[PATCH] powerpc/kprobes: Pass ppc_inst as a pointer to emulate_step() on ppc32

[Christophe Leroy]

Le 07/06/2021 à 13:34, Naveen N. Rao a écrit :
> Naveen N. Rao wrote:
>> Trying to use a kprobe on ppc32 results in the below splat:
>>     BUG: Unable to handle kernel data access on read at 0x7c0802a6
>>     Faulting instruction address: 0xc002e9f0
>>     Oops: Kernel access of bad area, sig: 11 [#1]
>>     BE PAGE_SIZE=4K PowerPC 44x Platform
>>     Modules linked in:
>>     CPU: 0 PID: 89 Comm: sh Not tainted 5.13.0-rc1-01824-g3a81c0495fdb #7
>>     NIP:  c002e9f0 LR: c0011858 CTR: 00008a47
>>     REGS: c292fd50 TRAP: 0300   Not tainted  (5.13.0-rc1-01824-g3a81c0495fdb)
>>     MSR:  00009000 <EE,ME>  CR: 24002002  XER: 20000000
>>     DEAR: 7c0802a6 ESR: 00000000
>>     <snip>
>>     NIP [c002e9f0] emulate_step+0x28/0x324
>>     LR [c0011858] optinsn_slot+0x128/0x10000
>>     Call Trace:
>>      opt_pre_handler+0x7c/0xb4 (unreliable)
>>      optinsn_slot+0x128/0x10000
>>      ret_from_syscall+0x0/0x28
>>
>> The offending instruction is:
>>     81 24 00 00     lwz     r9,0(r4)
>>
>> Here, we are trying to load the second argument to emulate_step():
>> struct ppc_inst, which is the instruction to be emulated. On ppc64,
>> structures are passed in registers when passed by value. However, per
>> the ppc32 ABI, structures are always passed to functions as pointers.
>> This isn't being adhered to when setting up the call to emulate_step()
>> in the optprobe trampoline. Fix the same.
>>
>> Fixes: eacf4c0202654a ("powerpc: Enable OPTPROBES on PPC32")
>> Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
>> ---
>>  arch/powerpc/kernel/optprobes.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> Christophe,
> Can you confirm if this patch works for you? It would be good if this can go in v5.13.
> 

I'm trying to use kprobes, but I must be missing something. I have tried to follow the exemple in 
kernel's documentation:

  # echo 'p:myprobe do_sys_open dfd=%r3' > /sys/kernel/debug/tracing/kprobe_events

  # echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable

  # cat /sys/kernel/debug/kprobes/list

  c00122e4  k  kretprobe_trampoline+0x0    [OPTIMIZED]
  c018a1b4  k  do_sys_open+0x0    [OPTIMIZED]

  # cat /sys/kernel/debug/tracing/tracing_on

  1

  # cat /sys/kernel/debug/tracing/trace

# tracer: nop
#
# entries-in-buffer/entries-written: 0/0   #P:1
#
#                                _-----=> irqs-off
#                               / _----=> need-resched
#                              | / _---=> hardirq/softirq
#                              || / _--=> preempt-depth
#                              ||| /     delay
#           TASK-PID     CPU#  ||||   TIMESTAMP  FUNCTION
#              | |         |   ||||      |         |



So it looks like I get no event. I can't believe that do_sys_open() is never hit.

This is without your patch, so it should Oops ?


Then it looks like something is locked up somewhere, because I can't do anything else:

  # echo 'p:myprobe2 do_sys_openat2 dfd=%r3' >/sys/kernel/debug/tracing/kprobe_events

  -sh: can't create /sys/kernel/debug/tracing/kprobe_events: Device or resource busy

  # echo '-:myprobe' > /sys/kernel/debug/tracing/kprobe_events

  -sh: can't create /sys/kernel/debug/tracing/kprobe_events: Device or resource busy

  # echo > /sys/kernel/debug/tracing/kprobe_events

  -sh: can't create /sys/kernel/debug/tracing/kprobe_events: Device or resource busy


Christophe