Re: [PATCH bpf-next v3 01/10] bpf: btf: Add btf_type_by_name_kind

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

On Thu, Jan 23, 2020 at 7:25 AM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> - The LSM code does the combination of btf_find_by_name_kind and
>   btf_type_by_id a couple of times to figure out the BTF type for
>   security_hook_heads and security_list_options.
> - Add an extern for btf_vmlinux in btf.h
>
> Signed-off-by: KP Singh <kpsingh@google.com>
> Reviewed-by: Brendan Jackman <jackmanb@google.com>
> Reviewed-by: Florent Revest <revest@google.com>
> Reviewed-by: Thomas Garnier <thgarnie@google.com>
> ---
>  include/linux/btf.h |  3 +++
>  kernel/bpf/btf.c    | 12 ++++++++++++
>  2 files changed, 15 insertions(+)
>
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index 5c1ea99b480f..d4e859f90a39 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -15,6 +15,7 @@ struct btf_type;
>  union bpf_attr;
>
>  extern const struct file_operations btf_fops;
> +extern struct btf *btf_vmlinux;
>
>  void btf_put(struct btf *btf);
>  int btf_new_fd(const union bpf_attr *attr);
> @@ -66,6 +67,8 @@ const struct btf_type *
>  btf_resolve_size(const struct btf *btf, const struct btf_type *type,
>                  u32 *type_size, const struct btf_type **elem_type,
>                  u32 *total_nelems);
> +const struct btf_type *btf_type_by_name_kind(
> +       struct btf *btf, const char *name, u8 kind);
>
>  #define for_each_member(i, struct_type, member)                        \
>         for (i = 0, member = btf_type_member(struct_type);      \
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 32963b6d5a9c..ea53c16802cb 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -441,6 +441,18 @@ const struct btf_type *btf_type_resolve_func_ptr(const struct btf *btf,
>         return NULL;
>  }
>
> +const struct btf_type *btf_type_by_name_kind(
> +       struct btf *btf, const char *name, u8 kind)
> +{
> +       s32 type_id;
> +
> +       type_id = btf_find_by_name_kind(btf, name, kind);
> +       if (type_id < 0)
> +               return ERR_PTR(type_id);
> +
> +       return btf_type_by_id(btf, type_id);
> +}
> +

is it worth having this as a separate global function? If
btf_find_by_name_kind returns valid ID, then you don't really need to
check btf_type_by_id result, it is always going to be valid. So the
pattern becomes:

type_id = btf_find_by_name_kind(btf, name, kind);
if (type_id < 0)
  goto handle_error;
t = btf_type_by_id(btf, type_id);
/* now just use t */

which is not much more verbose than:

t = btf_type_by_name_kind(btf, name, kind);
if (IS_ERR(t))
  goto handle_error
/* now use t */


>  /* Types that act only as a source, not sink or intermediate
>   * type when resolving.
>   */
> --
> 2.20.1
>