linux-kernel - Re: [PATCH v2 bpf-next] libbpf: Add bpf_obj_get

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Message-ID: <CAEf4BzbWpQS6js5LfS80PkqwDwcLc+NgzfqqUTG-CkLP16shCg@mail.gmail.com>
Date:   Wed, 27 Jul 2022 16:02:43 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Joe Burton <jevburton.kernel@...il.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Joe Burton <jevburton@...gle.com>
Subject: Re: [PATCH v2 bpf-next] libbpf: Add bpf_obj_get_opts()

On Tue, Jul 19, 2022 at 12:40 PM Joe Burton <jevburton.kernel@...il.com> wrote:
>
> From: Joe Burton <jevburton@...gle.com>
>
> Add an extensible variant of bpf_obj_get() capable of setting the
> `file_flags` parameter.
>
> This parameter is needed to enable unprivileged access to BPF maps.
> Without a method like this, users must manually make the syscall.
>
> Signed-off-by: Joe Burton <jevburton@...gle.com>
> ---
>  tools/lib/bpf/bpf.c      | 10 ++++++++++
>  tools/lib/bpf/bpf.h      |  9 +++++++++
>  tools/lib/bpf/libbpf.map |  1 +
>  3 files changed, 20 insertions(+)
>

I agree that bpf_obj_get_opts should be separate from bpf_get_fd_opts.
Just because both currently have file_flags in them doesn't mean that
they should/will always stay in sync. So two separate opts for two
separate APIs makes sense to me.

So I'd accept this patch, but please see a few small things below and
send v3. Thanks!


> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> index 5eb0df90eb2b..5acb0e8bd13c 100644
> --- a/tools/lib/bpf/bpf.c
> +++ b/tools/lib/bpf/bpf.c
> @@ -578,12 +578,22 @@ int bpf_obj_pin(int fd, const char *pathname)
>  }
>
>  int bpf_obj_get(const char *pathname)
> +{
> +       LIBBPF_OPTS(bpf_obj_get_opts, opts);

if you were doing it this way, here should be an empty line. But
really you can/should just pass NULL instead of opts in this case.

> +       return bpf_obj_get_opts(pathname, &opts);
> +}
> +
> +int bpf_obj_get_opts(const char *pathname, const struct bpf_obj_get_opts *opts)
>  {
>         union bpf_attr attr;
>         int fd;
>
> +       if (!OPTS_VALID(opts, bpf_obj_get_opts))
> +               return libbpf_err(-EINVAL);
> +
>         memset(&attr, 0, sizeof(attr));
>         attr.pathname = ptr_to_u64((void *)pathname);
> +       attr.file_flags = OPTS_GET(opts, file_flags, 0);
>
>         fd = sys_bpf_fd(BPF_OBJ_GET, &attr, sizeof(attr));
>         return libbpf_err_errno(fd);
> diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> index 88a7cc4bd76f..f31b493b5f9a 100644
> --- a/tools/lib/bpf/bpf.h
> +++ b/tools/lib/bpf/bpf.h
> @@ -270,8 +270,17 @@ LIBBPF_API int bpf_map_update_batch(int fd, const void *keys, const void *values
>                                     __u32 *count,
>                                     const struct bpf_map_batch_opts *opts);
>
> +struct bpf_obj_get_opts {
> +       size_t sz; /* size of this struct for forward/backward compatibility */
> +
> +       __u32 file_flags;

please add size_t :0; to avoid non-zero-initialized padding  (we do it
in a lot of other opts structs)


> +};
> +#define bpf_obj_get_opts__last_field file_flags
> +
>  LIBBPF_API int bpf_obj_pin(int fd, const char *pathname);
>  LIBBPF_API int bpf_obj_get(const char *pathname);
> +LIBBPF_API int bpf_obj_get_opts(const char *pathname,
> +                               const struct bpf_obj_get_opts *opts);
>
>  struct bpf_prog_attach_opts {
>         size_t sz; /* size of this struct for forward/backward compatibility */
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 0625adb9e888..119e6e1ea7f1 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -355,6 +355,7 @@ LIBBPF_0.8.0 {
>
>  LIBBPF_1.0.0 {
>         global:
> +               bpf_obj_get_opts;
>                 bpf_prog_query_opts;
>                 bpf_program__attach_ksyscall;
>                 btf__add_enum64;
> --
> 2.37.0.170.g444d1eabd0-goog
>