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

[Shrikanth Hegde]

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.



Seen this on tip/master:
commit 5d3b0106245d467fd5ba0bd9a373a13356684f6e (HEAD -> master, origin/master, origin/HEAD)
Merge: 93368366738f e21279b73ef6
Author: Ingo Molnar <mingo at 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?