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

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Message-ID: <ZgXallkHApJC-adM@slm.duckdns.org>
Date: Thu, 28 Mar 2024 11:01:10 -1000
From: Tejun Heo <tj@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
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

Hello,

On Thu, Mar 28, 2024 at 01:45:56PM -0700, Alexei Starovoitov wrote:
> 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.

Would it matter? If the user has root perm, they can do whatever with the
files anyway, so I'm not sure why we'd restrict any specific knob. Maybe we
wanna make sure @filename doesn't include '/'? Or is it that you don't want
to go through the usual file name look up?

> 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.

That all sound good.

Thanks.

-- 
tejun