Re: [PATCH bpf-next v1 2/3] bpf: add bpf_relay_output kfunc

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Philo Lu <lulie@linux.alibaba.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	 Daniel Borkmann <daniel@iogearbox.net>,
	John Fastabend <john.fastabend@gmail.com>,
	 Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>,
	 Yonghong Song <yonghong.song@linux.dev>,
	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>,
	Joanne Koong <joannelkoong@gmail.com>,
	 Yafang Shao <laoar.shao@gmail.com>,
	Kui-Feng Lee <kuifeng@meta.com>,
	 Hou Tao <houtao@huaweicloud.com>,
	Shung-Hsi Yu <shung-hsi.yu@suse.com>,
	 Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	Dust Li <dust.li@linux.alibaba.com>,
	 "D. Wythe" <alibuda@linux.alibaba.com>,
	guwen@linux.alibaba.com,  hengqi@linux.alibaba.com
Subject: Re: [PATCH bpf-next v1 2/3] bpf: add bpf_relay_output kfunc
Date: Wed, 27 Dec 2023 10:07:05 -0800	[thread overview]
Message-ID: <CAADnVQLZS4Do63myRNqL-1ho7SBm+fc2si9i3Km5spkxRtzKfw@mail.gmail.com> (raw)
In-Reply-To: <20231227100130.84501-3-lulie@linux.alibaba.com>

On Wed, Dec 27, 2023 at 2:01 AM Philo Lu <lulie@linux.alibaba.com> wrote:
>
> +__bpf_kfunc int bpf_relay_output(struct bpf_map *map,
> +                                  void *data, u64 data__sz, u32 flags)
> +{
> +       struct bpf_relay_map *rmap;
> +
> +       /* not support any flag now */
> +       if (unlikely(!map || flags))
> +               return -EINVAL;
> +
> +       rmap = container_of(map, struct bpf_relay_map, map);
> +       if (!rmap->relay_chan->has_base_filename)
> +               return -ENOENT;
> +
> +       relay_write(rmap->relay_chan, data, data__sz);
> +       return 0;

This just opens a can of worms.
Above is not nmi safe. relay_write() can be used only out of
known context which effectively makes it unusable out of bpf tracing
progs that can kprobe attach anywhere in the kernel.
perf_event buffer is the only sure way to deliver events to user
space with overwrite.
bpf ringbuf is a best effort due to
if (in_nmi()) if (!spin_trylock_irqsave

Sorry, but it's a nack to allow bpf progs interface with relay.