Re: [RFC] ftrace: Add support to keep some functions out of ftrace

Björn Töpel
2022-08-15T12:20:15Z

On Mon, 15 Aug 2022 at 13:29, Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Mon, Aug 15, 2022 at 01:01:06PM +0200, Björn Töpel wrote:
> > On Mon, 15 Aug 2022 at 10:04, Jiri Olsa <olsajiri@gmail.com> wrote:
> > [...]
> > > > > >
> > > > > > Today, objtool has also got involved, and added an "--mcount" option
> > > > > > that will create the section too.
> > > > > I overlooked that objtool is involved as well,
> > > > > will check on that
> > > >
> > > > objtool --mcount option only involves mcount_loc generation (see
> > > > annotate_call_site) and other validation check call destination directly
> > > > (see is_fentry_call).
> > > >
> > > > Some simply removing --mcount option dose work for this.
> > > >
> > > >
> > > > Another question, it seems we can export and use DEFINE_BPF_DISPATCHER out
> > > > of kernel, does that means we should add NO_MCOUNT_FILES for these single
> > > > uages as well?
> > >
> > > yes, cc-ing Björn to make sure it's valid use case for dispatcher
> > >
> >
> > Hmm, could you expand a bit on how this would work?
>
> the goal here is to remove bpf_dispatcher_<FUNC>_func functions from
> ftrace, because it's updated by dispatcher code with bpf_arch_text_poke,
> but it's also visible and attachable to ftrace.. and will cause problems
> when these 2 updates will race
>
> question was if DEFINE_BPF_DISPATCHER can be used in kernel module,
> which would bring another realm of problems ;-)
>

Oh, now I follow. AFAIK there is only one flavor of BPF dispatcher in
use, and that's the XDP dispatcher, which does not reside in module
code, but is typically *called* by module code.

A module could define a BPF dispatcher, but it wouldn't be able to
update it, since bpf_arch_text_poke() does not support poking in
modules.

Björn