Re: [PATCHv2] ftrace: Show all functions with addresses in available_filter_functions_addrs

[Andrii Nakryiko]

On Mon, Jun 12, 2023 at 1:25 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Mon, Jun 12, 2023 at 11:02:22AM -0400, Steven Rostedt wrote:
> > On Mon, 12 Jun 2023 07:49:53 -0700
> > Yonghong Song <yhs@xxxxxxxx> wrote:
> >
> > > I am actually interested in how available_filter_functions_addrs
> > > will be used. For example, bpf_program__attach_kprobe_multi_opts()
> > > can already take addresses from kallsyms. How to use
> > > available_filter_functions_addrs to facilitate kprobe_multi?
>
> the problem is that we need to do 2 passes:
>
>  - through available_filter_functions and find out if the function is traceable
>  - through /proc/kallsyms to get the address for traceable function
>
> having available_filter_functions symbols together with addresses allow
> us to skip the kallsyms step
>
> and we are ok with the address in available_filter_functions_addr not being the
> function entry, because kprobe_multi uses fprobe and that handles both entry and
> patch-site address properly
>
> > > Do we need to change kernel APIs? It would be great at least we
> > > got a RFC patch to answer these questions.
> >
> > I agree, having that information would also be useful to me.
> >
> > Jiri? Andrii?
>
> so we have 2 interfaces how to create kprobe_multi link:
>
>   a) passing symbols to kernel
>
>      1) user gathers symbols and need to ensure that they are
>         trace-able -> pass through available_filter_functions file
>
>      2) kernel takes those symbols and translates them to addresses
>         through kallsyms api
>
>      3) addresses are passed to fprobe/ftrace through:
>
>          register_fprobe_ips
>          -> ftrace_set_filter_ips
>
>   b) passing addresses to kernel
>
>      1) user gathers symbols and needs to ensure that they are
>         trace-able -> pass through available_filter_functions file
>
>      2) user takes those symbols and translates them to addresses
>        through /proc/kallsyms
>
>      3) addresses are passed to the kernel and kernel calls:
>
>          register_fprobe_ips
>          -> ftrace_set_filter_ips
>
>
> The new available_filter_functions_addrs file helps us with option b),
> because we can make 'b 1' and 'b 2' in one step - while filtering traceable
> functions, we get the address directly.
>
> I tested the new available_filter_functions_addrs changes with some hacked
> selftest changes, you can check it in here [1].
>
> I assume Jackie Liu will send new version of her patchset [2] based on this
> new available_filter_functions_addrs file.
>
> I think we should have these changes coming together and add some perf
> measurements from before and after to make the benefit apparent.
>

If Steven would be ok with it, can we land this change through the
bpf-next tree? Then we can have BPF selftest added in the same patch
set that parses a new file and uses bpf_program__attach_kprobe_multi()
to attach using explicit addresses.

This should make it clear to everyone how this is meant to be used and
will be a good test that everything works end-to-end.

> jirka
>
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git/commit/?h=bpf/avail_addrs&id=fecaeeaf40bae034715ab2e9a46ca1dc16371e8e
> [2] https://lore.kernel.org/bpf/20230526155026.1419390-1-liu.yun@xxxxxxxxx/#r