This is an archive of past FreeBSD releases; it's part of the FreeBSD Documentation Archive.
Once a dump has been obtained, getting useful information out of the dump is relatively easy for simple problems. Before launching into the internals of gdb to debug the crash dump, locate the debug version of your kernel (normally called kernel.debug) and the path to the source files used to build your kernel (normally /usr/obj/usr/src/sys/KERNCONF). With those two pieces of info, let the debugging commence!
# cd /usr/obj/usr/src/sys/KERNCONF # gdb -k /boot/kernel/kernel.debug /var/crash/vmcore.0and voila! You can debug the crash dump using the kernel sources just like you can for any other program.
Here is a script log of a gdb session illustrating the procedure. Long lines have been folded to improve readability, and the lines are numbered for reference. Despite this, it is 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:# cd /sys/compile/URIAH
3:# gdb -k 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:# exit
78:exit
79:
80:Script done on Fri Dec 30 23:18:04 1994
Comments to the above script:
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.
This is the location of function trap() in the stack
trace.
Force usage of a new stack frame; this is no longer necessary. The stack frames are supposed to point to the right locations now, even in case of a trap. From looking at the code in source line 403, there is a high probability that either the pointer access for ``tp'' was messed up, or the array access was out of bounds.
The pointer looks suspicious, but happens to be a valid address.
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.)
This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.
For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.