libbpf: Add enum64 sanitization

When old kernel does not support enum64 but user space btf
contains non-zero enum kflag or enum64, libbpf needs to
do proper sanitization so modified btf can be accepted
by the kernel.

Sanitization for enum kflag can be achieved by clearing
the kflag bit. For enum64, the type is replaced with an
union of integer member types and the integer member size
must be smaller than enum64 size. If such an integer
type cannot be found, a new type is created and used
for union members.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220607062636.3721375-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 52 insertions, 4 deletions

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index b03165687936..a0f5aae8626b 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2242,6 +2242,7 @@ static const char *__btf_kind_str(__u16 kind)
 	case BTF_KIND_FLOAT: return "float";
 	case BTF_KIND_DECL_TAG: return "decl_tag";
 	case BTF_KIND_TYPE_TAG: return "type_tag";
+	case BTF_KIND_ENUM64: return "enum64";
 	default: return "unknown";
 	}
 }
@@ -2770,12 +2771,13 @@ static bool btf_needs_sanitization(struct bpf_object *obj)
 	bool has_func = kernel_supports(obj, FEAT_BTF_FUNC);
 	bool has_decl_tag = kernel_supports(obj, FEAT_BTF_DECL_TAG);
 	bool has_type_tag = kernel_supports(obj, FEAT_BTF_TYPE_TAG);
+	bool has_enum64 = kernel_supports(obj, FEAT_BTF_ENUM64);
 
 	return !has_func || !has_datasec || !has_func_global || !has_float ||
-	       !has_decl_tag || !has_type_tag;
+	       !has_decl_tag || !has_type_tag || !has_enum64;
 }
 
-static void bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
+static int bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
 {
 	bool has_func_global = kernel_supports(obj, FEAT_BTF_GLOBAL_FUNC);
 	bool has_datasec = kernel_supports(obj, FEAT_BTF_DATASEC);
@@ -2783,6 +2785,8 @@ static void bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
 	bool has_func = kernel_supports(obj, FEAT_BTF_FUNC);
 	bool has_decl_tag = kernel_supports(obj, FEAT_BTF_DECL_TAG);
 	bool has_type_tag = kernel_supports(obj, FEAT_BTF_TYPE_TAG);
+	bool has_enum64 = kernel_supports(obj, FEAT_BTF_ENUM64);
+	__u32 enum64_placeholder_id = 0;
 	struct btf_type *t;
 	int i, j, vlen;
 
@@ -2845,8 +2849,32 @@ static void bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
 			/* replace TYPE_TAG with a CONST */
 			t->name_off = 0;
 			t->info = BTF_INFO_ENC(BTF_KIND_CONST, 0, 0);
-		}
+		} else if (!has_enum64 && btf_is_enum(t)) {
+			/* clear the kflag */
+			t->info = btf_type_info(btf_kind(t), btf_vlen(t), false);
+		} else if (!has_enum64 && btf_is_enum64(t)) {
+			/* replace ENUM64 with a union */
+			struct btf_member *m;
+
+			if (enum64_placeholder_id == 0) {
+				enum64_placeholder_id = btf__add_int(btf, "enum64_placeholder", 1, 0);
+				if (enum64_placeholder_id < 0)
+					return enum64_placeholder_id;
+
+				t = (struct btf_type *)btf__type_by_id(btf, i);
+			}
+
+			m = btf_members(t);
+			vlen = btf_vlen(t);
+			t->info = BTF_INFO_ENC(BTF_KIND_UNION, 0, vlen);
+			for (j = 0; j < vlen; j++, m++) {
+				m->type = enum64_placeholder_id;
+				m->offset = 0;
+			}
+                }
 	}
+
+	return 0;
 }
 
 static bool libbpf_needs_btf(const struct bpf_object *obj)
@@ -3184,7 +3212,9 @@ static int bpf_object__sanitize_and_load_btf(struct bpf_object *obj)
 
 		/* enforce 8-byte pointers for BPF-targeted BTFs */
 		btf__set_pointer_size(obj->btf, 8);
-		bpf_object__sanitize_btf(obj, kern_btf);
+		err = bpf_object__sanitize_btf(obj, kern_btf);
+		if (err)
+			return err;
 	}
 
 	if (obj->gen_loader) {
@@ -3691,6 +3721,10 @@ static enum kcfg_type find_kcfg_type(const struct btf *btf, int id,
 		if (strcmp(name, "libbpf_tristate"))
 			return KCFG_UNKNOWN;
 		return KCFG_TRISTATE;
+	case BTF_KIND_ENUM64:
+		if (strcmp(name, "libbpf_tristate"))
+			return KCFG_UNKNOWN;
+		return KCFG_TRISTATE;
 	case BTF_KIND_ARRAY:
 		if (btf_array(t)->nelems == 0)
 			return KCFG_UNKNOWN;
@@ -4874,6 +4908,17 @@ static int probe_kern_bpf_cookie(void)
 	return probe_fd(ret);
 }
 
+static int probe_kern_btf_enum64(void)
+{
+	static const char strs[] = "\0enum64";
+	__u32 types[] = {
+		BTF_TYPE_ENC(1, BTF_INFO_ENC(BTF_KIND_ENUM64, 0, 0), 8),
+	};
+
+	return probe_fd(libbpf__load_raw_btf((char *)types, sizeof(types),
+					     strs, sizeof(strs)));
+}
+
 enum kern_feature_result {
 	FEAT_UNKNOWN = 0,
 	FEAT_SUPPORTED = 1,
@@ -4939,6 +4984,9 @@ static struct kern_feature_desc {
 	[FEAT_BPF_COOKIE] = {
 		"BPF cookie support", probe_kern_bpf_cookie,
 	},
+	[FEAT_BTF_ENUM64] = {
+		"BTF_KIND_ENUM64 support", probe_kern_btf_enum64,
+	},
 };
 
 bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)