Re: [PATCH bpf-next 2/6] libbpf: Add BTF_KIND_FLOAT support

[Yonghong Song <yhs@fb.com>]

On 2/15/21 5:12 PM, Ilya Leoshkevich wrote:
> The logic follows that of BTF_KIND_INT most of the time. Sanitization
> replaces BTF_KIND_FLOATs with equally-sized BTF_KIND_INTs on older
> kernels.
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>   tools/lib/bpf/btf.c             | 44 +++++++++++++++++++++++++++++++++
>   tools/lib/bpf/btf.h             |  8 ++++++
>   tools/lib/bpf/btf_dump.c        |  4 +++
>   tools/lib/bpf/libbpf.c          | 29 +++++++++++++++++++++-
>   tools/lib/bpf/libbpf.map        |  5 ++++
>   tools/lib/bpf/libbpf_internal.h |  2 ++
>   6 files changed, 91 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index d9c10830d749..07a30e98c3de 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -291,6 +291,7 @@ static int btf_type_size(const struct btf_type *t)
>   	case BTF_KIND_PTR:
>   	case BTF_KIND_TYPEDEF:
>   	case BTF_KIND_FUNC:
> +	case BTF_KIND_FLOAT:
>   		return base_size;
>   	case BTF_KIND_INT:
>   		return base_size + sizeof(__u32);
[...]
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index d43cc3f29dae..703988d68e73 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -178,6 +178,8 @@ enum kern_feature_id {
>   	FEAT_PROG_BIND_MAP,
>   	/* Kernel support for module BTFs */
>   	FEAT_MODULE_BTF,
> +	/* BTF_KIND_FLOAT support */
> +	FEAT_BTF_FLOAT,
>   	__FEAT_CNT,
>   };
>   
> @@ -1935,6 +1937,7 @@ static const char *btf_kind_str(const struct btf_type *t)
>   	case BTF_KIND_FUNC_PROTO: return "func_proto";
>   	case BTF_KIND_VAR: return "var";
>   	case BTF_KIND_DATASEC: return "datasec";
> +	case BTF_KIND_FLOAT: return "float";
>   	default: return "unknown";
>   	}
>   }
> @@ -2384,15 +2387,17 @@ static bool btf_needs_sanitization(struct bpf_object *obj)
>   {
>   	bool has_func_global = kernel_supports(FEAT_BTF_GLOBAL_FUNC);
>   	bool has_datasec = kernel_supports(FEAT_BTF_DATASEC);
> +	bool has_float = kernel_supports(FEAT_BTF_FLOAT);
>   	bool has_func = kernel_supports(FEAT_BTF_FUNC);
>   
> -	return !has_func || !has_datasec || !has_func_global;
> +	return !has_func || !has_datasec || !has_func_global || !has_float;
>   }
>   
>   static void bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
>   {
>   	bool has_func_global = kernel_supports(FEAT_BTF_GLOBAL_FUNC);
>   	bool has_datasec = kernel_supports(FEAT_BTF_DATASEC);
> +	bool has_float = kernel_supports(FEAT_BTF_FLOAT);
>   	bool has_func = kernel_supports(FEAT_BTF_FUNC);
>   	struct btf_type *t;
>   	int i, j, vlen;
> @@ -2445,6 +2450,9 @@ static void bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
>   		} else if (!has_func_global && btf_is_func(t)) {
>   			/* replace BTF_FUNC_GLOBAL with BTF_FUNC_STATIC */
>   			t->info = BTF_INFO_ENC(BTF_KIND_FUNC, 0, 0);
> +		} else if (!has_float && btf_is_float(t)) {
> +			/* replace FLOAT with INT */
> +			t->info = BTF_INFO_ENC(BTF_KIND_FLOAT, 0, 0);

You can replace float with a "pointer to void" type.

>   		}
>   	}
>   }
> @@ -3882,6 +3890,18 @@ static int probe_kern_btf_datasec(void)
>   					     strs, sizeof(strs)));
>   }
>   
> +static int probe_kern_btf_float(void)
> +{
> +	static const char strs[] = "\0float";
> +	__u32 types[] = {
> +		/* float */
> +		BTF_TYPE_FLOAT_ENC(1, 4),
> +	};
> +
> +	return probe_fd(libbpf__load_raw_btf((char *)types, sizeof(types),
> +					     strs, sizeof(strs)));
> +}
> +
>   static int probe_kern_array_mmap(void)
>   {
>   	struct bpf_create_map_attr attr = {
[...]