linux-kernel - Re: [PATCH] tracepoint: Add tracepoint_probe_register_may

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Message-ID: <CAEf4BzYRxRW8qR3oENuVEMBYtcvK0bUDEkoq+e4TRT5Hh0pV_Q@mail.gmail.com>
Date:   Wed, 7 Jul 2021 17:23:54 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        syzbot+721aa903751db87aa244@...kaller.appspotmail.com,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Ingo Molnar <mingo@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH] tracepoint: Add tracepoint_probe_register_may_exist() for
 BPF tracing

On Wed, Jul 7, 2021 at 5:05 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 7 Jul 2021 16:49:26 -0700
> Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
>
> > As for why the user might need that, it's up to the user and I don't
> > want to speculate because it will always sound contrived without a
> > specific production use case. But people are very creative and we try
> > not to dictate how and what can be done if it doesn't break any
> > fundamental assumption and safety.
>
> I guess it doesn't matter, because if they try to do it, the second
> attachment will simply fail to attach.
>

But not for the kprobe case.

And it might not always be possible to know that the same BPF program
is being attached. It could be attached by different processes that
re-use pinned program (without being aware of each other). Or it could
be done from some generic library that just accepts prog_fd and
doesn't really know the exact BPF program and whether it was already
attached.

Not sure why it doesn't matter that attachment will fail where it is
expected to succeed. The question is rather why such restriction?

> -- Steve