Re: [PATCH bpf v1] Add `core_btf_path` to `bpf_object_open_opts` to pass BTF path from skeleton program

[Vamsi Kodavanty <vamsi@araalinetworks.com>]

Andrii,
   Thank you for the detailed review. I will address them as well as
the self tests. And will send out a new patch addressing them and
conforming to style/expectations.

Cheers
Vamsi.

On Mon, Jan 11, 2021 at 2:02 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Fri, Jan 8, 2021 at 6:36 PM Vamsi Kodavanty <vamsi@araalinetworks.com> wrote:
> >
> > Andrii,
> >      I have made the following changes as discussed to add an option to the `open_opts`
> > to take in the BTF.
> >      Please do take a look. Also, I am not sure what the procedure is for submitting patches/reviews.
> > If anyone has any pointers to a webpage where this is described I can go through it. But, below are
> > the proposed changes.
> >
>
> Daniel already gave you pointers. Also make sure you add [PATCH
> bpf-next] prefix to email subject to identify the patch is for
> bpf-next kernel tree.
> And with all changes like this we should also add selftests,
> exercising new features. Please take a look at
> tools/testing/selftests/bpf. I think updating
> test_progs/test_core_reloc.c in there to use this instead of
> bpf_object__load_xattr() might be enough of the testing.
>
> > Best Regards,
> > Vamsi.
> >
> > ---
> >  src/libbpf.c | 56 +++++++++++++++++++++++++++++++++++++---------------
> >  src/libbpf.h |  4 +++-
> >  2 files changed, 43 insertions(+), 17 deletions(-)
> >
> > diff --git a/src/libbpf.c b/src/libbpf.c
> > index 6ae748f..35d7254 100644
> > --- a/src/libbpf.c
> > +++ b/src/libbpf.c
> > @@ -2538,9 +2538,12 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
> >         struct bpf_program *prog;
> >         int i;
> >
> > -       /* CO-RE relocations need kernel BTF */
> > +       /* CO-RE relocations need kernel BTF or an override BTF.
> > +        * If override BTF present CO-RE can use it.
>
> nit: "CO-RE relocations need kernel BTF, unless custom BTF override is
> specified"
>
> > +        */
> >         if (obj->btf_ext && obj->btf_ext->core_relo_info.len)
> > -               return true;
> > +               if (!obj->btf_vmlinux_override)
>
> please combine this into a single if
>
> > +                       return true;
> >
> >         /* Support for typed ksyms needs kernel BTF */
> >         for (i = 0; i < obj->nr_extern; i++) {
> > @@ -2561,6 +2564,27 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
> >         return false;
> >  }
> >
> > +static int bpf_object__load_override_btf(struct bpf_object *obj,
> > +                                                                                const char *targ_btf_path)
>
> formatting is off? scripts/checkpatch.pl -f <path to file> will report
> issues like this
>
> > +{
> > +       /* Could have been be set via `bpf_object_open_opts` */
> > +       if (obj->btf_vmlinux_override)
> > +               return 0;
>
> see below, let's make sure we load btf_vmlinux_override in one place
> (and cleanup somewhere close)
>
> > +
> > +       if (!targ_btf_path)
> > +               return 0;
> > +
> > +       obj->btf_vmlinux_override = btf__parse(targ_btf_path, NULL);
> > +       if (IS_ERR_OR_NULL(obj->btf_vmlinux_override)) {
> > +               int err = PTR_ERR(obj->btf_vmlinux_override);
> > +               obj->btf_vmlinux_override = NULL;
> > +               pr_warn("failed to parse target BTF: %d\n", err);
> > +               return err;
> > +       }
> > +
> > +       return 0;
> > +}
> > +
> >  static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
> >  {
> >         int err;
> > @@ -6031,7 +6055,7 @@ patch_insn:
> >  }
> >
> >  static int
> > -bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
> > +bpf_object__relocate_core(struct bpf_object *obj)
> >  {
> >         const struct btf_ext_info_sec *sec;
> >         const struct bpf_core_relo *rec;
> > @@ -6045,15 +6069,6 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
> >         if (obj->btf_ext->core_relo_info.len == 0)
> >                 return 0;
> >
> > -       if (targ_btf_path) {
> > -               obj->btf_vmlinux_override = btf__parse(targ_btf_path, NULL);
> > -               if (IS_ERR_OR_NULL(obj->btf_vmlinux_override)) {
> > -                       err = PTR_ERR(obj->btf_vmlinux_override);
> > -                       pr_warn("failed to parse target BTF: %d\n", err);
> > -                       return err;
> > -               }
> > -       }
> > -
>
> given we are moving out btf_vmlinux_override loading from
> bpf_object__relocate_core, we need to move out its destruction and
> cleanup to the same function that does BTF parsing. That will keep the
> logic simpler.
>
> >         cand_cache = hashmap__new(bpf_core_hash_fn, bpf_core_equal_fn, NULL);
> >         if (IS_ERR(cand_cache)) {
> >                 err = PTR_ERR(cand_cache);
> > @@ -6556,14 +6571,14 @@ bpf_object__relocate_calls(struct bpf_object *obj, struct bpf_program *prog)
> >  }
> >
> >  static int
> > -bpf_object__relocate(struct bpf_object *obj, const char *targ_btf_path)
> > +bpf_object__relocate(struct bpf_object *obj)
> >  {
> >         struct bpf_program *prog;
> >         size_t i;
> >         int err;
> >
> >         if (obj->btf_ext) {
> > -               err = bpf_object__relocate_core(obj, targ_btf_path);
> > +               err = bpf_object__relocate_core(obj);
> >                 if (err) {
> >                         pr_warn("failed to perform CO-RE relocations: %d\n",
> >                                 err);
> > @@ -7088,7 +7103,7 @@ static struct bpf_object *
> >  __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
> >                    const struct bpf_object_open_opts *opts)
> >  {
> > -       const char *obj_name, *kconfig;
> > +       const char *obj_name, *kconfig, *core_btf_path;
> >         struct bpf_program *prog;
> >         struct bpf_object *obj;
> >         char tmp_name[64];
> > @@ -7126,6 +7141,14 @@ __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
> >                         return ERR_PTR(-ENOMEM);
> >         }
> >
> > +       core_btf_path = OPTS_GET(opts, core_btf_path, NULL);
> > +       if (core_btf_path) {
> > +               pr_debug("parse btf '%s' for CO-RE relocations\n", core_btf_path);
>
> Move this right after btf__parse(), so you can report success or
> failure in one log statement; see how libbpf_find_kernel_btf() does
> it. Please use similar wording (just "target BTF" instead of "kernel
> BTF" to distinguish them).
>
> > +               obj->btf_vmlinux_override = btf__parse(core_btf_path, NULL);
>
> This BTF is not needed until the load phase, so let's not attempt to
> load it on open() unnecessarily. Just remember the path and defer till
> bpf_object__load() time.
>
> > +               if (IS_ERR_OR_NULL(obj->btf_vmlinux_override))
> > +                       pr_warn("can't parse btf at '%s'\n", core_btf_path);
>
> if BTF can't be loaded, load should fail
>
> > +       }
> > +
> >         err = bpf_object__elf_init(obj);
> >         err = err ? : bpf_object__check_endianness(obj);
> >         err = err ? : bpf_object__elf_collect(obj);
> > @@ -7481,13 +7504,14 @@ int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
> >         }
> >
> >         err = bpf_object__probe_loading(obj);
> > +       err = err ? : bpf_object__load_override_btf(obj, attr->target_btf_path);
> >         err = err ? : bpf_object__load_vmlinux_btf(obj, false);
> >         err = err ? : bpf_object__resolve_externs(obj, obj->kconfig);
> >         err = err ? : bpf_object__sanitize_and_load_btf(obj);
> >         err = err ? : bpf_object__sanitize_maps(obj);
> >         err = err ? : bpf_object__init_kern_struct_ops_maps(obj);
> >         err = err ? : bpf_object__create_maps(obj);
> > -       err = err ? : bpf_object__relocate(obj, attr->target_btf_path);
>
> For backwards compatibility, we need to still respect
> attr->target_btf_path. I'd say let attr->target_btf_path override
> opts->core_btf_path, if specified. Later on we'll probably just
> deprecate bpf_object__load_xattr() and target_btf_path will go away.
>
> > +       err = err ? : bpf_object__relocate(obj);
> >         err = err ? : bpf_object__load_progs(obj, attr->log_level);
> >
> >         /* clean up module BTFs */
> > diff --git a/src/libbpf.h b/src/libbpf.h
> > index 3c35eb4..40c4ee9 100644
> > --- a/src/libbpf.h
> > +++ b/src/libbpf.h
> > @@ -93,8 +93,10 @@ struct bpf_object_open_opts {
> >          * system Kconfig for CONFIG_xxx externs.
> >          */
> >         const char *kconfig;
> > +       /* Path to ELF file with BTF section to be used for relocations. */
>
> Given you use btf__parse() when opening this BTF, it handles both ELF
> and raw BTF data. So let's reword this comment to mention BTF in
> general.
>
> > +       const char *core_btf_path;
> >  };
> > -#define bpf_object_open_opts__last_field kconfig
> > +#define bpf_object_open_opts__last_field core_btf_path
> >
> >  LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
> >  LIBBPF_API struct bpf_object *
> > --
> > 2.23.3
> >