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Old 10-11-2010, 09:15 AM
Hu Tao
Default Another backtrace problem when running into exception stack(x86_64)

Hi Dave,

When we run into exception stack(test module attached) and take a
dumpfile by virsh dump, it seems crash doesn't show backtrace

crash shows:

crash> bt -a
PID: 1115 TASK: ffff88001e082d60 CPU: 0 COMMAND: "bash"
#0 [ffff88001f8a3c58] schedule at ffffffff813e9a41
#1 [ffff88001f8a3c60] _raw_spin_unlock at ffffffff813eb486
#2 [ffff88001f8a3c70] vt_console_print at ffffffff8123fc17
#3 [ffff88001f8a3cf0] _raw_spin_unlock_irqrestore at ffffffff813eb4c3
#4 [ffff88001f8a3d10] _raw_spin_unlock_irqrestore at ffffffff813eb4c3
#5 [ffff88001f8a3d20] release_console_sem at ffffffff8103c6a6
#6 [ffff88001f8a3d50] vprintk at ffffffff8103cca0
#7 [ffff88001f8a3df0] printk at ffffffff813e90e4
#8 [ffff88001f8a3e50] __handle_sysrq at ffffffff8124585d
#9 [ffff88001f8a3e90] write_sysrq_trigger at ffffffff8124593f
#10 [ffff88001f8a3eb0] proc_reg_write at ffffffff8112b9f0
#11 [ffff88001f8a3f00] vfs_write at ffffffff810e9101
#12 [ffff88001f8a3f40] sys_write at ffffffff810e9213
#13 [ffff88001f8a3f80] system_call_fastpath at ffffffff81002a82
RIP: 00007f26509b2200 RSP: 00007fff188c9900 RFLAGS: 00010206
RAX: 0000000000000001 RBX: ffffffff81002a82 RCX: 0000000000000400
RDX: 0000000000000002 RSI: 00007f2651293000 RDI: 0000000000000001
RBP: 00007f2651293000 R8: 000000000000000a R9: 00007f2651296700
R10: 00000000ffffffff R11: 0000000000000246 R12: 00007f2650c57780
R13: 0000000000000002 R14: 0000000000000002 R15: 0000000000000000
ORIG_RAX: 0000000000000001 CS: 0033 SS: 002b

The module's output:

[root@localhost ~]# insmod km_kprobe.ko
kprobe registered
[root@localhost ~]# echo q > /proc/sysrq-trigger
SysRq : Show clockevent devices & pending hrtimers (no others)
post_handler will loop.
Pid: 1116, comm: bash Not tainted 2.6.36-rc6-00140-g183b469-dirty #5
Call Trace:
<#DB> ffff880001a09da0 [<ffffffff8124552e>] ? sysrq_handle_show_timers+0x1/0xb
ffff880001a09dc0 [<ffffffffa00ea017>] handler_post+0x17/0x1c [km_kprobe]
ffff880001a09dd0 [<ffffffff813edb44>] kprobe_exceptions_notify+0x376/0x460
ffff880001a09df0 [<ffffffff8124552d>] ? sysrq_handle_show_timers+0x0/0xb
ffff880001a09e00 [<ffffffff813ed971>] ? kprobe_exceptions_notify+0x1a3/0x460
ffff880001a09e40 [<ffffffff813ee7e7>] notifier_call_chain+0x32/0x5e
ffff880001a09e80 [<ffffffff813ee850>] __atomic_notifier_call_chain+0x3d/0x6b
ffff880001a09ec0 [<ffffffff813ee88d>] atomic_notifier_call_chain+0xf/0x11
ffff880001a09ed0 [<ffffffff813ee8bd>] notify_die+0x2e/0x30
ffff880001a09f00 [<ffffffff813ec035>] do_debug+0x93/0x156
ffff880001a09f50 [<ffffffff813ebbb8>] debug+0x28/0x40
ffff880001a09fd8 [<ffffffff8124552e>] ? sysrq_handle_show_timers+0x1/0xb
<<EOE>> ffff88001d99fe50 [<ffffffff8124585d>] ? __handle_sysrq+0xba/0x156
ffff88001d99fe90 [<ffffffff8124593f>] write_sysrq_trigger+0x46/0x4e
ffff88001d99fea0 [<ffffffff812458f9>] ? write_sysrq_trigger+0x0/0x4e
ffff88001d99feb0 [<ffffffff8112b9f0>] proc_reg_write+0x8d/0xac
ffff88001d99ff00 [<ffffffff810e9101>] vfs_write+0xa9/0x105
ffff88001d99ff40 [<ffffffff810e9213>] sys_write+0x45/0x69
ffff88001d99ff80 [<ffffffff81002a82>] system_call_fastpath+0x16/0x1b

Notice that two backtrace output differ from <<EOE>>, crash doesn't show
exception stack frames. Although crash does check exception stack
against sp, but the problem seems to be we can't get right sp value
(ffff880001a09da0 in this example) from the dump file. Am I right? Is
there any we can get right sp value from dump file?

I did a manually check on these stack frames using crash on the dump
file(rd then sym), and the stack contents are the same as the module's

BTW, bt -S ffff880001a09da0 causes crash to seg fault.

Hu Tao
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kprobes.h>
#include <linux/sched.h>

static struct kprobe kp;

void handler_post(struct kprobe *p, struct pt_regs *regs, unsigned long flags)
printk("post_handler will loop.
for (;;

static int __init kprobe_init(void)
int ret;
kp.post_handler = handler_post;
kp.symbol_name = "sysrq_handle_show_timers";

if ((ret = register_kprobe(&kp) < 0)) {
printk("register_kprobe failed, returned %d
", ret);
return -1;
printk("kprobe registered
return 0;

static void __exit kprobe_exit(void)
printk("kprobe unregistered

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