libbpf: Add generic BTF type shallow copy API

Add btf__add_type() API that performs shallow copy of a given BTF type from
the source BTF into the destination BTF. All the information and type IDs are
preserved, but all the strings encountered are added into the destination BTF
and corresponding offsets are rewritten. BTF type IDs are assumed to be
correct or such that will be (somehow) modified afterwards.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210318194036.3521577-6-andrii@kernel.org

tools/lib/bpf/btf.c

@@ -1629,6 +1629,54 @@ static int btf_commit_type(struct btf *btf, int data_sz)
 	return btf->start_id + btf->nr_types - 1;
 }
 
+struct btf_pipe {
+	const struct btf *src;
+	struct btf *dst;
+};
+
+static int btf_rewrite_str(__u32 *str_off, void *ctx)
+{
+	struct btf_pipe *p = ctx;
+	int off;
+
+	if (!*str_off) /* nothing to do for empty strings */
+		return 0;
+
+	off = btf__add_str(p->dst, btf__str_by_offset(p->src, *str_off));
+	if (off < 0)
+		return off;
+
+	*str_off = off;
+	return 0;
+}
+
+int btf__add_type(struct btf *btf, const struct btf *src_btf, const struct btf_type *src_type)
+{
+	struct btf_pipe p = { .src = src_btf, .dst = btf };
+	struct btf_type *t;
+	int sz, err;
+
+	sz = btf_type_size(src_type);
+	if (sz < 0)
+		return sz;
+
+	/* deconstruct BTF, if necessary, and invalidate raw_data */
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	t = btf_add_type_mem(btf, sz);
+	if (!t)
+		return -ENOMEM;
+
+	memcpy(t, src_type, sz);
+
+	err = btf_type_visit_str_offs(t, btf_rewrite_str, &p);
+	if (err)
+		return err;
+
+	return btf_commit_type(btf, sz);
+}
+
 /*
  * Append new BTF_KIND_INT type with:
  *   - *name* - non-empty, non-NULL type name;

tools/lib/bpf/btf.h

@@ -93,6 +93,8 @@ 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_type(struct btf *btf, const struct btf *src_btf,
+			     const struct btf_type *src_type);
 
 LIBBPF_API int btf__add_int(struct btf *btf, const char *name, size_t byte_sz, int encoding);
 LIBBPF_API int btf__add_float(struct btf *btf, const char *name, size_t byte_sz);

tools/lib/bpf/libbpf.map

@@ -354,4 +354,5 @@ LIBBPF_0.3.0 {
 LIBBPF_0.4.0 {
 	global:
 		btf__add_float;
+		btf__add_type;
 } LIBBPF_0.3.0;