linux-kernel - Re: [PATCH] kprobes: Print an error if probe is rejected

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Message-Id: <1623419180.o4u5xf72jm.naveen@linux.ibm.com>
Date:   Fri, 11 Jun 2021 19:25:38 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] kprobes: Print an error if probe is rejected

Hi Masami,
Thanks for the review.


Masami Hiramatsu wrote:
> Hi Naveen,
> 
> On Thu, 10 Jun 2021 14:26:17 +0530
> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:
> 
>> When probing at different locations in the kernel, it is not always
>> evident if the location can be probed or not. As an example:
>> 
>>     $ perf probe __radix__flush_tlb_range:35
>>     Failed to write event: Invalid argument
>>       Error: Failed to add events.
>> 
>> The probed line above is:
>>      35         if (!mmu_has_feature(MMU_FTR_GTSE) && type == FLUSH_TYPE_GLOBAL) {
>> 
>> This ends up trying to probe on BUILD_BUG_ON(), which is rejected.
>> However, the user receives no indication at all as to why the probe
>> failed. Print an error in such cases so that it is clear that the probe
>> was rejected.
> 
> Hmm, Nack for this way, but I understand that is a problem.
> If you got the error in perf probe, which uses ftrace dynamic-event interface.
> In that case, the errors should not be output in the dmesg, but are reported
> via error_log in tracefs.

That would be a nice thing to add to perf, but I don't see why this 
should be a either/or. I still think it is good to have the core kprobe 
infrastructure print such errors in the kernel log. It is easier to look 
up such error strings in the kernel source to understand why a probe was 
rejected.

We also have perf_event_open() as an interface to add probes, and I 
don't think it would be helpful to require all tools to utilize the 
error log from tracefs for this purpose.


- Naveen