[PATCH 4/5] powerpc/jprobes: Disable preemption when triggered through ftrace

[Naveen N. Rao]

On 2017/09/13 05:05PM, Masami Hiramatsu wrote:
> On Thu, 14 Sep 2017 02:50:35 +0530
> "Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> wrote:
> 
> > KPROBES_SANITY_TEST throws the below splat when CONFIG_PREEMPT is
> > enabled:
> > 
> > [    3.140410] Kprobe smoke test: started
> > [    3.149680] DEBUG_LOCKS_WARN_ON(val > preempt_count())
> > [    3.149684] ------------[ cut here ]------------
> > [    3.149695] WARNING: CPU: 19 PID: 1 at kernel/sched/core.c:3094 preempt_count_sub+0xcc/0x140
> > [    3.149699] Modules linked in:
> > [    3.149705] CPU: 19 PID: 1 Comm: swapper/0 Not tainted 4.13.0-rc7-nnr+ #97
> > [    3.149709] task: c0000000fea80000 task.stack: c0000000feb00000
> > [    3.149713] NIP:  c00000000011d3dc LR: c00000000011d3d8 CTR: c000000000a090d0
> > [    3.149718] REGS: c0000000feb03400 TRAP: 0700   Not tainted  (4.13.0-rc7-nnr+)
> > [    3.149722] MSR:  8000000000021033 <SF,ME,IR,DR,RI,LE>  CR: 28000282  XER: 00000000
> > [    3.149732] CFAR: c00000000015aa18 SOFTE: 0
> > <snip>
> > [    3.149786] NIP [c00000000011d3dc] preempt_count_sub+0xcc/0x140
> > [    3.149790] LR [c00000000011d3d8] preempt_count_sub+0xc8/0x140
> > [    3.149794] Call Trace:
> > [    3.149798] [c0000000feb03680] [c00000000011d3d8] preempt_count_sub+0xc8/0x140 (unreliable)
> > [    3.149804] [c0000000feb036e0] [c000000000046198] kprobe_handler+0x228/0x4b0
> > [    3.149810] [c0000000feb03750] [c0000000000269c8] program_check_exception+0x58/0x3b0
> > [    3.149816] [c0000000feb037c0] [c00000000000903c] program_check_common+0x16c/0x170
> > [    3.149822] --- interrupt: 0 at kprobe_target+0x8/0x20
> >                    LR = init_test_probes+0x248/0x7d0
> > [    3.149829] [c0000000feb03ab0] [c000000000e4f048] kp+0x0/0x80 (unreliable)
> > [    3.149835] [c0000000feb03b10] [c00000000004ea60] livepatch_handler+0x38/0x74
> > [    3.149841] [c0000000feb03ba0] [c000000000d0de54] init_kprobes+0x1d8/0x208
> > [    3.149846] [c0000000feb03c40] [c00000000000daa8] do_one_initcall+0x68/0x1d0
> > [    3.149852] [c0000000feb03d00] [c000000000ce44f0] kernel_init_freeable+0x298/0x374
> > [    3.149857] [c0000000feb03dc0] [c00000000000dd84] kernel_init+0x24/0x160
> > [    3.149863] [c0000000feb03e30] [c00000000000bfec] ret_from_kernel_thread+0x5c/0x70
> > [    3.149867] Instruction dump:
> > [    3.149871] 419effdc 3d22001b 39299240 81290000 2f890000 409effc8 3c82ffcb 3c62ffcb
> > [    3.149879] 3884bc68 3863bc18 4803d5fd 60000000 <0fe00000> 4bffffa8 60000000 60000000
> > [    3.149890] ---[ end trace 432dd46b4ce3d29f ]---
> > [    3.166003] Kprobe smoke test: passed successfully
> > 
> > The issue is that we aren't disabling preemption in
> > kprobe_ftrace_handler(). Disable it.
> 
> Oops, right! Similar patch may need for x86 too.

Indeed, I will send a patch for that.

On a related note, I've been looking into why we have !PREEMPT for 
CONFIG_OPTPROBES. It looks like the primary reason is x86 having to deal 
with replacing multiple instructions. However, that isn't true with arm 
and powerpc. So, does it make sense to move 'depends on !PREEMPT' to the 
x86 code? Are there other scenarios where it might cause issues for 
arm/powerpc?

Thanks!
- Naveen