Re: [PATCH v2 bpf-next 2/6] libbpf: Add BTF_KIND_FLOAT support

[Yonghong Song <yhs@fb.com>]

On 2/18/21 6:25 PM, Ilya Leoshkevich wrote:
> The logic follows that of BTF_KIND_INT most of the time. Sanitization
> replaces BTF_KIND_FLOATs with BTF_KIND_TYPEDEFs pointing to
> equally-sized BTF_KIND_ARRAYs 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          | 45 ++++++++++++++++++++++++++++++++-
>   tools/lib/bpf/libbpf.map        |  5 ++++
>   tools/lib/bpf/libbpf_internal.h |  2 ++
>   6 files changed, 107 insertions(+), 1 deletion(-)
> 
[...]
> diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> index 2f9d685bd522..5e957fcceee6 100644
> --- a/tools/lib/bpf/btf_dump.c
> +++ b/tools/lib/bpf/btf_dump.c
> @@ -279,6 +279,7 @@ static int btf_dump_mark_referenced(struct btf_dump *d)
>   		case BTF_KIND_INT:
>   		case BTF_KIND_ENUM:
>   		case BTF_KIND_FWD:
> +		case BTF_KIND_FLOAT:
>   			break;
>   
>   		case BTF_KIND_VOLATILE:
> @@ -453,6 +454,7 @@ static int btf_dump_order_type(struct btf_dump *d, __u32 id, bool through_ptr)
>   
>   	switch (btf_kind(t)) {
>   	case BTF_KIND_INT:
> +	case BTF_KIND_FLOAT:
>   		tstate->order_state = ORDERED;
>   		return 0;
>   
> @@ -1133,6 +1135,7 @@ static void btf_dump_emit_type_decl(struct btf_dump *d, __u32 id,
>   		case BTF_KIND_STRUCT:
>   		case BTF_KIND_UNION:
>   		case BTF_KIND_TYPEDEF:
> +		case BTF_KIND_FLOAT:
>   			goto done;
>   		default:
>   			pr_warn("unexpected type in decl chain, kind:%u, id:[%u]\n",
> @@ -1247,6 +1250,7 @@ static void btf_dump_emit_type_chain(struct btf_dump *d,
>   
>   		switch (kind) {
>   		case BTF_KIND_INT:
> +		case BTF_KIND_FLOAT:
>   			btf_dump_emit_mods(d, decls);
>   			name = btf_name_of(d, t->name_off);
>   			btf_dump_printf(d, "%s", name);
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index d43cc3f29dae..3b170066d613 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";
>   	}
>   }
> @@ -2384,18 +2387,22 @@ 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;
> +	int t_u32 = 0;
> +	int t_u8 = 0;
>   
>   	for (i = 1; i <= btf__get_nr_types(btf); i++) {
>   		t = (struct btf_type *)btf__type_by_id(btf, i);
> @@ -2445,6 +2452,23 @@ 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 TYPEDEF(u8[]) */
> +			int t_array;
> +			__u32 size;
> +
> +			size = t->size;
> +			if (!t_u8)
> +				t_u8 = btf__add_int(btf, "u8", 1, 0);
> +			if (!t_u32)
> +				t_u32 = btf__add_int(btf, "u32", 4, 0);
> +			t_array = btf__add_array(btf, t_u32, t_u8, size);
> +
> +			/* adding new types may have invalidated t */
> +			t = (struct btf_type *)btf__type_by_id(btf, i);
> +
> +			t->info = BTF_INFO_ENC(BTF_KIND_TYPEDEF, 0, 0);

This won't work. The typedef must have a valid name. Otherwise, kernel 
will reject it. A const char array should be okay here.

> +			t->type = t_array;
>   		}
>   	}
>   }
[...]