsolete?
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Remember that you need to
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specify
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<tscreen><verb>
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options DODUMP
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</verb></tscreen>
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in your config file in order to get kernel core dumps.
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-->
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See <ref id="kernelconfig:" name="Kernel Configuration"> for
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details on configuring the FreeBSD kernel.
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<em><bf>Note:</bf> In the following, the term `<tt>kgdb</tt>' refers
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to <tt>gdb</tt> run in `kernel debug mode'. This can be accomplished by
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either starting the <tt>gdb</tt> with the option <tt>-k</tt>, or by linking
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and starting it under the name <tt>kgdb</tt>. This is not being
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done by default, however.</em>
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When the kernel has been built make a copy of it, say
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<tt>kernel.debug</tt>, and then run <tt>strip -x</tt> on the
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original. Install the original as normal. You may also install
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the unstripped kernel, but symbol table lookup time for some
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programs might drastically increase.
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If you are testing a new kernel, for example by typing the new
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kernel's name at the boot prompt, but need to boot a different
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one in order to get your system up and running again, boot it
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only into single user state using the <tt>-s</tt> flag at the
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boot prompt, and then perform the following steps:
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<tscreen><verb>
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fsck -p
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mount -a -t ufs # so your file system for /var/crash is writable
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savecore -N /kernel.panicked /var/crash
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exit # ...to multi-user
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</verb></tscreen>
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This instructs <tt>savecore(8)</tt> to use another kernel for symbol name
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extraction. It would otherwise default to the currently running kernel.
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Now, after a crash dump, go to <tt>/sys/compile/WHATEVER</tt> and run
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<tt>kgdb</tt>. From <tt>kgdb</tt> do:
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<tscreen><verb>
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symbol-file kernel.debug
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exec-file /var/crash/system.0
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core-file /var/crash/ram.0
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</verb></tscreen>
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and voila, you can debug the crash dump using the kernel sources
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just like you can for any other program.
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If your kernel panicked due to a trap, perhaps the most common
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case for getting a core dump, the following trick might help
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you. Examine the stack using <tt>kgdb</tt>'s `where' command,
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and look for the stack frame in the function <tt>trap()</tt>. Go `up'
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to that frame, and then type:
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<tscreen><verb>
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frame frame->tf_ebp frame->tf_eip
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</verb></tscreen>
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This will tell <tt>kgdb</tt> to go to the stack frame explicitly named by a
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frame pointer and instruction pointer, which is the location where
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the trap occured. There are still some bugs in <tt>kgdb</tt> (you can go
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`up' from there, but not `down'; the stack trace will still remain
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as it was before going to here), but generally this method will lead
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you much closer to the failing piece of code.
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Here's a script log of a <tt>kgdb</tt> session illustrating the above. Long
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lines have been folded to improve readability, and the lines are
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numbered for reference. Despite of this, it's a real-world error
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trace taken during the development of the pcvt console driver.
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<tscreen><verb>
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1:Script started on Fri Dec 30 23:15:22 1994
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2:uriah # cd /sys/compile/URIAH
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3:uriah # kgdb kernel /var/crash/vmcore.1
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4:Reading symbol data from /usr/src/sys/compile/URIAH/kernel...done.
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5:IdlePTD 1f3000
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6:panic: because you said to!
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7:current pcb at 1e3f70
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8:Reading in symbols for ../../i386/i386/machdep.c...done.
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9:(kgdb) where
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10:#0 boot (arghowto=256) (../../i386/i386/machdep.c line 767)
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11:#1 0xf0115159 in panic ()
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12:#2 0xf01955bd in diediedie () (../../i386/i386/machdep.c line 698)
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13:#3 0xf010185e in db_fncall ()
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14:#4 0xf0101586 in db_command (-266509132, -266509516, -267381073)
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15:#5 0xf0101711 in db_command_loop ()
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16:#6 0xf01040a0 in db_trap ()
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17:#7 0xf0192976 in kdb_trap (12, 0, -272630436, -266743723)
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18:#8 0xf019d2eb in trap_fatal (...)
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19:#9 0xf019ce60 in trap_pfault (...)
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20:#10 0xf019cb2f in trap (...)
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21:#11 0xf01932a1 in exception:calltrap ()
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22:#12 0xf0191503 in cnopen (...)
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23:#13 0xf0132c34 in spec_open ()
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24:#14 0xf012d014 in vn_open ()
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25:#15 0xf012a183 in open ()
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26:#16 0xf019d4eb in syscall (...)
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27:(kgdb) up 10
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28:Reading in symbols for ../../i386/i386/trap.c...done.
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29:#10 0xf019cb2f in trap (frame={tf_es = -260440048, tf_ds = 16, tf_\
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30:edi = 3072, tf_esi = -266445372, tf_ebp = -272630356, tf_isp = -27\
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31:2630396, tf_ebx = -266427884, tf_edx = 12, tf_ecx = -266427884, tf\
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32:_eax = 64772224, tf_trapno = 12, tf_err = -272695296, tf_eip = -26\
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33:6672343, tf_cs = -266469368, tf_eflags = 66066, tf_esp = 3072, tf_\
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34:ss = -266427884}) (../../i386/i386/trap.c line 283)
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35:283 (void) trap_pfault(&frame, FALSE);
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36:(kgdb) frame frame->tf_ebp frame->tf_eip
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37:Reading in symbols for ../../i386/isa/pcvt/pcvt_drv.c...done.
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38:#0 0xf01ae729 in pcopen (dev=3072, flag=3, mode=8192, p=(struct p\
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39:roc *) 0xf07c0c00) (../../i386/isa/pcvt/pcvt_drv.c line 403)
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40:403 return ((*linesw[tp->t_line].l_open)(dev, tp));
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41:(kgdb) list
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42:398
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43:399 tp->t_state |= TS_CARR_ON;
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44:400 tp->t_cflag |= CLOCAL; /* cannot be a modem (:-) */
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45:401
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46:402 #if PCVT_NETBSD || (PCVT_FREEBSD >= 200)
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47:403 return ((*linesw[tp->t_line].l_open)(dev, tp));
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48:404 #else
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49:405 return ((*linesw[tp->t_line].l_open)(dev, tp, flag));
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50:406 #endif /* PCVT_NETBSD || (PCVT_FREEBSD >= 200) */
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51:407 }
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52:(kgdb) print tp
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53:Reading in symbols for ../../i386/i386/cons.c...done.
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54:$1 = (struct tty *) 0x1bae
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55:(kgdb) print tp->t_line
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56:$2 = 1767990816
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57:(kgdb) up
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58:#1 0xf0191503 in cnopen (dev=0x00000000, flag=3, mode=8192, p=(st\
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59:ruct proc *) 0xf07c0c00) (../../i386/i386/cons.c line 126)
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60: return ((*cdevsw[major(dev)].d_open)(dev, flag, mode, p));
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61:(kgdb) up
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62:#2 0xf0132c34 in spec_open ()
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63:(kgdb) up
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64:#3 0xf012d014 in vn_open ()
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65:(kgdb) up
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66:#4 0xf012a183 in open ()
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67:(kgdb) up
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68:#5 0xf019d4eb in syscall (frame={tf_es = 39, tf_ds = 39, tf_edi =\
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69: 2158592, tf_esi = 0, tf_ebp = -272638436, tf_isp = -272629788, tf\
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70:_ebx = 7086, tf_edx = 1, tf_ecx = 0, tf_eax = 5, tf_trapno = 582, \
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71:tf_err = 582, tf_eip = 75749, tf_cs = 31, tf_eflags = 582, tf_esp \
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72:= -272638456, tf_ss = 39}) (../../i386/i386/trap.c line 673)
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73:673 error = (*callp->sy_call)(p, args, rval);
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74:(kgdb) up
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75:Initial frame selected; you cannot go up.
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76:(kgdb) quit
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77:uriah # exit
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78:exit
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79:
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80:Script done on Fri Dec 30 23:18:04 1994
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</verb></tscreen>
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Comments to the above script:
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<descrip>
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<tag/line 6:/ This is a dump taken from within DDB (see below), hence the
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panic comment ``because you said to!'', and a rather long
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stack trace; the initial reason for going into DDB has been
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a page fault trap though.
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<tag/line 20:/ This is the location of function <tt>trap()</tt>
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in the stack trace.
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<tag/line 36:/ Force usage of a new stack frame, kgdb responds and displays
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the source line where the trap happened; from looking at the
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code, there's a high probability that either the pointer
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access for ``tp'' was messed up, or the array access was
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out of bounds.
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<tag/line 52:/ The pointer looks suspicious, but happens to be a valid
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address.
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<tag/line 56:/ However, it obviously points to garbage, so we have found our
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error! (For those unfamiliar with that particular piece
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of code: <tt>tp->t_line</tt> refers to the line discipline
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of the console device here, which must be a rather small integer
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number.)
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</descrip>
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<sect><heading>Post-mortem analysis of a dump</heading>
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<p>What do you do if a kernel dumped core but you did not expect
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it, and it's therefore not compiled using <tt>config -g</tt>?
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Not everything is lost here. Don't panic!
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<!-- XXX obsolete?
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Of course, you still need to configure all your kernels with the
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DODUMP option being set, otherwise you won't get a core dump at all.
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(This is for safety reasons in the default kernels, to avoid them
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trying to dump e.g. during system installation where there's no
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FreeBSD partition at all and valuable data on the disk could be
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destroyed.)
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-->
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Go to your kernel compile directory, and edit the line
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containing <tt>COPTFLAGS?=-O</tt>. Add the <tt>-g</tt> option
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there (but <em>don't</em> change anything on the level of
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optimization). If you do already know roughly the probable
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location of the failing piece of code (e.g., the <tt>pcvt</tt>
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driver in the example above), remove all the object files for
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this code. Rebuild the kernel. Due to the time stamp change on
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the Makefile, there will be some other object files rebuild,
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for example <tt>trap.o</tt>. With a bit of luck, the added
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<tt>-g</tt> option won't change anything for the generated
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code, so you'll finally get a new kernel with similiar code to
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the faulting one but some debugging symbols. You should at
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least verify the old and new sizes with the <tt>size(1)</tt> command. If
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there is a mismatch, you probably need to give up here.
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Go and examine the dump as described above. The debugging
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symbols might be incomplete for some places, as can be seen in
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the stack trace in the example above where some functions are
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displayed without line numbers and argument lists. If you need
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more debugging symbols, remove the appropriate object files and
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repeat the <tt>kgdb</tt> session until you know enough.
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All this is not guaranteed to work, but it will do it fine in
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most cases.
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<sect><heading>On-line kernel debugging using DDB</heading>
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<p>While <tt>kgdb</tt> as an offline debugger provides a very
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high level of user interface, there are some things it cannot do.
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The most important ones being breakpointing and single-stepping
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kernel code.
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If you need to do low-level debugging on your kernel, there's
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an on- line debugger available called DDB. It allows to
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setting breakpoints, single-steping kernel functions, examining
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and changeing kernel variables, etc. However, it cannot not
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access kernel source files, and only has access to the global
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and static symbols, not to the full debug information like
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<tt>kgdb</tt>.
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To configure your kernel to include DDB, add the option line
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<tscreen><verb>
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options DDB
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</verb></tscreen>
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to your config file, and rebuild. (See <ref id="kernelconfig:"
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name="Kernel Configuration"> for details on configuring the
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FreeBSD kernel. Note that if you have an older version of the
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boot blocks, your debugger symbols might not be loaded at all.
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Update the boot blocks, the recent ones do load the DDB symbols
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automagically.)
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Once your DDB kernel is running, there are several ways to
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enter DDB. The first, and earliest way is to type the boot
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flag <tt>-d</tt> right at the boot prompt. The kernel will
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start up in debug mode and enter DDB prior to any device
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probing. Hence you are able to even debug the device
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probe/attach functions.
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The second scenario is a hot-key on the keyboard, usually
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Ctrl-Alt-ESC. For syscons, this can be remapped, and some of
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the distributed maps do this, so watch out. Patches for a
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COMCONSOLE kernel, are available from &a.joerg;.
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The third way is that any panic condition will branch to DDB if
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the kernel is configured to use it. It is not wise to
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configure a kernel with DDB for a machine running unattended
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for this reason.
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The DDB commands roughly resemble some <tt>gdb</tt> commands. The first you
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probably need is to set a breakpoint:
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<tscreen><verb>
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b function-name
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b address
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</verb></tscreen>
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Numbers are taken hexadecimal by default, but to make them
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distinct from symbol names, hexadecimal numbers starting with the
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letters <tt>a</tt>-<tt>f</tt> need to be preceded with
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<tt>0x</tt> (for other numbers, this is optional). Simple
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expressions are allowed, for example: <tt>function-name + 0x103</tt>.
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To continue the operation of an interrupted kernel, simply type
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<tscreen><verb>
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c
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</verb></tscreen>
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To get a stack trace, use
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<tscreen><verb>
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trace
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</verb></tscreen>
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Note that when entering DDB via a hot-key, the kernel is currently
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servicing an interrupt, so the stack trace might be not of much use
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for you.
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If you want to remove a breakpoint, use
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<tscreen><verb>
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del
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del address-expression
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</verb></tscreen>
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The first form will be accepted immediately after a breakpoint hit,
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and deletes the current breakpoint. The second form can remove any
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breakpoint, but you need to specify the exact address, as it can be
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obtained from
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<tscreen><verb>
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show b
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</verb></tscreen>
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To single-step the kernel, try
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<tscreen><verb>
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s
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</verb></tscreen>
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This will step into functions, but you can make DDB trace them until
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the matching return statement is reached by
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<tscreen><verb>
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n
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</verb></tscreen>
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Note: this is different from <tt>gdb</tt>'s `next' statement, it's like
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<tt>gdb</tt>'s `finish'.
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To examine data from memory, use (for example):
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<tscreen><verb>
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x/wx 0xf0133fe0,40
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x/hd db_symtab_space
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x/bc termbuf,10
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x/s stringbuf
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</verb></tscreen>
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for word/halfword/byte access, and hexadecimal/decimal/character/
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string display. The number after the comma is the object count.
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To display the next 0x10 items, simply use
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<tscreen><verb>
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x ,10
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</verb></tscreen>
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Similiarly, use
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<tscreen><verb>
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x/ia foofunc,10
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</verb></tscreen>
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to disassemble the first 0x10 instructions of <tt>foofunc</tt>, and display
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them along with their offset from the beginning of <tt>foofunc</tt>.
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To modify the memory, use the write command:
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<tscreen><verb>
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w/b termbuf 0xa 0xb 0
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w/w 0xf0010030 0 0
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</verb></tscreen>
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The command modifier (<tt>b</tt>/<tt>h</tt>/<tt>w</tt>)
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specifies the size of the data to be writtten, the first
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following expression is the address to write to, the remainder
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is interpreted as data to write to successive memory locations.
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If you need to know the current registers, use
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<tscreen><verb>
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show reg
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</verb></tscreen>
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Alternatively, you can display a single register value by e.g.
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<tscreen><verb>
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print $eax
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</verb></tscreen>
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and modify it by
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<tscreen><verb>
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set $eax new-value
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</verb></tscreen>
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Should you need to call some kernel functions from DDB, simply
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say
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<tscreen><verb>
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call func(arg1, arg2, ...)
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</verb></tscreen>
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The return value will be printed.
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For a <tt>ps(1)</tt> style summary of all running processes, use
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<tscreen><verb>
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ps
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</verb></tscreen>
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Now you have now examined why your kernel failed, and you wish to
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reboot. Remember that, depending on the severity of previous
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malfunctioning, not all parts of the kernel might still be working
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as expected. Perform one of the following actions to shut down and
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reboot your system:
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<tscreen><verb>
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call diediedie()
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</verb></tscreen>
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will cause your kernel to dump core and reboot, so you can
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later analyze the core on a higher level with kgdb. This
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command usually must be followed by another
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`<tt>continue</tt>' statement.
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There is now an alias for this: `<tt>panic</tt>'.
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<tscreen><verb>
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call boot(0)
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</verb></tscreen>
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might be a good way to cleanly shut down the running system, <tt>sync()</tt>
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all disks, and finally reboot. As long as the disk and file system
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interfaces of the kernel are not damaged, this might be a good way
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for an almost clean shutdown.
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<tscreen><verb>
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call cpu_reset()
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</verb></tscreen>
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is the final way out of disaster and almost the same as hitting
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the Big Red Button.
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<sect><heading>Debugging a console driver</heading>
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<p>Since you need a console driver to run DDB on, things are more
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complicated if the console driver itself is flakey. You might
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remember the <tt>options COMCONSOLE</tt> line, and hook up a standard
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terminal onto your first serial port. DDB works on any configured
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console driver, of course it also works on a <tt>COMCONSOLE</tt>.
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