Messages in this thread | | | From | Alexei Starovoitov <> | Date | Mon, 13 Jan 2020 21:17:49 -0800 | Subject | Re: [PATCH v4 2/9] perf/core: open access for CAP_SYS_PERFMON privileged process |
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On Mon, Jan 13, 2020 at 7:25 PM Masami Hiramatsu <mhiramat@kernel.org> wrote: > > 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.
I don't see how enable/disable is any safer than creation. If there are kernel bugs in kprobes the kernel will crash anyway. I think such partial CAP_SYS_PERFMON would be very confusing to the users. CAP_* is about delegation of root privileges to non-root. Delegating some of it is ok, but disallowing creation makes it useless for bpf tracing, so we would need to add another CAP later. Hence I suggest to do it right away instead of breaking sys_perf_even_open() access into two CAPs.
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