Subject: Re: BUG: unable to handle kernel NULL pointer dereference in qfq_dequeue()

From: Cong Wang <[email-masked]>
Date: Thu, 11 Oct 2012 16:38:29 +0800

On Mon, 2012-10-08 at 17:15 +0800, Cong Wang wrote:
> Hi, all,
> 
> We got the following kernel crash on RHEL6 and I confirmed upstream has
> the same problem (I didn't save this kernel log though):

Ok, I got the backtrace of the latest kernel, see below. Seems
qfq_slot_scan() in qfq_dequeue() returns something bad, 'cl' becomes
'0x10'.

(gdb) bt
#0  delay_tsc (__loops=<optimized out>) at arch/x86/lib/delay.c:69
#1  0xffffffff8143f4c9 in __delay (loops=<optimized out>) at
arch/x86/lib/delay.c:112
#2  0xffffffff8143f4ef in __const_udelay (xloops=<optimized out>) at
arch/x86/lib/delay.c:126
#3  0xffffffff81914536 in panic (fmt=<optimized out>) at
kernel/panic.c:182
#4  0xffffffff8193de42 in oops_end (flags=582, regs=0xffff88007d203d08,
signr=9) at arch/x86/kernel/dumpstack.c:248
#5  0xffffffff81913698 in no_context (regs=0xffff88007d203d08,
error_code=<optimized out>, address=16, signal=<optimized out>, 
    si_code=<optimized out>) at arch/x86/mm/fault.c:690
#6  0xffffffff81913885 in __bad_area_nosemaphore
(regs=0xffff88007d203d08, error_code=0, address=16, si_code=196609)
    at arch/x86/mm/fault.c:768
#7  0xffffffff819138b9 in bad_area_nosemaphore (regs=<optimized out>,
error_code=<optimized out>, address=<optimized out>)
    at arch/x86/mm/fault.c:775
#8  0xffffffff81940803 in __do_page_fault (regs=0xffff88007d203d08,
error_code=0) at arch/x86/mm/fault.c:1105
#9  0xffffffff81940883 in do_page_fault (regs=<optimized out>,
error_code=<optimized out>) at arch/x86/mm/fault.c:1237
#10 0xffffffff81940129 in do_async_page_fault (regs=<optimized out>,
error_code=<optimized out>) at arch/x86/kernel/kvm.c:246
#11 <signal handler called>
#12 0xffffffff8176c72c in qfq_round_down (shift=41, ts=Cannot access
memory at address 0x10
) at net/sched/sch_qfq.c:575
#13 qfq_dequeue (sch=0xffff8800757542c0) at net/sched/sch_qfq.c:819
#14 0xffffffff8175c0cb in cbq_dequeue_prio (prio=1,
sch=0xffff880076b15d00) at net/sched/sch_cbq.c:851
#15 cbq_dequeue_1 (sch=0xffff880076b15d00) at net/sched/sch_cbq.c:934
#16 cbq_dequeue (sch=0xffff880076b15d00) at net/sched/sch_cbq.c:973
#17 0xffffffff81750ff7 in dequeue_skb (q=0xffff880076b15d00) at
net/sched/sch_generic.c:69
#18 qdisc_restart (q=0xffff880076b15d00) at net/sched/sch_generic.c:178
#19 __qdisc_run (q=0xffff880076b15d00) at net/sched/sch_generic.c:193
#20 0xffffffff81725625 in qdisc_run (q=0xffff880076b15d00) at
include/net/pkt_sched.h:99
#21 net_tx_action (h=<optimized out>) at net/core/dev.c:3070
#22 0xffffffff81057069 in __do_softirq () at kernel/softirq.c:247
#23 0xffffffff81945b3c in ?? () at arch/x86/kernel/entry_64.S:1353
#24 0xffffffff81003f36 in do_softirq () at arch/x86/kernel/irq_64.c:106
#25 0xffffffff810572f8 in invoke_softirq () at kernel/softirq.c:329
#26 irq_exit () at kernel/softirq.c:348
#27 0xffffffff81081dd3 in scheduler_ipi () at kernel/sched/core.c:1355
#28 0xffffffff8101fee4 in smp_reschedule_interrupt (regs=<optimized
out>) at arch/x86/kernel/smp.c:256
#29 <signal handler called>
#30 0xffffffffffffff02 in ?? ()
Cannot access memory at address 0x246