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

[Jiri Olsa]

On Thu, Jun 08, 2023 at 04:55:40PM -0700, Andrii Nakryiko wrote:
> On Thu, Jun 8, 2023 at 4:27 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > On Thu, 8 Jun 2023 15:43:03 -0700
> > Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > > On Thu, Jun 8, 2023 at 2:26 PM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> > > >
> > > >
> > > > hi,
> > > > when ftrace based tracers we need to cross check available_filter_functions
> > > > with /proc/kallsyms. For example for kprobe_multi bpf link (based on fprobe)
> > > > we need to make sure that symbol regex resolves to traceable symbols and
> > > > that we get proper addresses for them.
> >
> > I forgot, what was the problem with doing the above?
> 
> More code, more memory, more CPU to parse all the text files. Parsing
> kallsyms is quite expensive, so avoiding this would be great.

yes, reading both kallsyms and available_filter_functions parsing often
shows up in perf profiles

> 
> >
> > > >
> > > > Looks like on the last last LSF/MM/BPF there was an agreement to add new
> > > > file that will have available_filter_functions symbols plus addresses.
> > > >
> > > > This RFC is to kick off the discussion, I'm not sure Steven wants to do
> > > > that differently ;-)
> >
> > I'm not totally against this, but I'd like to know the full issue its
> > solving. Perhaps I need to know more about what is being done, and what is
> > needed too.
> 
> There are BPF tools that allow user to specify regex/glob of kernel
> functions to attach to. This regex/glob is checked against
> available_filter_functions to check which functions are traceable. All
> good. But then also it's important to have corresponding memory
> addresses for selected functions (for many reasons, e.g., to have
> non-ambiguous and fast attachment by address instead of by name, or
> for some post-processing based on captured IP addresses, etc). And
> that means that now we need to also parse /proc/kallsyms and
> cross-join it with data fetched from available_filter_functions.
> 
> All this is unnecessary if avalable_filter_functions would just
> provide function address in the first place. It's a huge
> simplification. And saves memory and CPU.
> 
> >
> > > >
> > > > thanks,
> > > > jirka
> > > >
> > > >
> > > > ---
> > > > Adding new available_filter_functions_addrs file that shows all available
> > > > functions (same as available_filter_functions) together with addresses,
> > > > like:
> > > >
> > > >   # cat available_filter_functions_addrs | head
> > >
> > > nit: can we have some more succinct name, like "traceable_funcs" or
> >
> >
> > It's to match avaliable_filter_functions
> 
> it's minor, I'm fine with whatever name, I'm searching for it in my
> history every single time anyways :)
> 
> >
> > Another way is to add a tracing option to make the address show up in the
> > available_filter_functions file. That would be my preferred choice.
> >
> >   echo 1 > options/available_filter_addrs
> >
> > Or something like that.
> 
> This would modify behavior for entire system, right? I think this is
> very bad. Just because one application is aware of this option and
> wants to turn this on, doesn't mean that all other applications that
> might also use available_filter_functions should immediately break on
> that machine.
> 
> Please, let's have a separate file. There is no downside to that.

+1 for file, AFAIU the option would change that globaly so we could race
with another app and break each other

jirka