Re: powerpc: BUG: Kernel NULL pointer when running with bcc tools

[Venkat Rao Bagalkote <venkat88@linux.ibm.com>]

On 07/01/26 2:53 pm, Shrikanth Hegde wrote:
> Hello.
>
> While doing some tracing, observed this kernel panic on powerpc.
>
> It is quite easy reproduce this.
> - keep it preempt=full/lazy
> - enable some bcc tools. for example: ./stackcount nohz_balance_exit_idle
> - Run some workload such as "hackbench 10 process 1000 loops"
>
> I remember seeing this across other bcc tools.
>

Hello Shrikanth,


Thanks for the repro steps. I am hitting this issue on the latest 
linux-next kernel.


Regards,

Venkat.

>
>
> Seen this on tip/master:
> commit 5d3b0106245d467fd5ba0bd9a373a13356684f6e (HEAD -> master, 
> origin/master, origin/HEAD)
> Merge: 93368366738f e21279b73ef6
> Author: Ingo Molnar <mingo@kernel.org>
> Date:   Tue Jan 6 09:04:47 2026 +0100
>
>     Merge branch into tip/master: 'x86/sev'
>
>
>
> Panic log:
>  attempted to read user page (6b8) - exploit attempt? (uid: 0)
>  BUG: Kernel NULL pointer dereference on read at 0x000006b8
>  Faulting instruction address: 0xc00000000013cf10
>  Oops: Kernel access of bad area, sig: 11 [#1]
>  CPU: 38 UID: 0 PID: 2938 Comm: hackbench Kdump: loaded Not tainted 
> 6.19.0-rc4+ #326 PREEMPT(full)
>  NIP:  c00000000013cf10 LR: c00000000013ced0 CTR: c0000000004a7f08
>  REGS: c0000000a7f96b60 TRAP: 0300   Not tainted  (6.19.0-rc4+)
>  MSR:  8000000000001033 <SF,ME,IR,DR,RI,LE>  CR: 88022828 XER: 20040000
>  CFAR: c00000000050490c DAR: 00000000000006b8 DSISR: 40000000 IRQMASK: 3
>  GPR00: c00000000013ced0 c0000000a7f96e00 c000000001c38100 
> 0000000000000000
>  GPR04: 00007fffda01fea8 0000000000000008 0000000000000000 
> 00007fff94f05e84
>  GPR08: c0000000a7f90000 0000000000000000 00007fffda0205d0 
> 0000000000004000
>  GPR12: c000000002dbdddc c0000017fd891c80 c0000000001e08f4 
> 0000000000000000
>  GPR16: 0000000000000000 0000000000000000 0000000000000000 
> 00000d9842ecbda0
>  GPR20: c0000000c5c54a00 0000000000000026 0000000000000000 
> 0000000000000000
>  GPR24: 0000000000000001 fffffffffffffe00 c0000000c5c54a00 
> 000ffffffffffff8
>  GPR28: 00007fffffffffdf 0000000000000000 00007fffda01fea0 
> c0000000a7f96ef0
>  NIP [c00000000013cf10] perf_callchain_user_64+0x1b0/0x4f0
>  LR [c00000000013ced0] perf_callchain_user_64+0x170/0x4f0
>  Call Trace:
>  perf_callchain_user_64+0x170/0x4f0 (unreliable)
>  perf_callchain_user+0x20/0x3c
>  get_perf_callchain+0x1b0/0x3dc
>  bpf_get_stackid+0x94/0xec
>  bpf_prog_1ca8f9c0bc38eaa6_trace_count+0x70/0x1bc
>  trace_call_bpf+0x124/0x3c4
>  kprobe_perf_func+0x54/0x2f8
>  kprobe_ftrace_handler+0x1dc/0x298
>  ftrace_regs_call+0x4/0xa0
>  nohz_balance_exit_idle+0x10/0x120
>  nohz_balancer_kick+0x50/0x3e4
>  sched_tick+0x140/0x334
>  update_process_times+0xf0/0x144
>  tick_nohz_handler+0xc4/0x274
>  __hrtimer_run_queues+0x1c4/0x480
>  hrtimer_interrupt+0x12c/0x30c
>  timer_interrupt+0x140/0x394
>  __replay_soft_interrupts+0xa0/0x154
>  arch_local_irq_restore.part.0+0x1ac/0x224
>  _raw_spin_unlock_irqrestore+0x50/0xa0
>  try_to_wake_up+0x3e8/0xa58
>  wake_up_q+0x88/0xe4
>  rwsem_wake.isra.0+0x98/0xd8
>  up_write+0xa8/0xb4
>  unlink_file_vma_batch_process+0xc8/0xf0
>  unlink_file_vma_batch_add+0x5c/0xa4
>  free_pgtables+0x110/0x394
>  exit_mmap+0x1a0/0x5a8
>  __mmput+0x64/0x194
>  exit_mm+0xe0/0x170
>  do_exit+0x21c/0x5d0
>  do_group_exit+0x4c/0xc0
>  pid_child_should_wake+0x0/0x7c
>  system_call_exception+0x128/0x390
>  system_call_vectored_common+0x15c/0x2ec
>
>
>
> Looks like venkat had reported similar one earlier.
> https://lore.kernel.org/all/49cb29fc-003c-422a-98bd-dee8f13aa0b4@linux.ibm.com/ 
>
>
> Venkat, can you give above mentioned steps a try on linux-next latest?
>