Re: [PATCH bpf-next v2 3/4] libbpf: Introduce 'bpf_object__prepare()'

["Mauricio Vásquez Bernal" <mauricio@kinvolk.io>]

On Tue, Nov 16, 2021 at 11:42 AM Mauricio Vásquez <mauricio@kinvolk.io> wrote:
>
> BTFGen[0] requires access to the result of the CO-RE relocations without
> actually loading the bpf programs. The current libbpf API doesn't allow
> it because all the object preparation (subprogs, relocations: co-re,
> elf, maps) happens inside bpf_object__load().
>
> This commit introduces a new bpf_object__prepare() function to perform
> all the preparation steps than an ebpf object requires, allowing users
> to access the result of those preparation steps without having to load
> the program. Almost all the steps that were done in bpf_object__load()
> are now done in bpf_object__prepare(), except map creation and program
> loading.
>
> Map relocations require a bit more attention as maps are only created in
> bpf_object__load(). For this reason bpf_object__prepare() relocates maps
> using BPF_PSEUDO_MAP_IDX, if someone dumps the instructions before
> loading the program they get something meaningful. Map relocations are
> completed in bpf_object__load() once the maps are created and we have
> their fd to use with BPF_PSEUDO_MAP_FD.
>
> Users won’t see any visible changes if they’re using bpf_object__open()
> + bpf_object__load() because this commit keeps backwards compatibility
> by calling bpf_object__prepare() in bpf_object_load() if it wasn’t
> called by the user.
>
> bpf_object__prepare_xattr() is not implemented as their counterpart
> bpf_object__load_xattr() will be deprecated[1]. New options will be
> added only to bpf_object_open_opts.
>
> [0]: https://github.com/kinvolk/btfgen/
> [1]: https://github.com/libbpf/libbpf/wiki/Libbpf:-the-road-to-v1.0#libbpfh-high-level-apis
>
> Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>
> Signed-off-by: Rafael David Tinoco <rafael.tinoco@aquasec.com>
> Signed-off-by: Lorenzo Fontana <lorenzo.fontana@elastic.co>
> Signed-off-by: Leonardo Di Donato <leonardo.didonato@elastic.co>
> ---
>  tools/lib/bpf/libbpf.c   | 130 ++++++++++++++++++++++++++++-----------
>  tools/lib/bpf/libbpf.h   |   2 +
>  tools/lib/bpf/libbpf.map |   1 +
>  3 files changed, 98 insertions(+), 35 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 6ca76365c6da..f50f9428bb03 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -514,6 +514,7 @@ struct bpf_object {
>         int nr_extern;
>         int kconfig_map_idx;
>
> +       bool prepared;
>         bool loaded;
>         bool has_subcalls;
>         bool has_rodata;
> @@ -5576,34 +5577,19 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
>
>                 switch (relo->type) {
>                 case RELO_LD64:
> -                       if (obj->gen_loader) {
> -                               insn[0].src_reg = BPF_PSEUDO_MAP_IDX;
> -                               insn[0].imm = relo->map_idx;
> -                       } else {
> -                               insn[0].src_reg = BPF_PSEUDO_MAP_FD;
> -                               insn[0].imm = obj->maps[relo->map_idx].fd;
> -                       }
> +                       insn[0].src_reg = BPF_PSEUDO_MAP_IDX;
> +                       insn[0].imm = relo->map_idx;
>                         break;
>                 case RELO_DATA:
>                         insn[1].imm = insn[0].imm + relo->sym_off;
> -                       if (obj->gen_loader) {
> -                               insn[0].src_reg = BPF_PSEUDO_MAP_IDX_VALUE;
> -                               insn[0].imm = relo->map_idx;
> -                       } else {
> -                               insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
> -                               insn[0].imm = obj->maps[relo->map_idx].fd;
> -                       }
> +                       insn[0].src_reg = BPF_PSEUDO_MAP_IDX_VALUE;
> +                       insn[0].imm = relo->map_idx;
>                         break;
>                 case RELO_EXTERN_VAR:
>                         ext = &obj->externs[relo->sym_off];
>                         if (ext->type == EXT_KCFG) {
> -                               if (obj->gen_loader) {
> -                                       insn[0].src_reg = BPF_PSEUDO_MAP_IDX_VALUE;
> -                                       insn[0].imm = obj->kconfig_map_idx;
> -                               } else {
> -                                       insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
> -                                       insn[0].imm = obj->maps[obj->kconfig_map_idx].fd;
> -                               }
> +                               insn[0].src_reg = BPF_PSEUDO_MAP_IDX_VALUE;
> +                               insn[0].imm = obj->kconfig_map_idx;
>                                 insn[1].imm = ext->kcfg.data_off;
>                         } else /* EXT_KSYM */ {
>                                 if (ext->ksym.type_id && ext->is_set) { /* typed ksyms */
> @@ -6144,8 +6130,50 @@ bpf_object__relocate(struct bpf_object *obj, const char *targ_btf_path)
>                         return err;
>                 }
>         }
> -       if (!obj->gen_loader)
> -               bpf_object__free_relocs(obj);
> +
> +       return 0;
> +}
> +
> +/* relocate instructions that refer to map fds */
> +static int
> +bpf_object__finish_relocate(struct bpf_object *obj)
> +{
> +       int i, j;
> +
> +       if (obj->gen_loader)
> +               return 0;
> +
> +       for (i = 0; i < obj->nr_programs; i++) {
> +               struct bpf_program *prog = &obj->programs[i];
> +
> +               if (prog_is_subprog(obj, prog))
> +                       continue;
> +               for (j = 0; j < prog->nr_reloc; j++) {
> +                       struct reloc_desc *relo = &prog->reloc_desc[j];
> +                       struct bpf_insn *insn = &prog->insns[relo->insn_idx];
> +                       struct extern_desc *ext;
> +
> +                       switch (relo->type) {
> +                       case RELO_LD64:
> +                               insn[0].src_reg = BPF_PSEUDO_MAP_FD;
> +                               insn[0].imm = obj->maps[relo->map_idx].fd;
> +                               break;
> +                       case RELO_DATA:
> +                               insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
> +                               insn[0].imm = obj->maps[relo->map_idx].fd;
> +                               break;
> +                       case RELO_EXTERN_VAR:
> +                               ext = &obj->externs[relo->sym_off];
> +                               if (ext->type == EXT_KCFG) {
> +                                       insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
> +                                       insn[0].imm = obj->maps[obj->kconfig_map_idx].fd;
> +                               }
> +                       default:
> +                               break;
> +                       }
> +               }
> +       }
> +
>         return 0;
>  }
>
> @@ -6706,8 +6734,8 @@ bpf_object__load_progs(struct bpf_object *obj, int log_level)
>                 if (err)
>                         return err;
>         }
> -       if (obj->gen_loader)
> -               bpf_object__free_relocs(obj);
> +
> +       bpf_object__free_relocs(obj);
>         return 0;
>  }
>
> @@ -7258,6 +7286,39 @@ static int bpf_object__resolve_externs(struct bpf_object *obj,
>         return 0;
>  }
>
> +static int __bpf_object__prepare(struct bpf_object *obj, int log_level,
> +                                const char *target_btf_path)
> +{
> +       int err;
> +
> +       if (obj->prepared) {
> +               pr_warn("object '%s': prepare can't be attempted twice\n", obj->name);
> +               return libbpf_err(-EINVAL);
> +       }
> +
> +       if (obj->gen_loader)
> +               bpf_gen__init(obj->gen_loader, log_level);
> +
> +       err = bpf_object__probe_loading(obj);
> +       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__relocate(obj, obj->btf_custom_path ? : target_btf_path);
> +
> +       obj->prepared = true;
> +
> +       return err;
> +}
> +
> +LIBBPF_API int bpf_object__prepare(struct bpf_object *obj)
> +{
> +       if (!obj)
> +               return libbpf_err(-EINVAL);
> +       return __bpf_object__prepare(obj, 0, NULL);
> +}
> +
>  int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
>  {
>         struct bpf_object *obj;
> @@ -7274,17 +7335,14 @@ int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
>                 return libbpf_err(-EINVAL);
>         }
>
> -       if (obj->gen_loader)
> -               bpf_gen__init(obj->gen_loader, attr->log_level);
> +       if (!obj->prepared) {
> +               err = __bpf_object__prepare(obj, attr->log_level, attr->target_btf_path);
> +               if (err)
> +                       return err;
> +       }
>
> -       err = bpf_object__probe_loading(obj);

After sending the patches we realized they weren't working without
root privileges in systems with unprivileged BPF disabled. This line
should not be moved to bpf_object__prepare indeed. We'll fix it in the
next iteration.


> -       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, obj->btf_custom_path ? : attr->target_btf_path);
> +       err = bpf_object__create_maps(obj);
> +       err = err ? : bpf_object__finish_relocate(obj);
>         err = err ? : bpf_object__load_progs(obj, attr->log_level);
>
>         if (obj->gen_loader) {
> @@ -7940,6 +7998,8 @@ void bpf_object__close(struct bpf_object *obj)
>         bpf_object__elf_finish(obj);
>         bpf_object_unload(obj);
>         btf__free(obj->btf);
> +       if (!obj->user_provided_btf_vmlinux)
> +               btf__free(obj->btf_vmlinux_override);
>         btf_ext__free(obj->btf_ext);
>
>         for (i = 0; i < obj->nr_maps; i++)
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 908ab04dc9bd..d206b4400a4d 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -148,6 +148,8 @@ LIBBPF_API int bpf_object__unpin_programs(struct bpf_object *obj,
>  LIBBPF_API int bpf_object__pin(struct bpf_object *object, const char *path);
>  LIBBPF_API void bpf_object__close(struct bpf_object *object);
>
> +LIBBPF_API int bpf_object__prepare(struct bpf_object *obj);
> +
>  struct bpf_object_load_attr {
>         struct bpf_object *obj;
>         int log_level;
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index c9555f8655af..459b41228933 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -415,4 +415,5 @@ LIBBPF_0.6.0 {
>                 perf_buffer__new_raw;
>                 perf_buffer__new_raw_deprecated;
>                 btf__save_raw;
> +               bpf_object__prepare;
>  } LIBBPF_0.5.0;
> --
> 2.25.1
>