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From:   Jiri Olsa <olsajiri@gmail.com>
Date:   Tue, 14 Jun 2022 21:21:53 +0200
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        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>
Subject: Re: [PATCHv2 bpf 3/3] bpf: Force cookies array to follow symbols
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On Thu, Jun 09, 2022 at 11:32:59AM -0700, Andrii Nakryiko wrote:
> 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 :)

ok, I'l resend with the test workaround

jirka