linux-kernel - Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for 'perf

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Message-ID: <CAPhsuW73yT+D9HhLhi8pafYZsgT=qsqk5foAwGRTvStnWCZwNA@mail.gmail.com>
Date:   Sun, 20 Aug 2023 03:02:18 -0700
From:   Song Liu <song@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Francis Laniel <flaniel@...ux.microsoft.com>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for
 'perf_kprobe' PMU

On Sun, Aug 20, 2023 at 2:32 AM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
[...]
> > >
> > > perf_event_attr::kprobe_func = "_text";
> > > perf_event_attr::probe_offset = OFFSET;
> > >
> > > Then, it should be able to specify the correct one. Of course you can use
> > > other unique symbols around the target symbol.
> >
> > Trying to catch up with the thread.
>
> Thanks for your reply :)
>
> >
> > Besides the CAP_* issue, we can do this with
> >
> > perf_event_attr::kprobe_func = NULL;
> > perf_event_attr::kprobe_addr = address;
>
> As I pointed, you don't need actual address, instead, you can specify the
> probe point via "unique symbol" + offset.

Technically, this works. But it is weird to me.

> >
> > Then for the CAP_*, I think we should give CAP_PERFMON access to
> > /proc/kallsyms. Would this work?
>
> For the "unique symbol" + offset, you don't need the kallsyms, but need to
> access the System.map or vmlinux image. In this case, we don't need to expand
> the CAP_PERFMON capabilities.

I agree this is not needed in this case. But I wonder whether it makes sense
to give CAP_PERFMON access to /proc/kallsyms. Will this change make
CAP_PERFMON less secure?

Thanks,
Song