Merge branch 'libbpf: BTF writer APIs'

Andrii Nakryiko says:

====================
This patch set introduces a new set of BTF APIs to libbpf that allow to
conveniently produce BTF types and strings. These APIs will allow libbpf to do
more intrusive modifications of program's BTF (by rewriting it, at least as of
right now), which is necessary for the upcoming libbpf static linking. But
they are complete and generic, so can be adopted by anyone who has a need to
produce BTF type information.

One such example outside of libbpf is pahole, which was actually converted to
these APIs (locally, pending landing of these changes in libbpf) completely
and shows reduction in amount of custom pahole code necessary and brings nice
savings in memory usage (about 370MB reduction at peak for my kernel
configuration) and even BTF deduplication times (one second reduction,
23.7s -> 22.7s). Memory savings are due to avoiding pahole's own copy of
"uncompressed" raw BTF data. Time reduction comes from faster string
search and deduplication by relying on hashmap instead of BST used by pahole's
own code. Consequently, these APIs are already tested on real-world
complicated kernel BTF, but there is also pretty extensive selftest doing
extra validations.

Selftests in patch #3 add a set of generic ASSERT_{EQ,STREQ,ERR,OK} macros
that are useful for writing shorter and less repretitive selftests. I decided
to keep them local to that selftest for now, but if they prove to be useful in
more contexts we should move them to test_progs.h. And few more (e.g.,
inequality tests) macros are probably necessary to have a more complete set.

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>

v2->v3:
  - resending original patches #7-9 as patches #1-3 due to merge conflict;

v1->v2:
  - fixed comments (John);
  - renamed btf__append_xxx() into btf__add_xxx() (Alexei);
  - added btf__find_str() in addition to btf__add_str();
  - btf__new_empty() now sets kernel FD to -1 initially.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

4 files changed, 1128 insertions, 5 deletions

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 54861a5ba9d9..c25f49fad5a6 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -31,10 +31,10 @@ struct btf {
 	__u32 raw_size;
 
 	/*
-	 * When BTF is loaded from ELF or raw memory it is stored
-	 * in contiguous memory block, pointed to by raw_data pointer, and
-	 * hdr, types_data, and strs_data point inside that memory region to
-	 * respective parts of BTF representation:
+	 * When BTF is loaded from an ELF or raw memory it is stored
+	 * in a contiguous memory block. The hdr, type_data, and, strs_data
+	 * point inside that memory region to their respective parts of BTF
+	 * representation:
 	 *
 	 * +--------------------------------+
 	 * |  Header  |  Types  |  Strings  |
@@ -1025,7 +1025,7 @@ const void *btf__get_raw_data(const struct btf *btf_ro, __u32 *size)
 	return btf->raw_data;
 }
 
-const char *btf__name_by_offset(const struct btf *btf, __u32 offset)
+const char *btf__str_by_offset(const struct btf *btf, __u32 offset)
 {
 	if (offset < btf->hdr->str_len)
 		return btf->strs_data + offset;
@@ -1033,6 +1033,11 @@ const char *btf__name_by_offset(const struct btf *btf, __u32 offset)
 		return NULL;
 }
 
+const char *btf__name_by_offset(const struct btf *btf, __u32 offset)
+{
+	return btf__str_by_offset(btf, offset);
+}
+
 int btf__get_from_id(__u32 id, struct btf **btf)
 {
 	struct bpf_btf_info btf_info = { 0 };
@@ -1343,6 +1348,787 @@ int btf__add_str(struct btf *btf, const char *s)
 	return new_off;
 }
 
+static void *btf_add_type_mem(struct btf *btf, size_t add_sz)
+{
+	return btf_add_mem(&btf->types_data, &btf->types_data_cap, 1,
+			   btf->hdr->type_len, UINT_MAX, add_sz);
+}
+
+static __u32 btf_type_info(int kind, int vlen, int kflag)
+{
+	return (kflag << 31) | (kind << 24) | vlen;
+}
+
+static void btf_type_inc_vlen(struct btf_type *t)
+{
+	t->info = btf_type_info(btf_kind(t), btf_vlen(t) + 1, btf_kflag(t));
+}
+
+/*
+ * Append new BTF_KIND_INT type with:
+ *   - *name* - non-empty, non-NULL type name;
+ *   - *sz* - power-of-2 (1, 2, 4, ..) size of the type, in bytes;
+ *   - encoding is a combination of BTF_INT_SIGNED, BTF_INT_CHAR, BTF_INT_BOOL.
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_int(struct btf *btf, const char *name, size_t byte_sz, int encoding)
+{
+	struct btf_type *t;
+	int sz, err, name_off;
+
+	/* non-empty name */
+	if (!name || !name[0])
+		return -EINVAL;
+	/* byte_sz must be power of 2 */
+	if (!byte_sz || (byte_sz & (byte_sz - 1)) || byte_sz > 16)
+		return -EINVAL;
+	if (encoding & ~(BTF_INT_SIGNED | BTF_INT_CHAR | BTF_INT_BOOL))
+		return -EINVAL;
+
+	/* deconstruct BTF, if necessary, and invalidate raw_data */
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_type) + sizeof(int);
+	t = btf_add_type_mem(btf, sz);
+	if (!t)
+		return -ENOMEM;
+
+	/* if something goes wrong later, we might end up with an extra string,
+	 * but that shouldn't be a problem, because BTF can't be constructed
+	 * completely anyway and will most probably be just discarded
+	 */
+	name_off = btf__add_str(btf, name);
+	if (name_off < 0)
+		return name_off;
+
+	t->name_off = name_off;
+	t->info = btf_type_info(BTF_KIND_INT, 0, 0);
+	t->size = byte_sz;
+	/* set INT info, we don't allow setting legacy bit offset/size */
+	*(__u32 *)(t + 1) = (encoding << 24) | (byte_sz * 8);
+
+	err = btf_add_type_idx_entry(btf, btf->hdr->type_len);
+	if (err)
+		return err;
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	btf->nr_types++;
+	return btf->nr_types;
+}
+
+/* it's completely legal to append BTF types with type IDs pointing forward to
+ * types that haven't been appended yet, so we only make sure that id looks
+ * sane, we can't guarantee that ID will always be valid
+ */
+static int validate_type_id(int id)
+{
+	if (id < 0 || id > BTF_MAX_NR_TYPES)
+		return -EINVAL;
+	return 0;
+}
+
+/* generic append function for PTR, TYPEDEF, CONST/VOLATILE/RESTRICT */
+static int btf_add_ref_kind(struct btf *btf, int kind, const char *name, int ref_type_id)
+{
+	struct btf_type *t;
+	int sz, name_off = 0, err;
+
+	if (validate_type_id(ref_type_id))
+		return -EINVAL;
+
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_type);
+	t = btf_add_type_mem(btf, sz);
+	if (!t)
+		return -ENOMEM;
+
+	if (name && name[0]) {
+		name_off = btf__add_str(btf, name);
+		if (name_off < 0)
+			return name_off;
+	}
+
+	t->name_off = name_off;
+	t->info = btf_type_info(kind, 0, 0);
+	t->type = ref_type_id;
+
+	err = btf_add_type_idx_entry(btf, btf->hdr->type_len);
+	if (err)
+		return err;
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	btf->nr_types++;
+	return btf->nr_types;
+}
+
+/*
+ * Append new BTF_KIND_PTR type with:
+ *   - *ref_type_id* - referenced type ID, it might not exist yet;
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_ptr(struct btf *btf, int ref_type_id)
+{
+	return btf_add_ref_kind(btf, BTF_KIND_PTR, NULL, ref_type_id);
+}
+
+/*
+ * Append new BTF_KIND_ARRAY type with:
+ *   - *index_type_id* - type ID of the type describing array index;
+ *   - *elem_type_id* - type ID of the type describing array element;
+ *   - *nr_elems* - the size of the array;
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_array(struct btf *btf, int index_type_id, int elem_type_id, __u32 nr_elems)
+{
+	struct btf_type *t;
+	struct btf_array *a;
+	int sz, err;
+
+	if (validate_type_id(index_type_id) || validate_type_id(elem_type_id))
+		return -EINVAL;
+
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_type) + sizeof(struct btf_array);
+	t = btf_add_type_mem(btf, sz);
+	if (!t)
+		return -ENOMEM;
+
+	t->name_off = 0;
+	t->info = btf_type_info(BTF_KIND_ARRAY, 0, 0);
+	t->size = 0;
+
+	a = btf_array(t);
+	a->type = elem_type_id;
+	a->index_type = index_type_id;
+	a->nelems = nr_elems;
+
+	err = btf_add_type_idx_entry(btf, btf->hdr->type_len);
+	if (err)
+		return err;
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	btf->nr_types++;
+	return btf->nr_types;
+}
+
+/* generic STRUCT/UNION append function */
+static int btf_add_composite(struct btf *btf, int kind, const char *name, __u32 bytes_sz)
+{
+	struct btf_type *t;
+	int sz, err, name_off = 0;
+
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_type);
+	t = btf_add_type_mem(btf, sz);
+	if (!t)
+		return -ENOMEM;
+
+	if (name && name[0]) {
+		name_off = btf__add_str(btf, name);
+		if (name_off < 0)
+			return name_off;
+	}
+
+	/* start out with vlen=0 and no kflag; this will be adjusted when
+	 * adding each member
+	 */
+	t->name_off = name_off;
+	t->info = btf_type_info(kind, 0, 0);
+	t->size = bytes_sz;
+
+	err = btf_add_type_idx_entry(btf, btf->hdr->type_len);
+	if (err)
+		return err;
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	btf->nr_types++;
+	return btf->nr_types;
+}
+
+/*
+ * Append new BTF_KIND_STRUCT type with:
+ *   - *name* - name of the struct, can be NULL or empty for anonymous structs;
+ *   - *byte_sz* - size of the struct, in bytes;
+ *
+ * Struct initially has no fields in it. Fields can be added by
+ * btf__add_field() right after btf__add_struct() succeeds. 
+ *
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_struct(struct btf *btf, const char *name, __u32 byte_sz)
+{
+	return btf_add_composite(btf, BTF_KIND_STRUCT, name, byte_sz);
+}
+
+/*
+ * Append new BTF_KIND_UNION type with:
+ *   - *name* - name of the union, can be NULL or empty for anonymous union;
+ *   - *byte_sz* - size of the union, in bytes;
+ *
+ * Union initially has no fields in it. Fields can be added by
+ * btf__add_field() right after btf__add_union() succeeds. All fields
+ * should have *bit_offset* of 0.
+ *
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_union(struct btf *btf, const char *name, __u32 byte_sz)
+{
+	return btf_add_composite(btf, BTF_KIND_UNION, name, byte_sz);
+}
+
+/*
+ * Append new field for the current STRUCT/UNION type with:
+ *   - *name* - name of the field, can be NULL or empty for anonymous field;
+ *   - *type_id* - type ID for the type describing field type;
+ *   - *bit_offset* - bit offset of the start of the field within struct/union;
+ *   - *bit_size* - bit size of a bitfield, 0 for non-bitfield fields;
+ * Returns:
+ *   -  0, on success;
+ *   - <0, on error.
+ */
+int btf__add_field(struct btf *btf, const char *name, int type_id,
+		   __u32 bit_offset, __u32 bit_size)
+{
+	struct btf_type *t;
+	struct btf_member *m;
+	bool is_bitfield;
+	int sz, name_off = 0;
+
+	/* last type should be union/struct */
+	if (btf->nr_types == 0)
+		return -EINVAL;
+	t = btf_type_by_id(btf, btf->nr_types);
+	if (!btf_is_composite(t))
+		return -EINVAL;
+
+	if (validate_type_id(type_id))
+		return -EINVAL;
+	/* best-effort bit field offset/size enforcement */
+	is_bitfield = bit_size || (bit_offset % 8 != 0);
+	if (is_bitfield && (bit_size == 0 || bit_size > 255 || bit_offset > 0xffffff))
+		return -EINVAL;
+
+	/* only offset 0 is allowed for unions */
+	if (btf_is_union(t) && bit_offset)
+		return -EINVAL;
+
+	/* decompose and invalidate raw data */
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_member);
+	m = btf_add_type_mem(btf, sz);
+	if (!m)
+		return -ENOMEM;
+
+	if (name && name[0]) {
+		name_off = btf__add_str(btf, name);
+		if (name_off < 0)
+			return name_off;
+	}
+
+	m->name_off = name_off;
+	m->type = type_id;
+	m->offset = bit_offset | (bit_size << 24);
+
+	/* btf_add_type_mem can invalidate t pointer */
+	t = btf_type_by_id(btf, btf->nr_types);
+	/* update parent type's vlen and kflag */
+	t->info = btf_type_info(btf_kind(t), btf_vlen(t) + 1, is_bitfield || btf_kflag(t));
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	return 0;
+}
+
+/*
+ * Append new BTF_KIND_ENUM type with:
+ *   - *name* - name of the enum, can be NULL or empty for anonymous enums;
+ *   - *byte_sz* - size of the enum, in bytes.
+ *
+ * Enum initially has no enum values in it (and corresponds to enum forward
+ * declaration). Enumerator values can be added by btf__add_enum_value()
+ * immediately after btf__add_enum() succeeds.
+ *
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_enum(struct btf *btf, const char *name, __u32 byte_sz)
+{
+	struct btf_type *t;
+	int sz, err, name_off = 0;
+
+	/* byte_sz must be power of 2 */
+	if (!byte_sz || (byte_sz & (byte_sz - 1)) || byte_sz > 8)
+		return -EINVAL;
+
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_type);
+	t = btf_add_type_mem(btf, sz);
+	if (!t)
+		return -ENOMEM;
+
+	if (name && name[0]) {
+		name_off = btf__add_str(btf, name);
+		if (name_off < 0)
+			return name_off;
+	}
+
+	/* start out with vlen=0; it will be adjusted when adding enum values */
+	t->name_off = name_off;
+	t->info = btf_type_info(BTF_KIND_ENUM, 0, 0);
+	t->size = byte_sz;
+
+	err = btf_add_type_idx_entry(btf, btf->hdr->type_len);
+	if (err)
+		return err;
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	btf->nr_types++;
+	return btf->nr_types;
+}
+
+/*
+ * Append new enum value for the current ENUM type with:
+ *   - *name* - name of the enumerator value, can't be NULL or empty;
+ *   - *value* - integer value corresponding to enum value *name*;
+ * Returns:
+ *   -  0, on success;
+ *   - <0, on error.
+ */
+int btf__add_enum_value(struct btf *btf, const char *name, __s64 value)
+{
+	struct btf_type *t;
+	struct btf_enum *v;
+	int sz, name_off;
+
+	/* last type should be BTF_KIND_ENUM */
+	if (btf->nr_types == 0)
+		return -EINVAL;
+	t = btf_type_by_id(btf, btf->nr_types);
+	if (!btf_is_enum(t))
+		return -EINVAL;
+
+	/* non-empty name */
+	if (!name || !name[0])
+		return -EINVAL;
+	if (value < INT_MIN || value > UINT_MAX)
+		return -E2BIG;
+
+	/* decompose and invalidate raw data */
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_enum);
+	v = btf_add_type_mem(btf, sz);
+	if (!v)
+		return -ENOMEM;
+
+	name_off = btf__add_str(btf, name);
+	if (name_off < 0)
+		return name_off;
+
+	v->name_off = name_off;
+	v->val = value;
+
+	/* update parent type's vlen */
+	t = btf_type_by_id(btf, btf->nr_types);
+	btf_type_inc_vlen(t);
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	return 0;
+}
+
+/*
+ * Append new BTF_KIND_FWD type with:
+ *   - *name*, non-empty/non-NULL name;
+ *   - *fwd_kind*, kind of forward declaration, one of BTF_FWD_STRUCT,
+ *     BTF_FWD_UNION, or BTF_FWD_ENUM;
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_fwd(struct btf *btf, const char *name, enum btf_fwd_kind fwd_kind)
+{
+	if (!name || !name[0])
+		return -EINVAL;
+
+	switch (fwd_kind) {
+	case BTF_FWD_STRUCT:
+	case BTF_FWD_UNION: {
+		struct btf_type *t;
+		int id;
+
+		id = btf_add_ref_kind(btf, BTF_KIND_FWD, name, 0);
+		if (id <= 0)
+			return id;
+		t = btf_type_by_id(btf, id);
+		t->info = btf_type_info(BTF_KIND_FWD, 0, fwd_kind == BTF_FWD_UNION);
+		return id;
+	}
+	case BTF_FWD_ENUM:
+		/* enum forward in BTF currently is just an enum with no enum
+		 * values; we also assume a standard 4-byte size for it
+		 */
+		return btf__add_enum(btf, name, sizeof(int));
+	default:
+		return -EINVAL;
+	}
+}
+
+/*
+ * Append new BTF_KING_TYPEDEF type with:
+ *   - *name*, non-empty/non-NULL name;
+ *   - *ref_type_id* - referenced type ID, it might not exist yet;
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_typedef(struct btf *btf, const char *name, int ref_type_id)
+{
+	if (!name || !name[0])
+		return -EINVAL;
+
+	return btf_add_ref_kind(btf, BTF_KIND_TYPEDEF, name, ref_type_id);
+}
+
+/*
+ * Append new BTF_KIND_VOLATILE type with:
+ *   - *ref_type_id* - referenced type ID, it might not exist yet;
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_volatile(struct btf *btf, int ref_type_id)
+{
+	return btf_add_ref_kind(btf, BTF_KIND_VOLATILE, NULL, ref_type_id);
+}
+
+/*
+ * Append new BTF_KIND_CONST type with:
+ *   - *ref_type_id* - referenced type ID, it might not exist yet;
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_const(struct btf *btf, int ref_type_id)
+{
+	return btf_add_ref_kind(btf, BTF_KIND_CONST, NULL, ref_type_id);
+}
+
+/*
+ * Append new BTF_KIND_RESTRICT type with:
+ *   - *ref_type_id* - referenced type ID, it might not exist yet;
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_restrict(struct btf *btf, int ref_type_id)
+{
+	return btf_add_ref_kind(btf, BTF_KIND_RESTRICT, NULL, ref_type_id);
+}
+
+/*
+ * Append new BTF_KIND_FUNC type with:
+ *   - *name*, non-empty/non-NULL name;
+ *   - *proto_type_id* - FUNC_PROTO's type ID, it might not exist yet;
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_func(struct btf *btf, const char *name,
+		  enum btf_func_linkage linkage, int proto_type_id)
+{
+	int id;
+
+	if (!name || !name[0])
+		return -EINVAL;
+	if (linkage != BTF_FUNC_STATIC && linkage != BTF_FUNC_GLOBAL &&
+	    linkage != BTF_FUNC_EXTERN)
+		return -EINVAL;
+
+	id = btf_add_ref_kind(btf, BTF_KIND_FUNC, name, proto_type_id);
+	if (id > 0) {
+		struct btf_type *t = btf_type_by_id(btf, id);
+
+		t->info = btf_type_info(BTF_KIND_FUNC, linkage, 0);
+	}
+	return id;
+}
+
+/*
+ * Append new BTF_KIND_FUNC_PROTO with:
+ *   - *ret_type_id* - type ID for return result of a function.
+ *
+ * Function prototype initially has no arguments, but they can be added by
+ * btf__add_func_param() one by one, immediately after
+ * btf__add_func_proto() succeeded.
+ *
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_func_proto(struct btf *btf, int ret_type_id)
+{
+	struct btf_type *t;
+	int sz, err;
+
+	if (validate_type_id(ret_type_id))
+		return -EINVAL;
+
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_type);
+	t = btf_add_type_mem(btf, sz);
+	if (!t)
+		return -ENOMEM;
+
+	/* start out with vlen=0; this will be adjusted when adding enum
+	 * values, if necessary
+	 */
+	t->name_off = 0;
+	t->info = btf_type_info(BTF_KIND_FUNC_PROTO, 0, 0);
+	t->type = ret_type_id;
+
+	err = btf_add_type_idx_entry(btf, btf->hdr->type_len);
+	if (err)
+		return err;
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	btf->nr_types++;
+	return btf->nr_types;
+}
+
+/*
+ * Append new function parameter for current FUNC_PROTO type with:
+ *   - *name* - parameter name, can be NULL or empty;
+ *   - *type_id* - type ID describing the type of the parameter.
+ * Returns:
+ *   -  0, on success;
+ *   - <0, on error.
+ */
+int btf__add_func_param(struct btf *btf, const char *name, int type_id)
+{
+	struct btf_type *t;
+	struct btf_param *p;
+	int sz, name_off = 0;
+
+	if (validate_type_id(type_id))
+		return -EINVAL;
+
+	/* last type should be BTF_KIND_FUNC_PROTO */
+	if (btf->nr_types == 0)
+		return -EINVAL;
+	t = btf_type_by_id(btf, btf->nr_types);
+	if (!btf_is_func_proto(t))
+		return -EINVAL;
+
+	/* decompose and invalidate raw data */
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_param);
+	p = btf_add_type_mem(btf, sz);
+	if (!p)
+		return -ENOMEM;
+
+	if (name && name[0]) {
+		name_off = btf__add_str(btf, name);
+		if (name_off < 0)
+			return name_off;
+	}
+
+	p->name_off = name_off;
+	p->type = type_id;
+
+	/* update parent type's vlen */
+	t = btf_type_by_id(btf, btf->nr_types);
+	btf_type_inc_vlen(t);
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	return 0;
+}
+
+/*
+ * Append new BTF_KIND_VAR type with:
+ *   - *name* - non-empty/non-NULL name;
+ *   - *linkage* - variable linkage, one of BTF_VAR_STATIC,
+ *     BTF_VAR_GLOBAL_ALLOCATED, or BTF_VAR_GLOBAL_EXTERN;
+ *   - *type_id* - type ID of the type describing the type of the variable.
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_var(struct btf *btf, const char *name, int linkage, int type_id)
+{
+	struct btf_type *t;
+	struct btf_var *v;
+	int sz, err, name_off;
+
+	/* non-empty name */
+	if (!name || !name[0])
+		return -EINVAL;
+	if (linkage != BTF_VAR_STATIC && linkage != BTF_VAR_GLOBAL_ALLOCATED &&
+	    linkage != BTF_VAR_GLOBAL_EXTERN)
+		return -EINVAL;
+	if (validate_type_id(type_id))
+		return -EINVAL;
+
+	/* deconstruct BTF, if necessary, and invalidate raw_data */
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_type) + sizeof(struct btf_var);
+	t = btf_add_type_mem(btf, sz);
+	if (!t)
+		return -ENOMEM;
+
+	name_off = btf__add_str(btf, name);
+	if (name_off < 0)
+		return name_off;
+
+	t->name_off = name_off;
+	t->info = btf_type_info(BTF_KIND_VAR, 0, 0);
+	t->type = type_id;
+
+	v = btf_var(t);
+	v->linkage = linkage;
+
+	err = btf_add_type_idx_entry(btf, btf->hdr->type_len);
+	if (err)
+		return err;
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	btf->nr_types++;
+	return btf->nr_types;
+}
+
+/*
+ * Append new BTF_KIND_DATASEC type with:
+ *   - *name* - non-empty/non-NULL name;
+ *   - *byte_sz* - data section size, in bytes.
+ *
+ * Data section is initially empty. Variables info can be added with
+ * btf__add_datasec_var_info() calls, after btf__add_datasec() succeeds.
+ *
+ * Returns:
+ *   - >0, type ID of newly added BTF type;
+ *   - <0, on error.
+ */
+int btf__add_datasec(struct btf *btf, const char *name, __u32 byte_sz)
+{
+	struct btf_type *t;
+	int sz, err, name_off;
+
+	/* non-empty name */
+	if (!name || !name[0])
+		return -EINVAL;
+
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_type);
+	t = btf_add_type_mem(btf, sz);
+	if (!t)
+		return -ENOMEM;
+
+	name_off = btf__add_str(btf, name);
+	if (name_off < 0)
+		return name_off;
+
+	/* start with vlen=0, which will be update as var_secinfos are added */
+	t->name_off = name_off;
+	t->info = btf_type_info(BTF_KIND_DATASEC, 0, 0);
+	t->size = byte_sz;
+
+	err = btf_add_type_idx_entry(btf, btf->hdr->type_len);
+	if (err)
+		return err;
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	btf->nr_types++;
+	return btf->nr_types;
+}
+
+/*
+ * Append new data section variable information entry for current DATASEC type:
+ *   - *var_type_id* - type ID, describing type of the variable;
+ *   - *offset* - variable offset within data section, in bytes;
+ *   - *byte_sz* - variable size, in bytes.
+ *
+ * Returns:
+ *   -  0, on success;
+ *   - <0, on error.
+ */
+int btf__add_datasec_var_info(struct btf *btf, int var_type_id, __u32 offset, __u32 byte_sz)
+{
+	struct btf_type *t;
+	struct btf_var_secinfo *v;
+	int sz;
+
+	/* last type should be BTF_KIND_DATASEC */
+	if (btf->nr_types == 0)
+		return -EINVAL;
+	t = btf_type_by_id(btf, btf->nr_types);
+	if (!btf_is_datasec(t))
+		return -EINVAL;
+
+	if (validate_type_id(var_type_id))
+		return -EINVAL;
+
+	/* decompose and invalidate raw data */
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	sz = sizeof(struct btf_var_secinfo);
+	v = btf_add_type_mem(btf, sz);
+	if (!v)
+		return -ENOMEM;
+
+	v->type = var_type_id;
+	v->offset = offset;
+	v->size = byte_sz;
+
+	/* update parent type's vlen */
+	t = btf_type_by_id(btf, btf->nr_types);
+	btf_type_inc_vlen(t);
+
+	btf->hdr->type_len += sz;
+	btf->hdr->str_off += sz;
+	return 0;
+}
+
 struct btf_ext_sec_setup_param {
 	__u32 off;
 	__u32 len;
diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index b387d0b61f1c..f7dec0144c3c 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -49,6 +49,7 @@ LIBBPF_API int btf__fd(const struct btf *btf);
 LIBBPF_API void btf__set_fd(struct btf *btf, int fd);
 LIBBPF_API const void *btf__get_raw_data(const struct btf *btf, __u32 *size);
 LIBBPF_API const char *btf__name_by_offset(const struct btf *btf, __u32 offset);
+LIBBPF_API const char *btf__str_by_offset(const struct btf *btf, __u32 offset);
 LIBBPF_API int btf__get_from_id(__u32 id, struct btf **btf);
 LIBBPF_API int btf__get_map_kv_tids(const struct btf *btf, const char *map_name,
 				    __u32 expected_key_size,
@@ -77,6 +78,44 @@ LIBBPF_API struct btf *libbpf_find_kernel_btf(void);
 LIBBPF_API int btf__find_str(struct btf *btf, const char *s);
 LIBBPF_API int btf__add_str(struct btf *btf, const char *s);
 
+LIBBPF_API int btf__add_int(struct btf *btf, const char *name, size_t byte_sz, int encoding);
+LIBBPF_API int btf__add_ptr(struct btf *btf, int ref_type_id);
+LIBBPF_API int btf__add_array(struct btf *btf,
+			      int index_type_id, int elem_type_id, __u32 nr_elems);
+/* struct/union construction APIs */
+LIBBPF_API int btf__add_struct(struct btf *btf, const char *name, __u32 sz);
+LIBBPF_API int btf__add_union(struct btf *btf, const char *name, __u32 sz);
+LIBBPF_API int btf__add_field(struct btf *btf, const char *name, int field_type_id,
+			      __u32 bit_offset, __u32 bit_size);
+
+/* enum construction APIs */
+LIBBPF_API int btf__add_enum(struct btf *btf, const char *name, __u32 bytes_sz);
+LIBBPF_API int btf__add_enum_value(struct btf *btf, const char *name, __s64 value);
+
+enum btf_fwd_kind {
+	BTF_FWD_STRUCT = 0,
+	BTF_FWD_UNION = 1,
+	BTF_FWD_ENUM = 2,
+};
+
+LIBBPF_API int btf__add_fwd(struct btf *btf, const char *name, enum btf_fwd_kind fwd_kind);
+LIBBPF_API int btf__add_typedef(struct btf *btf, const char *name, int ref_type_id);
+LIBBPF_API int btf__add_volatile(struct btf *btf, int ref_type_id);
+LIBBPF_API int btf__add_const(struct btf *btf, int ref_type_id);
+LIBBPF_API int btf__add_restrict(struct btf *btf, int ref_type_id);
+
+/* func and func_proto construction APIs */
+LIBBPF_API int btf__add_func(struct btf *btf, const char *name,
+			     enum btf_func_linkage linkage, int proto_type_id);
+LIBBPF_API int btf__add_func_proto(struct btf *btf, int ret_type_id);
+LIBBPF_API int btf__add_func_param(struct btf *btf, const char *name, int type_id);
+
+/* var & datasec construction APIs */
+LIBBPF_API int btf__add_var(struct btf *btf, const char *name, int linkage, int type_id);
+LIBBPF_API int btf__add_datasec(struct btf *btf, const char *name, __u32 byte_sz);
+LIBBPF_API int btf__add_datasec_var_info(struct btf *btf, int var_type_id,
+					 __u32 offset, __u32 byte_sz);
+
 struct btf_dedup_opts {
 	unsigned int dedup_table_size;
 	bool dont_resolve_fwds;
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index d02d8f51e34f..6b10ebad69c6 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -305,9 +305,29 @@ LIBBPF_0.2.0 {
 		bpf_prog_bind_map;
 		bpf_prog_test_run_opts;
 		bpf_program__section_name;
+		btf__add_array;
+		btf__add_const;
+		btf__add_enum;
+		btf__add_enum_value;
+		btf__add_datasec;
+		btf__add_datasec_var_info;
+		btf__add_field;
+		btf__add_func;
+		btf__add_func_param;
+		btf__add_func_proto;
+		btf__add_fwd;
+		btf__add_int;
+		btf__add_ptr;
+		btf__add_restrict;
 		btf__add_str;
+		btf__add_struct;
+		btf__add_typedef;
+		btf__add_union;
+		btf__add_var;
+		btf__add_volatile;
 		btf__find_str;
 		btf__new_empty;
+		btf__str_by_offset;
 		perf_buffer__buffer_cnt;
 		perf_buffer__buffer_fd;
 		perf_buffer__epoll_fd;
diff --git a/tools/testing/selftests/bpf/prog_tests/btf_write.c b/tools/testing/selftests/bpf/prog_tests/btf_write.c
new file mode 100644
index 000000000000..88dce2cfa79b
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/btf_write.c
@@ -0,0 +1,278 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include <test_progs.h>
+#include <bpf/btf.h>
+
+#define ASSERT_EQ(actual, expected, name) ({				\
+	typeof(actual) ___act = (actual);				\
+	typeof(expected) ___exp = (expected);				\
+	bool ___ok = ___act == ___exp;					\
+	CHECK(!___ok, (name),						\
+	      "unexpected %s: actual %lld != expected %lld\n",		\
+	      (name), (long long)(___act), (long long)(___exp));	\
+	___ok;								\
+})
+
+#define ASSERT_STREQ(actual, expected, name) ({				\
+	const char *___act = actual;					\
+	const char *___exp = expected;					\
+	bool ___ok = strcmp(___act, ___exp) == 0;			\
+	CHECK(!___ok, (name),						\
+	      "unexpected %s: actual '%s' != expected '%s'\n",		\
+	      (name), ___act, ___exp);					\
+	___ok;								\
+})
+
+#define ASSERT_OK(res, name) ({						\
+	long long ___res = (res);					\
+	bool ___ok = ___res == 0;					\
+	CHECK(!___ok, (name), "unexpected error: %lld\n", ___res);	\
+	___ok;								\
+})
+
+#define ASSERT_ERR(res, name) ({					\
+	long long ___res = (res);					\
+	bool ___ok = ___res < 0;					\
+	CHECK(!___ok, (name), "unexpected success: %lld\n", ___res);	\
+	___ok;								\
+})
+
+static int duration = 0;
+
+void test_btf_write() {
+	const struct btf_var_secinfo *vi;
+	const struct btf_type *t;
+	const struct btf_member *m;
+	const struct btf_enum *v;
+	const struct btf_param *p;
+	struct btf *btf;
+	int id, err, str_off;
+
+	btf = btf__new_empty();
+	if (CHECK(IS_ERR(btf), "new_empty", "failed: %ld\n", PTR_ERR(btf)))
+		return;
+
+	str_off = btf__find_str(btf, "int");
+	ASSERT_EQ(str_off, -ENOENT, "int_str_missing_off");
+
+	str_off = btf__add_str(btf, "int");
+	ASSERT_EQ(str_off, 1, "int_str_off");
+
+	str_off = btf__find_str(btf, "int");
+	ASSERT_EQ(str_off, 1, "int_str_found_off");
+
+	/* BTF_KIND_INT */
+	id = btf__add_int(btf, "int", 4,  BTF_INT_SIGNED);
+	ASSERT_EQ(id, 1, "int_id");
+
+	t = btf__type_by_id(btf, 1);
+	/* should re-use previously added "int" string */
+	ASSERT_EQ(t->name_off, str_off, "int_name_off");
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "int", "int_name");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_INT, "int_kind");
+	ASSERT_EQ(t->size, 4, "int_sz");
+	ASSERT_EQ(btf_int_encoding(t), BTF_INT_SIGNED, "int_enc");
+	ASSERT_EQ(btf_int_bits(t), 32, "int_bits");
+
+	/* invalid int size */
+	id = btf__add_int(btf, "bad sz int", 7, 0);
+	ASSERT_ERR(id, "int_bad_sz");
+	/* invalid encoding */
+	id = btf__add_int(btf, "bad enc int", 4, 123);
+	ASSERT_ERR(id, "int_bad_enc");
+	/* NULL name */
+	id = btf__add_int(btf, NULL, 4, 0);
+	ASSERT_ERR(id, "int_bad_null_name");
+	/* empty name */
+	id = btf__add_int(btf, "", 4, 0);
+	ASSERT_ERR(id, "int_bad_empty_name");
+
+	/* PTR/CONST/VOLATILE/RESTRICT */
+	id = btf__add_ptr(btf, 1);
+	ASSERT_EQ(id, 2, "ptr_id");
+	t = btf__type_by_id(btf, 2);
+	ASSERT_EQ(btf_kind(t), BTF_KIND_PTR, "ptr_kind");
+	ASSERT_EQ(t->type, 1, "ptr_type");
+
+	id = btf__add_const(btf, 5); /* points forward to restrict */
+	ASSERT_EQ(id, 3, "const_id");
+	t = btf__type_by_id(btf, 3);
+	ASSERT_EQ(btf_kind(t), BTF_KIND_CONST, "const_kind");
+	ASSERT_EQ(t->type, 5, "const_type");
+
+	id = btf__add_volatile(btf, 3);
+	ASSERT_EQ(id, 4, "volatile_id");
+	t = btf__type_by_id(btf, 4);
+	ASSERT_EQ(btf_kind(t), BTF_KIND_VOLATILE, "volatile_kind");
+	ASSERT_EQ(t->type, 3, "volatile_type");
+
+	id = btf__add_restrict(btf, 4);
+	ASSERT_EQ(id, 5, "restrict_id");
+	t = btf__type_by_id(btf, 5);
+	ASSERT_EQ(btf_kind(t), BTF_KIND_RESTRICT, "restrict_kind");
+	ASSERT_EQ(t->type, 4, "restrict_type");
+
+	/* ARRAY */
+	id = btf__add_array(btf, 1, 2, 10); /* int *[10] */
+	ASSERT_EQ(id, 6, "array_id");
+	t = btf__type_by_id(btf, 6);
+	ASSERT_EQ(btf_kind(t), BTF_KIND_ARRAY, "array_kind");
+	ASSERT_EQ(btf_array(t)->index_type, 1, "array_index_type");
+	ASSERT_EQ(btf_array(t)->type, 2, "array_elem_type");
+	ASSERT_EQ(btf_array(t)->nelems, 10, "array_nelems");
+
+	/* STRUCT */
+	err = btf__add_field(btf, "field", 1, 0, 0);
+	ASSERT_ERR(err, "no_struct_field");
+	id = btf__add_struct(btf, "s1", 8);
+	ASSERT_EQ(id, 7, "struct_id");
+	err = btf__add_field(btf, "f1", 1, 0, 0);
+	ASSERT_OK(err, "f1_res");
+	err = btf__add_field(btf, "f2", 1, 32, 16);
+	ASSERT_OK(err, "f2_res");
+
+	t = btf__type_by_id(btf, 7);
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "s1", "struct_name");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_STRUCT, "struct_kind");
+	ASSERT_EQ(btf_vlen(t), 2, "struct_vlen");
+	ASSERT_EQ(btf_kflag(t), true, "struct_kflag");
+	ASSERT_EQ(t->size, 8, "struct_sz");
+	m = btf_members(t) + 0;
+	ASSERT_STREQ(btf__str_by_offset(btf, m->name_off), "f1", "f1_name");
+	ASSERT_EQ(m->type, 1, "f1_type");
+	ASSERT_EQ(btf_member_bit_offset(t, 0), 0, "f1_bit_off");
+	ASSERT_EQ(btf_member_bitfield_size(t, 0), 0, "f1_bit_sz");
+	m = btf_members(t) + 1;
+	ASSERT_STREQ(btf__str_by_offset(btf, m->name_off), "f2", "f2_name");
+	ASSERT_EQ(m->type, 1, "f2_type");
+	ASSERT_EQ(btf_member_bit_offset(t, 1), 32, "f2_bit_off");
+	ASSERT_EQ(btf_member_bitfield_size(t, 1), 16, "f2_bit_sz");
+
+	/* UNION */
+	id = btf__add_union(btf, "u1", 8);
+	ASSERT_EQ(id, 8, "union_id");
+
+	/* invalid, non-zero offset */
+	err = btf__add_field(btf, "field", 1, 1, 0);
+	ASSERT_ERR(err, "no_struct_field");
+
+	err = btf__add_field(btf, "f1", 1, 0, 16);
+	ASSERT_OK(err, "f1_res");
+
+	t = btf__type_by_id(btf, 8);
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "u1", "union_name");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_UNION, "union_kind");
+	ASSERT_EQ(btf_vlen(t), 1, "union_vlen");
+	ASSERT_EQ(btf_kflag(t), true, "union_kflag");
+	ASSERT_EQ(t->size, 8, "union_sz");
+	m = btf_members(t) + 0;
+	ASSERT_STREQ(btf__str_by_offset(btf, m->name_off), "f1", "f1_name");
+	ASSERT_EQ(m->type, 1, "f1_type");
+	ASSERT_EQ(btf_member_bit_offset(t, 0), 0, "f1_bit_off");
+	ASSERT_EQ(btf_member_bitfield_size(t, 0), 16, "f1_bit_sz");
+
+	/* ENUM */
+	id = btf__add_enum(btf, "e1", 4);
+	ASSERT_EQ(id, 9, "enum_id");
+	err = btf__add_enum_value(btf, "v1", 1);
+	ASSERT_OK(err, "v1_res");
+	err = btf__add_enum_value(btf, "v2", 2);
+	ASSERT_OK(err, "v2_res");
+
+	t = btf__type_by_id(btf, 9);
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "e1", "enum_name");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_ENUM, "enum_kind");
+	ASSERT_EQ(btf_vlen(t), 2, "enum_vlen");
+	ASSERT_EQ(t->size, 4, "enum_sz");
+	v = btf_enum(t) + 0;
+	ASSERT_STREQ(btf__str_by_offset(btf, v->name_off), "v1", "v1_name");
+	ASSERT_EQ(v->val, 1, "v1_val");
+	v = btf_enum(t) + 1;
+	ASSERT_STREQ(btf__str_by_offset(btf, v->name_off), "v2", "v2_name");
+	ASSERT_EQ(v->val, 2, "v2_val");
+
+	/* FWDs */
+	id = btf__add_fwd(btf, "struct_fwd", BTF_FWD_STRUCT);
+	ASSERT_EQ(id, 10, "struct_fwd_id");
+	t = btf__type_by_id(btf, 10);
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "struct_fwd", "fwd_name");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_FWD, "fwd_kind");
+	ASSERT_EQ(btf_kflag(t), 0, "fwd_kflag");
+
+	id = btf__add_fwd(btf, "union_fwd", BTF_FWD_UNION);
+	ASSERT_EQ(id, 11, "union_fwd_id");
+	t = btf__type_by_id(btf, 11);
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "union_fwd", "fwd_name");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_FWD, "fwd_kind");
+	ASSERT_EQ(btf_kflag(t), 1, "fwd_kflag");
+
+	id = btf__add_fwd(btf, "enum_fwd", BTF_FWD_ENUM);
+	ASSERT_EQ(id, 12, "enum_fwd_id");
+	t = btf__type_by_id(btf, 12);
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "enum_fwd", "fwd_name");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_ENUM, "enum_fwd_kind");
+	ASSERT_EQ(btf_vlen(t), 0, "enum_fwd_kind");
+	ASSERT_EQ(t->size, 4, "enum_fwd_sz");
+
+	/* TYPEDEF */
+	id = btf__add_typedef(btf, "typedef1", 1);
+	ASSERT_EQ(id, 13, "typedef_fwd_id");
+	t = btf__type_by_id(btf, 13);
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "typedef1", "typedef_name");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_TYPEDEF, "typedef_kind");
+	ASSERT_EQ(t->type, 1, "typedef_type");
+
+	/* FUNC & FUNC_PROTO */
+	id = btf__add_func(btf, "func1", BTF_FUNC_GLOBAL, 15);
+	ASSERT_EQ(id, 14, "func_id");
+	t = btf__type_by_id(btf, 14);
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "func1", "func_name");
+	ASSERT_EQ(t->type, 15, "func_type");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_FUNC, "func_kind");
+	ASSERT_EQ(btf_vlen(t), BTF_FUNC_GLOBAL, "func_vlen");
+
+	id = btf__add_func_proto(btf, 1);
+	ASSERT_EQ(id, 15, "func_proto_id");
+	err = btf__add_func_param(btf, "p1", 1);
+	ASSERT_OK(err, "p1_res");
+	err = btf__add_func_param(btf, "p2", 2);
+	ASSERT_OK(err, "p2_res");
+
+	t = btf__type_by_id(btf, 15);
+	ASSERT_EQ(btf_kind(t), BTF_KIND_FUNC_PROTO, "func_proto_kind");
+	ASSERT_EQ(btf_vlen(t), 2, "func_proto_vlen");
+	ASSERT_EQ(t->type, 1, "func_proto_ret_type");
+	p = btf_params(t) + 0;
+	ASSERT_STREQ(btf__str_by_offset(btf, p->name_off), "p1", "p1_name");
+	ASSERT_EQ(p->type, 1, "p1_type");
+	p = btf_params(t) + 1;
+	ASSERT_STREQ(btf__str_by_offset(btf, p->name_off), "p2", "p2_name");
+	ASSERT_EQ(p->type, 2, "p2_type");
+
+	/* VAR */
+	id = btf__add_var(btf, "var1", BTF_VAR_GLOBAL_ALLOCATED, 1);
+	ASSERT_EQ(id, 16, "var_id");
+	t = btf__type_by_id(btf, 16);
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "var1", "var_name");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_VAR, "var_kind");
+	ASSERT_EQ(t->type, 1, "var_type");
+	ASSERT_EQ(btf_var(t)->linkage, BTF_VAR_GLOBAL_ALLOCATED, "var_type");
+
+	/* DATASECT */
+	id = btf__add_datasec(btf, "datasec1", 12);
+	ASSERT_EQ(id, 17, "datasec_id");
+	err = btf__add_datasec_var_info(btf, 1, 4, 8);
+	ASSERT_OK(err, "v1_res");
+
+	t = btf__type_by_id(btf, 17);
+	ASSERT_STREQ(btf__str_by_offset(btf, t->name_off), "datasec1", "datasec_name");
+	ASSERT_EQ(t->size, 12, "datasec_sz");
+	ASSERT_EQ(btf_kind(t), BTF_KIND_DATASEC, "datasec_kind");
+	ASSERT_EQ(btf_vlen(t), 1, "datasec_vlen");
+	vi = btf_var_secinfos(t) + 0;
+	ASSERT_EQ(vi->type, 1, "v1_type");
+	ASSERT_EQ(vi->offset, 4, "v1_off");
+	ASSERT_EQ(vi->size, 8, "v1_sz");
+
+	btf__free(btf);
+}