Re: [RFC 0/3] pahole: Workaround dwarf bug for function encoding

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Oct 27, 2020 at 04:13:46PM -0700, Andrii Nakryiko wrote:
> On Mon, Oct 26, 2020 at 5:07 PM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > hi,
> > because of gcc bug [1] we can no longer rely on DW_AT_declaration
> > attribute to filter out declarations and end up with just
> > one copy of the function in the BTF data.
> >
> > It seems this bug is not easy to fix, but regardless if the
> > it's coming soon, it's probably good idea not to depend so
> > much only on dwarf data and make some extra checks.
> >
> > Thus for function encoding we are now doing following checks:
> >   - argument names are defined for the function
> >   - there's symbol and address defined for the function
> >   - function is generated only once
> >
> > These checks ensure that we encode function with defined
> > symbol/address and argument names.
> >
> > I marked this post as RFC, because with this workaround in
> > place we are also encoding assembly functions, which were
> > not present when using the previous gcc version.
> >
> > Full functions diff to previous gcc working version:
> >
> >   http://people.redhat.com/~jolsa/functions.diff.txt
> >
> > I'm not sure this does not break some rule for functions in
> > BTF data, becuse those assembly functions are not attachable
> > by bpf trampolines, so I don't think there's any use for them.
> 
> What will happen if we do try to attach to those assembly functions?
> Will there be some corruption or crash, or will it just fail and

the attach code checks for the __fentry__ nop,
so it will fail probably with EBUSY

> return error cleanly? What we actually want in BTF is all the
> functions that are attachable through BPF trampoline, which is all the
> functions that ftrace subsystem can attach to, right? So how does
> ftrace system know what can or cannot be attached to?

not sure, I think it records all the functions with
__fentry__ calls, perhaps we could take these records
as base for FUNCs, I'll check

jirka

> 
> >
> > thoughts?
> > jirka
> >
> >
> > [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060
> > ---
> > Jiri Olsa (3):
> >       btf_encoder: Move find_all_percpu_vars in generic config function
> >       btf_encoder: Change functions check due to broken dwarf
> >       btf_encoder: Include static functions to BTF data
> >
> >  btf_encoder.c | 221 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-------------------------------------
> >  elf_symtab.h  |   8 +++++
> >  2 files changed, 170 insertions(+), 59 deletions(-)
> >
>