linux-kernel - Re: [PATCH bpf-next 1/4] selftests/bpf: convert get_current_cgroup_id_user to test

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Message-ID: <aa7a0a5e-7dc7-4f72-8bfc-75aafc768dbf@bootlin.com>
Date: Thu, 1 Aug 2024 10:57:28 +0200
From: Alexis Lothoré <alexis.lothore@...tlin.com>
To: Alan Maguire <alan.maguire@...cle.com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>,
 Shuah Khan <shuah@...nel.org>
Cc: ebpf@...uxfoundation.org, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>, linux-kernel@...r.kernel.org,
 bpf@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/4] selftests/bpf: convert
 get_current_cgroup_id_user to test_progs

Hello Alan,

On 8/1/24 10:17, Alan Maguire wrote:
> On 31/07/2024 19:53, Alexis Lothoré wrote:
>> Hello Alan,
>>
>> On 7/31/24 19:23, Alan Maguire wrote:
>>> On 31/07/2024 11:38, Alexis Lothoré (eBPF Foundation) wrote:
>>
>> [...]
>>
>>>> +	pid = getpid();
>>>> +	if (!ASSERT_OK(bpf_map__update_elem(skel->maps.pidmap, &key,
>>>> +					    sizeof(key), &pid, sizeof(pid), 0),
>>>> +		       "write pid"))
>>>> +		goto cleanup_progs;
>>>> +
>>>
>>> I think it would be worth using a global variable in the BPF program
>>> my_pid, and setting skel->bss->my_pid here as other more up-to-date
>>> tests do (example progs/test_usdt.c, prog_tests/usdt.c). No need for a
>>> separate map anymore.
>>
>> That sounds like a good improvement, thanks for the hint and the example :) I'll
>> spin a new revision with this, and make sure to use it in my next test
>> conversion patches too when relevant.
>>
>> TBH I am not familiar with global variables usage in ebpf/libbpf, so it is not
>> clear for me when I should prefer it over classic maps. From some quick search I
>> feel like it should be the default choice when needing basic controls
>> knobs/feedback on a bpf program from userspace ? Or maybe it should be used even
>> more broadly by default ?
>>
> 
> Yeah, it's certainly what I use by default, unless I need multiple
> instances of an object. Under the hood, the BPF skeleton creates
> single-element array maps for .bss, .data and .rodata sections which
> contain all the initialized, uninitialized and constant globals in the
> BPF object and mmaps() them so you can read/update the values in
> userspace via skel->bss/skel->data without needing a map-related syscalls.

Thanks a lot for the additional details, much appreciated :)

-- 
Alexis Lothoré, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com