On Sat, 19 Aug 2023 08:22:50 -0700
Song Liu <[email protected]> wrote:
> On Fri, Aug 18, 2023 at 6:16 PM Masami Hiramatsu <[email protected]> wrote:
> >
> > On Fri, 18 Aug 2023 14:20:33 -0400
> > Steven Rostedt <[email protected]> wrote:
> >
> > > On Fri, 18 Aug 2023 20:13:43 +0200
> > > Francis Laniel <[email protected]> wrote:
> > >
> > > > Hi.
> > > >
> > > > Le vendredi 18 août 2023, 17:41:41 CEST Steven Rostedt a écrit :
> > > > > On Fri, 18 Aug 2023 21:37:05 +0900
> > > > >
> > > > > Masami Hiramatsu (Google) <[email protected]> wrote:
> > > > > > That's why perf probe uses the offset from '_text'. Normal KASLR will just
> > > > > > moves all symbols. (Finer one will move all symbols randomely)
> > > > > > This should not need to access /proc/kallsyms but vmlinux or SystemMap.
> > > > >
> > > > > We could just pass in: "_text+offset" too.
> > > >
> > > > So, the idea would be to change the existing create_local_trace_kprobe() and
> > > > above functions to indicate the user's offset is to be used against _text and
> > > > not address?
> > >
> > > No, not to modify that function, but if you know the offset from _text (via
> > > the vmlinux), you can easily calculate it for that function.
> >
> > Note that the kprobe-event PMU interface itself allows you to specify
> > FUNC+OFFSET style;
> >
> > https://lore.kernel.org/lkml/[email protected]/
> >
> > 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.
>
> 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.
Thank you,
>
> Thanks,
> Song
--
Masami Hiramatsu (Google) <[email protected]>
On Sun, Aug 20, 2023 at 2:32 AM Masami Hiramatsu <[email protected]> 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