linux-kernel - Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf

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Date: Thu, 28 Mar 2024 13:45:56 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Tejun Heo <tj@...nel.org>
Cc: Djalal Harouni <tixxdz@...il.com>, Zefan Li <lizefan.x@...edance.com>, 
	Johannes Weiner <hannes@...xchg.org>, 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@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	"open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf

On Thu, Mar 28, 2024 at 1:02 PM Tejun Heo <tj@...nel.org> wrote:
>
> There's also cgroup.kill which would be useful for similar use cases. We can
> add interface for both but idk. Let's say we have something like the
> following (pardon the bad naming):
>
>   bpf_cgroup_knob_write(struct cgroup *cgrp, char *filename, char *buf)
>
> Would that work? I'm not necessarily in love with the idea or against adding
> separate helpers but the duplication still bothers me a bit.

I liked it.
So filename will be one of cgroup_base_files[].name ?
We probably don't want psi or cgroup1_base_files in there.

>From the verifier pov 2nd arg can be "char *knob__str" and
the verifier will make sure it's a constant NULL terminated string,
so at runtime it will be easier to search cgroup_base_files array.
And 'buf' can be: void *mem, int mem__sz with kfunc doing
run-time validation that there a null there.