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

Alexei Starovoitov
2023-07-27T15:39:20Z

On Wed, Jul 26, 2023 at 6:00 AM Masami Hiramatsu (Google)
<mhiramat@kernel.org> wrote:
>
> From: Masami Hiramatsu (Google) <mhiramat@kernel.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@kernel.org>
> Reviewed-by: Alan Maguire <alan.maguire@oracle.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)
>