Re: [PATCH] libbpf: add specific btf name info when do core

[Tao Chen]

Got it, thanks for your detailed reply.

在 2024/3/23 上午1:57, Andrii Nakryiko 写道:
> On Fri, Mar 22, 2024 at 6:37 AM Tao Chen <chen.dylane@xxxxxxxxx> wrote:
> > Hi, Nakryiko, thank you for your reply. I try to familiarize with core
> > in libbpf， the debug info is really helpful for me. But i use the old
> > kernel btf, so origion debuginfo like:
> >    struct task_struct___x: found target candidate [130] struct
> > task_struct in [vmlinux]
>
> the idea here is to not emit path to BTF (otherwise we normally should
> emit [/sys/kernel/btf/vmlinux], but rather distinguish whether it's a
> kernel BTF ([vmlinux]) or some kernel module BTF ([<module-name>]).
>
> In your case you are overriding vmlinux BTF by using btf_custom_path
> option (so you should know where to find it, if you need to debug
> something). It's still, conceptually, a [vmlinux], and I'd like to
> keep it this way.
>
> > I think it may be more clear, if we print btf name when we use old
> > kernel btf like:
> >    struct task_struct___x: found target candidate [130] struct
> > task_struct in [/boot/***.btf]
> > The patch just solve debug info show for newbies above.
> >
> > 在 2024/3/22 上午2:52, Andrii Nakryiko 写道:
> > > On Thu, Mar 21, 2024 at 10:04 AM Tao Chen <chen.dylane@xxxxxxxxx> wrote:
> > > > No logic changed, just add specific btf name when core info
> > > > print, maybe it seems more understandable.
> > > >
> > > > Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> > > > ---
> > > >    tools/lib/bpf/libbpf.c | 14 +++++++++-----
> > > >    1 file changed, 9 insertions(+), 5 deletions(-)
> > >
> > > Can you elaborate on what problem you are trying to solve?
> > > Conceptually libbpf does look for types in vmlinux (meaning main
> > > kernel BTF), even if user overrides BTF location (presumably because
> > > of old kernel). So even when we emit "vmlinux" in logs, it seems
> > > correct.
> > >
> > > pw-bot: cr
> > >
> > >
> > > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > > > index afd09571c482..b32154288b4a 100644
> > > > --- a/tools/lib/bpf/libbpf.c
> > > > +++ b/tools/lib/bpf/libbpf.c
> > > > @@ -5653,7 +5653,8 @@ static int load_module_btfs(struct bpf_object *obj)
> > > >    }
> > > >
> > > >    static struct bpf_core_cand_list *
> > > > -bpf_core_find_cands(struct bpf_object *obj, const struct btf *local_btf, __u32 local_type_id)
> > > > +bpf_core_find_cands(struct bpf_object *obj, const struct btf *local_btf, __u32 local_type_id,
> > > > +                   const char *targ_btf_path)
> > > >    {
> > > >           struct bpf_core_cand local_cand = {};
> > > >           struct bpf_core_cand_list *cands;
> > > > @@ -5680,7 +5681,8 @@ bpf_core_find_cands(struct bpf_object *obj, const struct btf *local_btf, __u32 l
> > > >
> > > >           /* Attempt to find target candidates in vmlinux BTF first */
> > > >           main_btf = obj->btf_vmlinux_override ?: obj->btf_vmlinux;
> > > > -       err = bpf_core_add_cands(&local_cand, local_essent_len, main_btf, "vmlinux", 1, cands);
> > > > +       err = bpf_core_add_cands(&local_cand, local_essent_len, main_btf,
> > > > +                                targ_btf_path ?: "vmlinux", 1, cands);
> > > >           if (err)
> > > >                   goto err_out;
> > > >
> > > > @@ -5793,7 +5795,8 @@ static int bpf_core_resolve_relo(struct bpf_program *prog,
> > > >                                    int relo_idx,
> > > >                                    const struct btf *local_btf,
> > > >                                    struct hashmap *cand_cache,
> > > > -                                struct bpf_core_relo_res *targ_res)
> > > > +                                struct bpf_core_relo_res *targ_res,
> > > > +                                const char *targ_btf_path)
> > > >    {
> > > >           struct bpf_core_spec specs_scratch[3] = {};
> > > >           struct bpf_core_cand_list *cands = NULL;
> > > > @@ -5813,7 +5816,7 @@ static int bpf_core_resolve_relo(struct bpf_program *prog,
> > > >
> > > >           if (relo->kind != BPF_CORE_TYPE_ID_LOCAL &&
> > > >               !hashmap__find(cand_cache, local_id, &cands)) {
> > > > -               cands = bpf_core_find_cands(prog->obj, local_btf, local_id);
> > > > +               cands = bpf_core_find_cands(prog->obj, local_btf, local_id, targ_btf_path);
> > > >                   if (IS_ERR(cands)) {
> > > >                           pr_warn("prog '%s': relo #%d: target candidate search failed for [%d] %s %s: %ld\n",
> > > >                                   prog_name, relo_idx, local_id, btf_kind_str(local_type),
> > > > @@ -5920,7 +5923,8 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
> > > >                           if (prog->obj->gen_loader)
> > > >                                   continue;
> > > >
> > > > -                       err = bpf_core_resolve_relo(prog, rec, i, obj->btf, cand_cache, &targ_res);
> > > > +                       err = bpf_core_resolve_relo(prog, rec, i, obj->btf, cand_cache, &targ_res,
> > > > +                                                   targ_btf_path);
> > > >                           if (err) {
> > > >                                   pr_warn("prog '%s': relo #%d: failed to relocate: %d\n",
> > > >                                           prog->name, i, err);
> > > > --
> > > > 2.34.1