Re: perf: fuzzer leads to trace_kprobe: Could not insert message flood

[Song Liu]

> On Apr 11, 2018, at 5:04 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> 
> 
> * Song Liu <songliubraving@xxxxxx> wrote:
> 
>> 
>> 
>>> On Apr 10, 2018, at 7:48 AM, Vince Weaver <vincent.weaver@xxxxxxxxx> wrote:
>>> 
>>> Author: Song Liu <songliubraving@xxxxxx>
>>> Date:   Wed Dec 6 14:45:15 2017 -0800
>>> 
>>> When running the perf_fuzzer on a current git checkout my logs are flooded 
>>> with messages such as this:
>>> [71487.869077] trace_kprobe: Could not insert probe at unknown+0: -22
>>> [71488.174479] trace_kprobe: Could not insert probe at unknown+0: -22
>>> 
>>> Presumably this is due to the introduction of the perf_kprobe PMU in
>>> 	commit e12f03d7031a977356e3d7b75a68c2185ff8d155
>>> 	Author: Song Liu <songliubraving@xxxxxx>
>>> 	Date:   Wed Dec 6 14:45:15 2017 -0800
>>> 
>>> Is there a way to get this error disabled, or else rate-limited?
>>> 
>>> Vince
>> 
>> Hi Vince, 
>> 
>> Thanks for the report. 
>> 
>> This is a new API that creates probe together with perf_event_open(). Based on 
>> my limited understanding of perf_fuzzer, it doesn't understand this API, and uses 
>> it in an abnormal way. [...]
> 
> Vince's point is valid: the kernel log should not be flooded with pointless 
> messages as a response to user-space ABI uses ...
> 
> Why is there a kernel log message at all, isn't an error returned?
> 
>> [...] I would recommend perf_fuzzer to understand this new API and test it. 
>> [...]
> 
> This bug needs to be fixed: a new API must not effectively DoS fuzzing efforts by 
> spamming the kernel log ...

Yeah, the new API allows non-root user to trigger this message. We should only 
allow root to create kprobe with perf_event_open(). 

On the other hand, do we need to fix this for root? In fact, a simple bash loop 
can create something similar through the text interface (with root):

root@virt-test:~# for x in {0..5} ; do echo p:xx xx+$x >> /sys/kernel/debug/tracing/kprobe_events ; done
-bash: echo: write error: No such file or directory
-bash: echo: write error: No such file or directory
-bash: echo: write error: No such file or directory
-bash: echo: write error: No such file or directory
-bash: echo: write error: No such file or directory
-bash: echo: write error: No such file or directory
root@virt-test:~# dmesg | tail -n 5
[  664.208374] trace_kprobe: Could not insert probe at xx+1: -2
[  664.237882] trace_kprobe: Could not insert probe at xx+2: -2
[  664.268067] trace_kprobe: Could not insert probe at xx+3: -2
[  664.297395] trace_kprobe: Could not insert probe at xx+4: -2
[  664.327614] trace_kprobe: Could not insert probe at xx+5: -2

This happens before the new API is introduced. 

The following patch does capable(CAP_SYS_ADMIN) for perf_kprobe and 
perf_uprobe at an earlier stage, so non-root user cannot trigger 
this error message. Please let me know whether we need to fix this 
for root. 

Thanks,
Song