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.
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
