Re: [PATCH bpf-next] libbpf: Introduce 'custom_btf_path' to 'bpf_obj_open_opts'.

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

On Wed, Jun 23, 2021 at 9:04 PM Shuyi Cheng
<chengshuyi@linux.alibaba.com> wrote:
>
> In order to enable the older kernel to use the CO-RE feature, load the
> vmlinux btf of the specified path.
>
> Learn from Andrii's comments in [0], add the custom_btf_path parameter
> to bpf_obj_open_opts, you can directly use the skeleton's
> <objname>_bpf__open_opts function to pass in the custom_btf_path
> parameter.
>
> Prior to this, there was also a developer who provided a patch with
> similar functions. It is a pity that the follow-up did not continue to
> advance. See [1].
>
>         [0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
>         [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/
>
> Signed-off-by: Shuyi Cheng <chengshuyi@linux.alibaba.com>
> ---
>  tools/lib/bpf/libbpf.c | 23 ++++++++++++++++++++---
>  tools/lib/bpf/libbpf.h |  6 +++++-
>  2 files changed, 25 insertions(+), 4 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 1e04ce7..518b19f 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -509,6 +509,8 @@ struct bpf_object {
>         void *priv;
>         bpf_object_clear_priv_t clear_priv;
>
> +       char *custom_btf_path;
> +
>         char path[];
>  };
>  #define obj_elf_valid(o)       ((o)->efile.elf)
> @@ -2679,8 +2681,15 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
>         if (!force && !obj_needs_vmlinux_btf(obj))
>                 return 0;
>
> -       obj->btf_vmlinux = libbpf_find_kernel_btf();
> -       err = libbpf_get_error(obj->btf_vmlinux);
> +       if (obj->custom_btf_path) {
> +               obj->btf_vmlinux = btf__parse(obj->custom_btf_path, NULL);
> +               err = libbpf_get_error(obj->btf_vmlinux);
> +               pr_debug("loading custom vmlinux BTF '%s': %d\n", obj->custom_btf_path, err);
> +       } else {
> +               obj->btf_vmlinux = libbpf_find_kernel_btf();
> +               err = libbpf_get_error(obj->btf_vmlinux);
> +       }


I think it will be more flexible to treat custom_btf as an vmlinux BTF
override, just like [1] did. I can see how in some situations users
might want to treat this custom BTF as either a replacement of vmlinux
BTF or as an augmentation of vmlinux BTF for the purpose of extra
custom CO-RE relocations (e.g., something along the XDP hints that
were discussed recently). For now it's probably enough to implement
"custom BTF is a replacement for vmlinux BTF" policy and, if
necessary, add "custom BTF is in addition to vmlinux BTF" later with
extra opts flag/field.

Keep in mind that this custom BTF is only useful for BPF CO-RE
relocation. Any other kernel feature relying on vmlinux BTF (e.g.,
fentry) won't work with custom BTF because it expects correct BTF type
IDs.


> +
>         if (err) {
>                 pr_warn("Error loading vmlinux BTF: %d\n", err);
>                 obj->btf_vmlinux = NULL;
> @@ -7554,7 +7563,7 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
>  __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, *tmp_btf_path;
>         struct bpf_program *prog;
>         struct bpf_object *obj;
>         char tmp_name[64];
> @@ -7584,6 +7593,13 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
>         obj = bpf_object__new(path, obj_buf, obj_buf_sz, obj_name);
>         if (IS_ERR(obj))
>                 return obj;
> +
> +       tmp_btf_path = OPTS_GET(opts, custom_btf_path, NULL);
> +       if (tmp_btf_path && strlen(tmp_btf_path) < PATH_MAX) {

if strlen() is >= PATH_MAX you'll just silently ignore it? We should
either truncate silently (because PATH_MAX is totally reasonable
assumption) or error out.

> +               obj->custom_btf_path = strdup(tmp_btf_path);
> +               if (!obj->custom_btf_path)
> +                       return ERR_PTR(-ENOMEM);
> +       }
>
>         kconfig = OPTS_GET(opts, kconfig, NULL);
>         if (kconfig) {
> @@ -8702,6 +8718,7 @@ void bpf_object__close(struct bpf_object *obj)
>         for (i = 0; i < obj->nr_maps; i++)
>                 bpf_map__destroy(&obj->maps[i]);
>
> +       zfree(&obj->custom_btf_path);
>         zfree(&obj->kconfig);
>         zfree(&obj->externs);
>         obj->nr_extern = 0;
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 6e61342..16e0f01 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -94,8 +94,12 @@ struct bpf_object_open_opts {
>          * system Kconfig for CONFIG_xxx externs.
>          */
>         const char *kconfig;
> +       /* Specify the path of vmlinux btf to facilitate the use of CO-RE features
> +        * in the old kernel.
> +        */
> +       char *custom_btf_path;
>  };
> -#define bpf_object_open_opts__last_field kconfig
> +#define bpf_object_open_opts__last_field custom_btf_path
>
>  LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
>  LIBBPF_API struct bpf_object *
> --
> 1.8.3.1
>