Messages in this thread | | | From | Song Liu <> | Date | Sun, 20 Aug 2023 23:09:22 -0700 | Subject | Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for 'perf_kprobe' PMU |
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On Sun, Aug 20, 2023 at 6:16 AM Masami Hiramatsu <mhiramat@kernel.org> wrote: > > On Sun, 20 Aug 2023 03:02:18 -0700 > Song Liu <song@kernel.org> wrote: > > > On Sun, Aug 20, 2023 at 2:32 AM Masami Hiramatsu <mhiramat@kernel.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. > > It's not so weired because it is a relative address, e.g. from _text, > this means "the address in the text section". And perf probe already > uses it a while. > > > > > > > > > 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? > > Yes, because /proc/kallsyms will expose the real address of the all > symbols, which makes KASLR useless. But on the other hand, it maybe > already useless because BPF program can read any real address, right? > Hmm, from this point of view, is the CAP_PERFMON meaningful? > (maybe it can avoid loading modules etc.)
kprobe BPF program has access to pt_regs, so it can read ip of the attached function. Can we do the same with regular kprobe (no bpf)?
Thanks, Song
> > Thank you, > > > > > Thanks, > > Song > > > -- > Masami Hiramatsu (Google) <mhiramat@kernel.org>
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