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

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Message-ID: <c5a9033b-8f47-4ae8-97ca-75c70bce88cd@oracle.com>
Date: Thu, 1 Aug 2024 09:17:59 +0100
From: Alan Maguire <alan.maguire@...cle.com>
To: Alexis Lothoré <alexis.lothore@...tlin.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

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.


>>> +	/* trigger the syscall on which is attached the tested prog */
>>> +	if (!ASSERT_OK(syscall(__NR_nanosleep, &req, NULL), "nanosleep"))
>>> +		goto cleanup_progs;
>>> +
>>> +	if (!ASSERT_OK(bpf_map__lookup_elem(skel->maps.cg_ids, &key,
>>> +					    sizeof(key), &kcgid, sizeof(kcgid),
>>> +					    0),
>>> +		       "read bpf cgroup id"))
>>> +		goto cleanup_progs;
>>> +
>>
>> ditto here, cg_ids could be a global var cg_id that the bpf prog sets
>> and we check here via skel->bss->cg_id.
> 
> ACK, I'll update this too.
>

Great, thank you!

Alan