Re: [PATCH 2/2] kprobe: fix: Add ftrace_ops_assist_func to kprobe blacklist

[Francis Deslauriers <francis.deslauriers@efficios.com>]

Hi Steven,

I completely forgot about this issue until recently when I encountered it again.
Instrumenting the ftrace_ops_assist_func symbol and some other symbol
seems to be causing problems.

Placing kretprobes like in the following configuration crashes my
kernel (4.16.0-rc5) on a Qemu/KVM virtual machine:

config 1:
echo "r:event_1 __fdget" >> kprobe_events
echo "r:event_2 ftrace_ops_assist_func" >> kprobe_events

config 2:
echo "r:event_1 __fdget_pos" >> kprobe_events
echo "r:event_2 ftrace_ops_assist_func" >> kprobe_events

config 3:
echo 'r:event_1 arch_dup_task_struct' >> kprobe_events
echo 'r:event_2 ftrace_ops_assist_func' >> kprobe_events

config 4:
echo 'r:event_1 sys_open' >> kprobe_events
echo 'r:event_2 ftrace_ops_assist_func' >> kprobe_events

Here is my kernel config [1]:

In a previous email [2], you mentioned that you would like to add the
ftrace-related symbols to a section to un-blacklist them all at once
on demand but wanted to discuss it at Linux Plumbers. Do you still
think that it's the right approach?

I can easily test any patch regarding this issue.

[1] http://paste.ubuntu.com/p/BJWvgMnW8z/
[2] https://lkml.org/lkml/2017/7/14/568

Thank you,

2017-07-14 14:29 GMT-04:00 Steven Rostedt <rostedt@goodmis.org>:
> On Fri, 14 Jul 2017 10:58:35 -0400
> Francis Deslauriers <francis.deslauriers@efficios.com> wrote:
>
>> This function is called when a kprobe is hit. Thus it should be
>> blacklisted to prevent kprobe to be triggered by kprobes.
>>
>> Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
>> ---
>>  kernel/trace/ftrace.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
>> index b308be3..c473d9b 100644
>> --- a/kernel/trace/ftrace.c
>> +++ b/kernel/trace/ftrace.c
>> @@ -36,6 +36,7 @@
>>
>>  #include <trace/events/sched.h>
>>
>> +#include <asm/kprobes.h>
>>  #include <asm/sections.h>
>>  #include <asm/setup.h>
>>
>> @@ -5739,6 +5740,7 @@ static void ftrace_ops_assist_func(unsigned long ip, unsigned long parent_ip,
>>       preempt_enable_notrace();
>>       trace_clear_recursion(bit);
>>  }
>> +NOKPROBE_SYMBOL(ftrace_ops_assist_func);
>
> Continuing from what I said in the other email, this is fixing a
> symptom and not the problem. The real fix will be much more involved. I
> have a good idea on how to accomplish it too.
>
> -- Steve
>
>
>>
>>  /**
>>   * ftrace_ops_get_func - get the function a trampoline should call
>



-- 
Francis Deslauriers
Software developer
EfficiOS inc.