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From:   Joe Burton <jevburton@google.com>
Date:   Tue, 26 Oct 2021 15:36:12 -0700
Message-ID: <CAL0ypaAyBSnuKD1ZSeWL6nofMJ343Hqdp5HTpxXOt6qsxurJ5g@mail.gmail.com>
Subject: Re: [PATCH] libbpf: Deprecate bpf_objects_list
To:     Joe Burton <jevburton.kernel@gmail.com>
Cc:     Andrii Nakryiko <andrii@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, trivial@kernel.org
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I messed up the mailing lists and will be resending this patch series
in a moment. Apologies for the noise.

Best,
Joe Burton


On Tue, Oct 26, 2021 at 3:27 PM Joe Burton <jevburton.kernel@gmail.com> wrote:
>
> From: Joe Burton <jevburton@google.com>
>
> Add a flag to `enum libbpf_strict_mode' to disable the global
> `bpf_objects_list', preventing race conditions when concurrent threads
> call bpf_object__open() or bpf_object__close().
>
> bpf_object__next() will return NULL if this option is set.
>
> Callers may achieve the same workflow by tracking bpf_objects in
> application code.
>
>   [0] Closes: https://github.com/libbpf/libbpf/issues/293
>
> Signed-off-by: Joe Burton <jevburton@google.com>
> ---
>  tools/lib/bpf/libbpf.c        | 8 +++++++-
>  tools/lib/bpf/libbpf.h        | 3 ++-
>  tools/lib/bpf/libbpf_legacy.h | 6 ++++++
>  3 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 2fbed2d4a645..59d39ce9f375 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1148,6 +1148,7 @@ static struct bpf_object *bpf_object__new(const char *path,
>                                           size_t obj_buf_sz,
>                                           const char *obj_name)
>  {
> +       bool strict = (libbpf_mode & LIBBPF_STRICT_NO_OBJECT_LIST);
>         struct bpf_object *obj;
>         char *end;
>
> @@ -1188,7 +1189,8 @@ static struct bpf_object *bpf_object__new(const char *path,
>         obj->loaded = false;
>
>         INIT_LIST_HEAD(&obj->list);
> -       list_add(&obj->list, &bpf_objects_list);
> +       if (!strict)
> +               list_add(&obj->list, &bpf_objects_list);
>         return obj;
>  }
>
> @@ -7935,6 +7937,10 @@ struct bpf_object *
>  bpf_object__next(struct bpf_object *prev)
>  {
>         struct bpf_object *next;
> +       bool strict = (libbpf_mode & LIBBPF_STRICT_NO_OBJECT_LIST);
> +
> +       if (strict)
> +               return NULL;
>
>         if (!prev)
>                 next = list_first_entry(&bpf_objects_list,
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index e1900819bfab..defabdbe7760 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -168,7 +168,8 @@ LIBBPF_API struct bpf_program *
>  bpf_object__find_program_by_name(const struct bpf_object *obj,
>                                  const char *name);
>
> -LIBBPF_API struct bpf_object *bpf_object__next(struct bpf_object *prev);
> +LIBBPF_API LIBBPF_DEPRECATED_SINCE(0, 7, "track bpf_objects in application code instead")
> +struct bpf_object *bpf_object__next(struct bpf_object *prev);
>  #define bpf_object__for_each_safe(pos, tmp)                    \
>         for ((pos) = bpf_object__next(NULL),            \
>                 (tmp) = bpf_object__next(pos);          \
> diff --git a/tools/lib/bpf/libbpf_legacy.h b/tools/lib/bpf/libbpf_legacy.h
> index 29ccafab11a8..5ba5c9beccfa 100644
> --- a/tools/lib/bpf/libbpf_legacy.h
> +++ b/tools/lib/bpf/libbpf_legacy.h
> @@ -57,6 +57,12 @@ enum libbpf_strict_mode {
>          * function name instead of section name.
>          */
>         LIBBPF_STRICT_SEC_NAME = 0x04,
> +       /*
> +        * Disable the global 'bpf_objects_list'. Maintaining this list adds
> +        * a race condition to bpf_object__open() and bpf_object__close().
> +        * Clients can maintain it on their own if it is valuable for them.
> +        */
> +       LIBBPF_STRICT_NO_OBJECT_LIST = 0x08,
>
>         __LIBBPF_STRICT_LAST,
>  };
> --
> 2.33.0.1079.g6e70778dc9-goog
>