linux-kernel - Re: [PATCH bpf] libbpf: eBPF fails on events with auxiliary data

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Message-ID: <82a66615-801a-40cd-a21c-ec82ff43b5d7@linux.ibm.com>
Date: Thu, 31 Jul 2025 09:41:27 +0200
From: Thomas Richter <tmricht@...ux.ibm.com>
To: Jiri Olsa <olsajiri@...il.com>, Ian Rogers <irogers@...gle.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-perf-users@...r.kernel.org, acme@...nel.org, namhyung@...nel.org,
        bpf@...r.kernel.org, agordeev@...ux.ibm.com, gor@...ux.ibm.com,
        sumanthk@...ux.ibm.com, hca@...ux.ibm.com, japo@...ux.ibm.com
Subject: Re: [PATCH bpf] libbpf: eBPF fails on events with auxiliary data

On 7/25/25 16:06, Jiri Olsa wrote:
> On Fri, Jul 25, 2025 at 11:34:05AM +0200, Thomas Richter wrote:
>> On linux-next
>> commit b4c658d4d63d61 ("perf target: Remove uid from target")
>> introduces a regression on s390. In fact the regression exists
>> on all platforms when the event supports auxiliary data gathering.
>>
>> Command
>>    # ./perf record -u 0 -aB --synth=no -- ./perf test -w thloop
>>    [ perf record: Woken up 1 times to write data ]
>>    [ perf record: Captured and wrote 0.011 MB perf.data ]
>>    # ./perf report --stats | grep SAMPLE
>>    #

....

>> -		goto err_out;
>> -	}
> 
> I think this might break existing users depending on this
> 
> could we instead add some 'enable' flag to bpf_perf_event_opts and perf
> would use bpf_program__attach_perf_event_opts function instead?
> 
> jirka
> 

Hi Jiri, Ilya and Ian,

Jiri recommended a more flexible approach and I submitted version 2
https://lore.kernel.org/all/20250728144340.711196-1-tmricht@linux.ibm.com

This version now also passed the eBPF ci-test suite.

I am blind on the eBPF topic and do not have enough knowledge to dig deeper into eBPF.
Please note that this is not an s390 specific issue. It happens on other platforms
when the event supports auxiliary data gathering, i. e. has_aux() returns true.

I would like to ask someone with more eBPF knowledge for some help to drive this further.
Thanks a lot.
-- 
Thomas Richter, Dept 3303, IBM s390 Linux Development, Boeblingen, Germany
--
IBM Deutschland Research & Development GmbH

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