Re: [PATCH v4 bpf-next 06/14] bpf: Use BTF_ID to resolve bpf_ctx_convert struct

[Yonghong Song <yhs@fb.com>]

On 6/25/20 3:12 PM, Jiri Olsa wrote:
> This way the ID is resolved during compile time,
> and we can remove the runtime name search.
> 
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>   kernel/bpf/btf.c | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 4da6b0770ff9..701a2cb5dfb2 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -18,6 +18,7 @@
>   #include <linux/sort.h>
>   #include <linux/bpf_verifier.h>
>   #include <linux/btf.h>
> +#include <linux/btf_ids.h>
>   #include <linux/skmsg.h>
>   #include <linux/perf_event.h>
>   #include <net/sock.h>
> @@ -3621,6 +3622,9 @@ static int btf_translate_to_vmlinux(struct bpf_verifier_log *log,
>   	return kern_ctx_type->type;
>   }
>   
> +BTF_ID_LIST(bpf_ctx_convert_btf_id)
> +BTF_ID(struct, bpf_ctx_convert)
> +
>   struct btf *btf_parse_vmlinux(void)
>   {
>   	struct btf_verifier_env *env = NULL;
> @@ -3659,10 +3663,10 @@ struct btf *btf_parse_vmlinux(void)
>   	if (err)
>   		goto errout;
>   
> -	/* find struct bpf_ctx_convert for type checking later */
> -	btf_id = btf_find_by_name_kind(btf, "bpf_ctx_convert", BTF_KIND_STRUCT);
> -	if (btf_id < 0) {
> -		err = btf_id;
> +	/* struct bpf_ctx_convert for type checking later */
> +	btf_id = bpf_ctx_convert_btf_id[0];
> +	if (btf_id <= 0) {

Just want to double check. Is it possible btf_id < 0 since previous 
patch did not check < 0?

> +		err = -EINVAL;
>   		goto errout;
>   	}
>   	/* btf_parse_vmlinux() runs under bpf_verifier_lock */
>