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

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Message-ID: <CAADnVQLhWDcX-7XCdo-W=jthU=9iPqODwrE6c9fvU8sfAJ5ARg@mail.gmail.com>
Date: Thu, 28 Mar 2024 12:46:03 -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 10:58 AM Tejun Heo <tj@...nel.org> wrote:
>
> Hello, Alexei.
>
> On Thu, Mar 28, 2024 at 10:32:24AM -0700, Alexei Starovoitov wrote:
> > > 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.
>
> Oh, I didn't know. Why is that?
>
> > The seq_file would be passed/pinned by user space?
>
> Would it work if it's just "open this file, write this and then close it"?

Continuing discussion...
To use kernel_file_open() it would need path, inode, cred.
None of that is available now.
Allocating all these structures just to wrap a cgroup pointer
feels like overkill.
Of course, it would solve the need to introduce other
cgroup apis that are already available via text based cgroupfs
read/write. So there are pros and cons in both approaches.
Maybe the 3rd option would be to expose:
cgroup_lock() as a special blend of acquire plus lock.
Then there will be no need for bpf_task_freeze_cgroup() with task
argument. Instead cgroup_freeze() will be such kfunc that
takes cgroup argument and the verifier will check that
cgroup was acquired/locked.
Sort-of what we check to access bpf_rb_root.