linux-kernel - Re: kernel panic: Attempted to kill init!

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Date:   Fri, 30 Dec 2022 08:55:15 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Hao Sun <sunhao.th@...il.com>
Cc:     Yonghong Song <yhs@...a.com>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kernel panic: Attempted to kill init!

On Fri, Dec 30, 2022 at 1:54 AM Hao Sun <sunhao.th@...il.com> wrote:
>
>
>
> > On 28 Dec 2022, at 2:35 PM, Yonghong Song <yhs@...a.com> wrote:
> >
> >
> >
> > On 12/21/22 8:35 PM, Hao Sun wrote:
> >> Hi,
> >> This crash can be triggered by executing the C reproducer for
> >> multiple times, which just keep loading the following prog as
> >> raw tracepoint into kmem_cache_free().
> >> The prog send SIGSEGV to current via bpf_send_signal_thread(),
> >> after load this, whoever tries to free mem would trigger this,
> >> kernel crashed when this happens to init.
> >> Seems we should filter init out in bpf_send_signal_common() by
> >> is_global_init(current), or maybe we should check this in the
> >> verifier?
> >
> > The helper is just to send a particular signal to *current*
> > thread. In typical use case, it is never a good idea to send
> > the signal to a *random* thread. In certain cases, maybe user
> > indeed wants to send the signal to init thread to observe
> > something. Note that such destructive side effect already
> > exists in the bpf land. For example, for a xdp program,
> > it could drop all packets to make machine not responsive
> > to ssh etc. Therefore, I recommend to keep the existing
> > bpf_send_signal_common() helper behavior.
>
> Sound the two are different cases. Not responsive in XDP seems like
> an intended behaviour, panic caused by killing init is buggy. If the
> last thread of global init was killed, kernel panic immediately.

I don't get it. How was it possible that this prog was
executed with current == pid 1 ?