Received-SPF: pass (google.com: domain of linux-kernel+bounces-123499-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Precedence: bulk
MIME-Version: 1.0
References: <20240327-ccb56fc7a6e80136db80876c@djalal> <20240327225334.58474-1-tixxdz@gmail.com>
 <ZgWnPZtwBYfHEFzf@slm.duckdns.org> <CAADnVQK6BUGZFCATD8Ejcfob5sKK-b8HUD_4o8Q6s9FM72L4iQ@mail.gmail.com>
 <ZgWv19ySvoACAll4@slm.duckdns.org>
In-Reply-To: <ZgWv19ySvoACAll4@slm.duckdns.org>
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Thu, 28 Mar 2024 12:46:03 -0700
Message-ID: <CAADnVQLhWDcX-7XCdo-W=jthU=9iPqODwrE6c9fvU8sfAJ5ARg@mail.gmail.com>
Subject: Re: [RFC PATCH bpf-next 0/3] bpf: freeze a task cgroup from bpf
To: Tejun Heo <tj@kernel.org>
Cc: Djalal Harouni <tixxdz@gmail.com>, Zefan Li <lizefan.x@bytedance.com>, 
	Johannes Weiner <hannes@cmpxchg.org>, Alexei Starovoitov <ast@kernel.org>, 
	Daniel Borkmann <daniel@iogearbox.net>, Andrii Nakryiko <andrii@kernel.org>, 
	Martin KaFai Lau <martin.lau@linux.dev>, Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>, 
	Yonghong Song <yonghong.song@linux.dev>, John Fastabend <john.fastabend@gmail.com>, 
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, 
	Jiri Olsa <jolsa@kernel.org>, Mykola Lysenko <mykolal@fb.com>, Shuah Khan <shuah@kernel.org>, 
	LKML <linux-kernel@vger.kernel.org>, 
	"open list:CONTROL GROUP (CGROUP)" <cgroups@vger.kernel.org>, bpf <bpf@vger.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Thu, Mar 28, 2024 at 10:58=E2=80=AFAM Tejun Heo <tj@kernel.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 someth=
ing
> > > which already has a defined userspace interface. Would it be better t=
o 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.