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

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Message-ID: <CAADnVQK6BUGZFCATD8Ejcfob5sKK-b8HUD_4o8Q6s9FM72L4iQ@mail.gmail.com>
Date: Thu, 28 Mar 2024 10:32:24 -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 10:22 AM Tejun Heo <tj@...nel.org> wrote:
>
> Hello, Djalal.
>
> On Wed, Mar 27, 2024 at 11:53:22PM +0100, Djalal Harouni wrote:
> > This patch series adds support to freeze the task cgroup hierarchy
> > that is on a default cgroup v2 without going through kernfs interface.
> >
> > For some cases we want to freeze the cgroup of a task based on some
> > signals, doing so from bpf is better than user space which could be
> > too late.
> >
> > Planned users of this feature are: tetragon and systemd when freezing
> > a cgroup hierarchy that could be a K8s pod, container, system service
> > or a user session.
> >
> > Patch 1: cgroup: add cgroup_freeze_no_kn() to freeze a cgroup from bpf
> > Patch 2: bpf: add bpf_task_freeze_cgroup() to freeze the cgroup of a task
> > Patch 3: selftests/bpf: add selftest for bpf_task_freeze_cgroup
>
> It bothers me a bit that it's adding a dedicated interface for something
> which already has a defined userspace interface. Would it be better to have
> kfunc wrappers for kernel_read() and kernel_write()?

How would that look ?
prog cannot and shouldn't open a file.
The seq_file would be passed/pinned by user space?