linux-kernel - Re: [PATCH v3 3/9] bpf/btf: Add a function to search a member of a struct/union

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Date:   Thu, 27 Jul 2023 08:39:02 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        bpf <bpf@...r.kernel.org>, Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH v3 3/9] bpf/btf: Add a function to search a member of a struct/union

On Wed, Jul 26, 2023 at 6:00 AM Masami Hiramatsu (Google)
<mhiramat@...nel.org> wrote:
>
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
>
> Add btf_find_struct_member() API to search a member of a given data structure
> or union from the member's name.
>
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> Reviewed-by: Alan Maguire <alan.maguire@...cle.com>
> ---
>  Changes in v3:
>   - Remove simple input check.
>   - Fix unneeded IS_ERR_OR_NULL() check for btf_type_by_id().
>   - Move the code next to btf_get_func_param().
>   - Use for_each_member() macro instead of for-loop.
>   - Use btf_type_skip_modifiers() instead of btf_type_by_id().
> ---
>  include/linux/btf.h |    3 +++
>  kernel/bpf/btf.c    |   35 +++++++++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+)
>
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index 20e3a07eef8f..4b10d57ceee0 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -226,6 +226,9 @@ const struct btf_type *btf_find_func_proto(const char *func_name,
>                                            struct btf **btf_p);
>  const struct btf_param *btf_get_func_param(const struct btf_type *func_proto,
>                                            s32 *nr);
> +const struct btf_member *btf_find_struct_member(struct btf *btf,
> +                                               const struct btf_type *type,
> +                                               const char *member_name);
>
>  #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 f7b25c615269..5258870030fc 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -958,6 +958,41 @@ const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s3
>                 return NULL;
>  }
>
> +/*
> + * Find a member of data structure/union by name and return it.
> + * Return NULL if not found, or -EINVAL if parameter is invalid.
> + */
> +const struct btf_member *btf_find_struct_member(struct btf *btf,
> +                                               const struct btf_type *type,
> +                                               const char *member_name)
> +{
> +       const struct btf_member *member, *ret;
> +       const char *name;
> +       int i;
> +
> +       if (!btf_type_is_struct(type))
> +               return ERR_PTR(-EINVAL);
> +
> +       for_each_member(i, type, member) {
> +               if (!member->name_off) {
> +                       /* unnamed member: dig deeper */
> +                       type = btf_type_skip_modifiers(btf, member->type, NULL);
> +                       if (type) {
> +                               ret = btf_find_struct_member(btf, type,
> +                                                            member_name);

Unbounded recursion in the kernel? Ouch. That might cause issues.
Please convert it to a loop. It doesn't have to be recursive.

> +                               if (!IS_ERR_OR_NULL(ret))
> +                                       return ret;
> +                       }
> +               } else {
> +                       name = btf_name_by_offset(btf, member->name_off);
> +                       if (name && !strcmp(member_name, name))
> +                               return member;
> +               }
> +       }
> +
> +       return NULL;
> +}
> +
>  #define BTF_SHOW_MAX_ITER      10
>
>  #define BTF_KIND_BIT(kind)     (1ULL << kind)
>