Re: [PATCH 2/2] btf_encoder: Change functions check due to broken dwarf

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Nov 03, 2020 at 10:58:58AM -0800, Andrii Nakryiko wrote:
> On Mon, Nov 2, 2020 at 2:57 PM Jiri Olsa <jolsa@redhat.com> wrote:
> >
> > On Mon, Nov 02, 2020 at 10:59:08PM +0100, Jiri Olsa wrote:
> > > On Sat, Oct 31, 2020 at 11:31:31PM +0100, Jiri Olsa wrote:
> > > > We need to generate just single BTF instance for the
> > > > function, while DWARF data contains multiple instances
> > > > of DW_TAG_subprogram tag.
> > > >
> > > > Unfortunately we can no longer rely on DW_AT_declaration
> > > > tag (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060)
> > > >
> > > > Instead we apply following checks:
> > > >   - argument names are defined for the function
> > > >   - there's symbol and address defined for the function
> > > >   - function is generated only once
> > > >
> > > > Also because we want to follow kernel's ftrace traceable
> > > > functions, this patchset is adding extra check that the
> > > > function is one of the ftrace's functions.
> > > >
> > > > All ftrace functions addresses are stored in vmlinux
> > > > binary within symbols:
> > > >   __start_mcount_loc
> > > >   __stop_mcount_loc
> > >
> > > hum, for some reason this does not pass through bpf internal
> > > functions like bpf_iter_bpf_map.. I learned it hard way ;-)
> 
> what's the exact name of the function that was missing?
> bpf_iter_bpf_map doesn't exist. And if it's __init function, why does
> it matter, it's not going to be even available at runtime, right?
> 

bpf_map iter definition:

DEFINE_BPF_ITER_FUNC(bpf_map, struct bpf_iter_meta *meta, struct bpf_map *map)

goes to:

#define DEFINE_BPF_ITER_FUNC(target, args...)                   \
        extern int bpf_iter_ ## target(args);                   \
        int __init bpf_iter_ ## target(args) { return 0; }

that creates __init bpf_iter_bpf_map function that will make
it into BTF where it's expected when opening iterator, but the
code will be freed because it's __init function

there are few iteratos functions like that, and I was going to
check if there's more

> 
> > > will check
> >
> > so it gets filtered out because it's __init function
> > I'll check if the fix below catches all internal functions,
> > but I guess we should do something more robust
> >
> > jirka
> >
> >
> > ---
> > diff --git a/btf_encoder.c b/btf_encoder.c
> > index 0a378aa92142..3cd94280c35b 100644
> > --- a/btf_encoder.c
> > +++ b/btf_encoder.c
> > @@ -143,7 +143,8 @@ static int filter_functions(struct btf_elf *btfe, struct mcount_symbols *ms)
> >                 /* Do not enable .init section functions. */
> >                 if (init_filter &&
> >                     func->addr >= ms->init_begin &&
> > -                   func->addr <  ms->init_end)
> > +                   func->addr <  ms->init_end &&
> > +                   strncmp("bpf_", func->name, 4))
> 
> this looks like a very wrong way to do this? Can you please elaborate
> on what's missing and why it shouldn't be missing?

yes, it's just a hack, we should do something more
robust as I mentioned above

it just allowed me to use iterators finaly ;-)

jirka