Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a struct/union

[Alexei Starovoitov]

On Mon, Jul 31, 2023 at 12:30 AM Masami Hiramatsu (Google)
<mhiramat@xxxxxxxxxx> wrote:
>
> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> ---
>  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().
>  Changes in v4:
>   - Use a stack for searching in anonymous members instead of nested call.
> ---
>  include/linux/btf.h |    3 +++
>  kernel/bpf/btf.c    |   40 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 43 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..8d81a4ffa67b 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -958,6 +958,46 @@ const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s3
>                 return NULL;
>  }
>
> +#define BTF_ANON_STACK_MAX     16
> +
> +/*
> + * 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_type *anon_stack[BTF_ANON_STACK_MAX];
> +       const struct btf_member *member;
> +       const char *name;
> +       int i, top = 0;
> +
> +retry:
> +       if (!btf_type_is_struct(type))
> +               return ERR_PTR(-EINVAL);
> +
> +       for_each_member(i, type, member) {
> +               if (!member->name_off) {
> +                       /* Anonymous union/struct: push it for later use */
> +                       type = btf_type_skip_modifiers(btf, member->type, NULL);
> +                       if (type && top < BTF_ANON_STACK_MAX)
> +                               anon_stack[top++] = type;
> +               } else {
> +                       name = btf_name_by_offset(btf, member->name_off);
> +                       if (name && !strcmp(member_name, name))
> +                               return member;
> +               }
> +       }
> +       if (top > 0) {
> +               /* Pop from the anonymous stack and retry */
> +               type = anon_stack[--top];
> +               goto retry;
> +       }

Looks good, but I don't see a test case for this.
The logic is a bit tricky. I'd like to have a selftest that covers it.

You probably need to drop Alan's reviewed-by, since the patch is quite
different from the time he reviewed it.

Assuming that is addressed. How do we merge the series?
The first 3 patches have serious conflicts with bpf trees.

Maybe send the first 3 with extra selftest for above recursion
targeting bpf-next then we can have a merge commit that Steven can pull
into tracing?

Or if we can have acks for patches 4-9 we can pull the whole set into bpf-next.