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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Thu, 9 Jun 2022 11:32:59 -0700
Message-ID: <CAEf4BzakYUnM3ZNgRbix=Z4bnPpzGiazAffMVh239476qH_c7A@mail.gmail.com>
Subject: Re: [PATCHv2 bpf 3/3] bpf: Force cookies array to follow symbols sorting
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Jiri Olsa <olsajiri@gmail.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, lkml <linux-kernel@vger.kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Masami Hiramatsu <mhiramat@kernel.org>
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On Wed, Jun 8, 2022 at 9:08 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Wed, 8 Jun 2022 08:59:50 -0700
> Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>
> > Would it be possible to preprocess ftrace_pages to remove such invalid
> > records (so that by the time we have to report
> > available_filter_functions there are no invalid records)? Or that data
> > is read-only when kernel is running?
>
> It's possible, but will be time consuming (slow down boot up) and racy. In
> other words, I didn't feel it was worth it.
>
> We can add it. How much of an issue is it to have these place holders for
> you? Currently, I only see it causes issues with tests. Is it really an
> issue for use cases?

I have the tool (retsnoop) that uses available_filter_functions, I'll
have to update it to ignore such entries. It's a small inconvenience,
once you know about this change, but multiply that for multiple users
that use available_filter_functions for some sort of generic tooling
doing kprobes/tracing, and it adds up. So while it's not horrible,
ideally user-visible data shouldn't have non-usable placeholders.

How much slowdown would you expect on start up? Not clear what would
be racy about this start up preprocessing, but I believe you.

So in summary, it's not the end of the world, but as a user I'd prefer
not to know about this quirk, of course :)

>
> -- Steve