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Date:   Tue, 14 Jan 2020 12:25:06 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Alexey Budankov <alexey.budankov@linux.intel.com>
Cc:     arnaldo.melo@gmail.com, Song Liu <songliubraving@fb.com>,
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Subject: Re: [PATCH v4 2/9] perf/core: open access for CAP_SYS_PERFMON
 privileged process
Message-Id: <20200114122506.3cf442dc189a649d4736f86e@kernel.org>
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On Sat, 11 Jan 2020 12:57:18 +0300
Alexey Budankov <alexey.budankov@linux.intel.com> wrote:

> 
> On 11.01.2020 3:35, arnaldo.melo@gmail.com wrote:

> > Message-ID: <A7F0BF73-9189-44BA-9264-C88F2F51CBF3@kernel.org>
> > 
> > On January 10, 2020 9:23:27 PM GMT-03:00, Song Liu <songliubraving@fb.com> wrote:
> >>
> >>
> >>> On Jan 10, 2020, at 3:47 PM, Masami Hiramatsu <mhiramat@kernel.org>
> >> wrote:
> >>>
> >>> On Fri, 10 Jan 2020 13:45:31 -0300
> >>> Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
> >>>
> >>>> Em Sat, Jan 11, 2020 at 12:52:13AM +0900, Masami Hiramatsu escreveu:
> >>>>> On Fri, 10 Jan 2020 15:02:34 +0100 Peter Zijlstra
> >> <peterz@infradead.org> wrote:
> >>>>>> Again, this only allows attaching to previously created kprobes,
> >> it does
> >>>>>> not allow creating kprobes, right?
> >>>>
> >>>>>> That is; I don't think CAP_SYS_PERFMON should be allowed to create
> >>>>>> kprobes.
> >>>>
> >>>>>> As might be clear; I don't actually know what the user-ABI is for
> >>>>>> creating kprobes.
> >>>>
> >>>>> There are 2 ABIs nowadays, ftrace and ebpf. perf-probe uses ftrace
> >> interface to
> >>>>> define new kprobe events, and those events are treated as
> >> completely same as
> >>>>> tracepoint events. On the other hand, ebpf tries to define new
> >> probe event
> >>>>> via perf_event interface. Above one is that interface. IOW, it
> >> creates new kprobe.
> >>>>
> >>>> Masami, any plans to make 'perf probe' use the perf_event_open()
> >>>> interface for creating kprobes/uprobes?
> >>>
> >>> Would you mean perf probe to switch to perf_event_open()?
> >>> No, perf probe is for setting up the ftrace probe events. I think we
> >> can add an
> >>> option to use perf_event_open(). But current kprobe creation from
> >> perf_event_open()
> >>> is separated from ftrace by design.
> >>
> >> I guess we can extend event parser to understand kprobe directly.
> >> Instead of
> >>
> >> 	perf probe kernel_func
> >> 	perf stat/record -e probe:kernel_func ...
> >>
> >> We can just do 
> >>
> >> 	perf stat/record -e kprobe:kernel_func ...
> > 
> > 
> > You took the words from my mouth, exactly, that is a perfect use case, an alternative to the 'perf probe' one of making a disabled event that then gets activated via record/stat/trace, in many cases it's better, removes the explicit probe setup case.
> 
> Arnaldo, Masami, Song,
> 
> What do you think about making this also open to CAP_SYS_PERFMON privileged processes?
> Could you please also review and comment on patch 5/9 for bpf_trace.c?

As we talked at RFC series of CAP_SYS_TRACING last year, I just expected
to open it for enabling/disabling kprobes, not for creation.

If we can accept user who has no admin priviledge but the CAP_SYS_PERFMON,
to shoot their foot by their own risk, I'm OK to allow it. (Even though,
it should check the max number of probes to be created by something like
ulimit)
I think nowadays we have fixed all such kernel crash problems on x86,
but not sure for other archs, especially on the devices I can not reach.
I need more help to stabilize it.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>