Re: [PATCH bpf-next] libbpf: add libbpf_load_vmlinux_btf/libbpf_load_module_btf APIs

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, Jul 23, 2021 at 10:13 PM Hengqi Chen <hengqi.chen@gmail.com> wrote:
>
> Add libbpf_load_vmlinux_btf/libbpf_load_module_btf APIs.
> This is part of the libbpf v1.0. [1]
>
> [1] https://github.com/libbpf/libbpf/issues/280

Saying it's part of libbpf 1.0 effort and given a link to Github PR is
not really a sufficient commit message. Please expand on what you are
doing in the patch and why.

>
> Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
> ---
>  tools/lib/bpf/btf.c      | 24 +++++++++++++++++++++++-
>  tools/lib/bpf/btf.h      |  2 ++
>  tools/lib/bpf/libbpf.c   |  8 ++++----
>  tools/lib/bpf/libbpf.map |  2 ++
>  4 files changed, 31 insertions(+), 5 deletions(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index b46760b93bb4..414e1c5635ef 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -4021,7 +4021,7 @@ static void btf_dedup_merge_hypot_map(struct btf_dedup *d)
>                  */
>                 if (d->hypot_adjust_canon)
>                         continue;
> -
> +
>                 if (t_kind == BTF_KIND_FWD && c_kind != BTF_KIND_FWD)
>                         d->map[t_id] = c_id;
>
> @@ -4395,6 +4395,11 @@ static int btf_dedup_remap_types(struct btf_dedup *d)
>   * data out of it to use for target BTF.
>   */
>  struct btf *libbpf_find_kernel_btf(void)
> +{
> +       return libbpf_load_vmlinux_btf();
> +}
> +
> +struct btf *libbpf_load_vmlinux_btf(void)
>  {
>         struct {
>                 const char *path_fmt;
> @@ -4440,6 +4445,23 @@ struct btf *libbpf_find_kernel_btf(void)
>         return libbpf_err_ptr(-ESRCH);
>  }
>
> +struct btf *libbpf_load_module_btf(const char *mod)

So we probably need to allow user to pre-load and re-use vmlinux BTF
for efficiency, especially if they have some use-case to load a lot of
BTFs.

> +{
> +       char path[80];
> +       struct btf *base;
> +       int err;
> +
> +       base = libbpf_load_vmlinux_btf();
> +       err = libbpf_get_error(base);
> +       if (err) {
> +               pr_warn("Error loading vmlinux BTF: %d\n", err);
> +               return base;

libbpf_err_ptr() needs to be used here, pr_warn() could have destroyed
errno already

> +       }
> +
> +       snprintf(path, sizeof(path), "/sys/kernel/btf/%s", mod);
> +       return btf__parse_split(path, base);

so who's freeing base BTF in this case?

> +}
> +
>  int btf_type_visit_type_ids(struct btf_type *t, type_id_visit_fn visit, void *ctx)
>  {
>         int i, n, err;

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