Date:   Thu, 22 Jul 2021 17:39:43 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH bpf-next v2 2/5] libbpf: rename btf__get_from_id() as btf__load_from_kernel_by_id()

On Wed, Jul 21, 2021 at 8:38 AM Quentin Monnet <quentin@...valent.com> wrote:
>
> Rename function btf__get_from_id() as btf__load_from_kernel_by_id() to
> better indicate what the function does. Change the new function so that,
> instead of requiring a pointer to the pointer to update and returning
> with an error code, it takes a single argument (the id of the BTF
> object) and returns the corresponding pointer. This is more in line with
> the existing constructors.
>
> The other tools calling the deprecated btf__get_from_id() function will
> be updated in a future commit.
>
> References:
>
> - https://github.com/libbpf/libbpf/issues/278
> - https://github.com/libbpf/libbpf/wiki/Libbpf:-the-road-to-v1.0#btfh-apis
>
> v2:
> - Instead of a simple renaming, change the new function to make it
>   return the pointer to the btf struct.
> - API v0.5.0 instead of v0.6.0.

We generally keep such version changes to cover letters. It keeps each
individual commit clean and collects full history in the cover letter
which becomes a body of merge commit when the whole patch set is
applied. For next revision please consolidate the history in the cover
letter. Thanks!

>
> Signed-off-by: Quentin Monnet <quentin@...valent.com>
> Acked-by: John Fastabend <john.fastabend@...il.com>
> ---
>  tools/lib/bpf/btf.c      | 25 +++++++++++++++++--------
>  tools/lib/bpf/btf.h      |  1 +
>  tools/lib/bpf/libbpf.c   |  5 +++--
>  tools/lib/bpf/libbpf.map |  1 +
>  4 files changed, 22 insertions(+), 10 deletions(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 7e0de560490e..6654bdee7ad7 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -1383,21 +1383,30 @@ struct btf *btf_get_from_fd(int btf_fd, struct btf *base_btf)
>         return btf;
>  }
>
> +struct btf *btf__load_from_kernel_by_id(__u32 id)
> +{
> +       struct btf *btf;
> +       int btf_fd;
> +
> +       btf_fd = bpf_btf_get_fd_by_id(id);
> +       if (btf_fd < 0)
> +               return ERR_PTR(-errno);

please use libbpf_err_ptr() for consistency, see
bpf_object__open_mem() for an example

> +
> +       btf = btf_get_from_fd(btf_fd, NULL);
> +       close(btf_fd);
> +
> +       return libbpf_ptr(btf);
> +}
> +
>  int btf__get_from_id(__u32 id, struct btf **btf)
>  {
>         struct btf *res;
> -       int err, btf_fd;
> +       int err;
>
>         *btf = NULL;
> -       btf_fd = bpf_btf_get_fd_by_id(id);
> -       if (btf_fd < 0)
> -               return libbpf_err(-errno);
> -
> -       res = btf_get_from_fd(btf_fd, NULL);
> +       res = btf__load_from_kernel_by_id(id);
>         err = libbpf_get_error(res);
>
> -       close(btf_fd);
> -
>         if (err)
>                 return libbpf_err(err);
>
> diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
> index fd8a21d936ef..3db9446bc133 100644
> --- a/tools/lib/bpf/btf.h
> +++ b/tools/lib/bpf/btf.h
> @@ -68,6 +68,7 @@ LIBBPF_API const void *btf__get_raw_data(const struct btf *btf, __u32 *size);
>  LIBBPF_API const char *btf__name_by_offset(const struct btf *btf, __u32 offset);
>  LIBBPF_API const char *btf__str_by_offset(const struct btf *btf, __u32 offset);
>  LIBBPF_API int btf__get_from_id(__u32 id, struct btf **btf);
> +LIBBPF_API struct btf *btf__load_from_kernel_by_id(__u32 id);

let's move this definition to after btf__parse() to keep all
"constructors" together (we can move btf__get_from_id() there for
completeness as well, I suppose).

>  LIBBPF_API int btf__get_map_kv_tids(const struct btf *btf, const char *map_name,
>                                     __u32 expected_key_size,
>                                     __u32 expected_value_size,
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 242e97892043..eff005b1eba1 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -9576,8 +9576,8 @@ static int libbpf_find_prog_btf_id(const char *name, __u32 attach_prog_fd)
>  {
>         struct bpf_prog_info_linear *info_linear;
>         struct bpf_prog_info *info;
> -       struct btf *btf = NULL;
>         int err = -EINVAL;
> +       struct btf *btf;
>
>         info_linear = bpf_program__get_prog_info_linear(attach_prog_fd, 0);
>         err = libbpf_get_error(info_linear);
> @@ -9591,7 +9591,8 @@ static int libbpf_find_prog_btf_id(const char *name, __u32 attach_prog_fd)
>                 pr_warn("The target program doesn't have BTF\n");
>                 goto out;
>         }
> -       if (btf__get_from_id(info->btf_id, &btf)) {
> +       btf = btf__load_from_kernel_by_id(info->btf_id);
> +       if (libbpf_get_error(btf)) {

there seems to be a bug in existing code and you are keeping it. On
error err will be 0. Let's fix it. Same for above if (!info->btf_id),
please fix that as well while you are at it.

>                 pr_warn("Failed to get BTF of the program\n");
>                 goto out;
>         }
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index f7d52d76ca3a..ca8cc7a7faad 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -373,6 +373,7 @@ LIBBPF_0.5.0 {
>                 bpf_map__initial_value;
>                 bpf_map_lookup_and_delete_elem_flags;
>                 bpf_object__gen_loader;
> +               btf__load_from_kernel_by_id;
>                 btf__load_into_kernel;
>                 btf_dump__dump_type_data;
>                 libbpf_set_strict_mode;
> --
> 2.30.2
>

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