Re: [PATCH v2 bpf-next 09/16] libbpf: Support for fd_idx

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

On Thu, Apr 22, 2021 at 5:27 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> From: Alexei Starovoitov <ast@kernel.org>
>
> Add support for FD_IDX make libbpf prefer that approach to loading programs.
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---
>  tools/lib/bpf/bpf.c             |  1 +
>  tools/lib/bpf/libbpf.c          | 70 +++++++++++++++++++++++++++++----
>  tools/lib/bpf/libbpf_internal.h |  1 +
>  3 files changed, 65 insertions(+), 7 deletions(-)
>

[...]

> +static int probe_kern_fd_idx(void)
> +{
> +       struct bpf_load_program_attr attr;
> +       struct bpf_insn insns[] = {
> +               BPF_LD_IMM64_RAW(BPF_REG_0, BPF_PSEUDO_MAP_IDX, 0),
> +               BPF_EXIT_INSN(),
> +       };
> +
> +       memset(&attr, 0, sizeof(attr));
> +       attr.prog_type = BPF_PROG_TYPE_SOCKET_FILTER;
> +       attr.insns = insns;
> +       attr.insns_cnt = ARRAY_SIZE(insns);
> +       attr.license = "GPL";
> +
> +       probe_fd(bpf_load_program_xattr(&attr, NULL, 0));

probe_fd() calls close(fd) internally, which technically can interfere
with errno, though close() shouldn't be called because syscall has to
fail on correct kernels... So this should work, but I feel like
open-coding that logic is better than ignoring probe_fd() result.

> +       return errno == EPROTO;
> +}
> +

[...]

> @@ -7239,6 +7279,8 @@ bpf_object__load_progs(struct bpf_object *obj, int log_level)
>         struct bpf_program *prog;
>         size_t i;
>         int err;
> +       struct bpf_map *map;
> +       int *fd_array = NULL;
>
>         for (i = 0; i < obj->nr_programs; i++) {
>                 prog = &obj->programs[i];
> @@ -7247,6 +7289,16 @@ bpf_object__load_progs(struct bpf_object *obj, int log_level)
>                         return err;
>         }
>
> +       if (kernel_supports(FEAT_FD_IDX) && obj->nr_maps) {
> +               fd_array = malloc(sizeof(int) * obj->nr_maps);
> +               if (!fd_array)
> +                       return -ENOMEM;
> +               for (i = 0; i < obj->nr_maps; i++) {
> +                       map = &obj->maps[i];
> +                       fd_array[i] = map->fd;

nit: obj->maps[i].fd will keep it a single line

> +               }
> +       }
> +
>         for (i = 0; i < obj->nr_programs; i++) {
>                 prog = &obj->programs[i];
>                 if (prog_is_subprog(obj, prog))
> @@ -7256,10 +7308,14 @@ bpf_object__load_progs(struct bpf_object *obj, int log_level)
>                         continue;
>                 }
>                 prog->log_level |= log_level;
> +               prog->fd_array = fd_array;

you are not freeing this memory on success, as far as I can see. And
given multiple programs are sharing fd_array, it's a bit problematic
for prog to have fd_array. This is per-object properly, so let's add
it at bpf_object level and clean it up on bpf_object__close()? And by
assigning to obj->fd_array at malloc() site, you won't need to do all
the error-handling free()s below.

>                 err = bpf_program__load(prog, obj->license, obj->kern_version);
> -               if (err)
> +               if (err) {
> +                       free(fd_array);
>                         return err;
> +               }
>         }
> +       free(fd_array);
>         return 0;
>  }
>
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index 6017902c687e..9114c7085f2a 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -204,6 +204,7 @@ struct bpf_prog_load_params {
>         __u32 log_level;
>         char *log_buf;
>         size_t log_buf_sz;
> +       int *fd_array;
>  };
>
>  int libbpf__bpf_prog_load(const struct bpf_prog_load_params *load_attr);
> --
> 2.30.2
>