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

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Date: Tue, 2 Apr 2024 19:20:45 +0100
From: Djalal Harouni <tixxdz@...il.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: Tejun Heo <tj@...nel.org>, 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>, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, bpf@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf

Hello Michal,

On 4/2/24 18:16, Michal Koutný wrote:
> Hello.
> 
> On Wed, Mar 27, 2024 at 11:53:22PM +0100, Djalal Harouni <tixxdz@...il.com> wrote:
>> ...
>> 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.
> 
> Notice that freezer itself is not immediate -- tasks are frozen as if a
> signal (kill(2)) was delivered to them (i.e. returning to userspace).

Thanks yes, I would expect freeze to behave like signal, and if one
wants to block immediately there is the LSM override return. The
selftest attached tries to do exactly that.

> What kind of signals (also kill?) are you talking about for
> illustration?

Could be security signals, reading sensitive files or related to any
operation management, for X reasons this user session should be freezed
or killed.

The kill is an effective defense against fork-bombs as an example.

>> 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.
> 
> It sounds like the signals are related to a particular process. If so
> what is it good for to freeze unrelated processes in the same cgroup?

Today some container/pod operations are performed at bpf level, having
the freeze and kill available is straightforward to perform this.


> I think those answers better clarify why this is needed.

Alright will add those in v2.

> 
> As for the generalization to any cgroup attribute (or kernfs). Can this
> be compared with sysctls -- I see there are helpers to intercept user
> writes but no helpers to affect sysctl values without an outer writer.
> What would justify different approaches between kernfs attributes and
> sysctls (direct writes vs modified writes)?

For generalizing this, haven't thought about it that much. First use
case is to try to get freeze and possibly kill support, and use a common
interface as requested.

Thank you!

> 
> Thanks,
> Michal