From 3cfff166114e460b971ff712f176333c48f699b0 Mon Sep 17 00:00:00 2001
From: Andrii Nakryiko <andriin@fb.com>
Date: Fri, 25 Sep 2020 18:13:54 -0700
Subject: [PATCH] libbpf: Add btf__new_empty() to create an empty BTF object

Add an ability to create an empty BTF object from scratch. This is going to be
used by pahole for BTF encoding. And also by selftest for convenient creation
of BTF objects.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200926011357.2366158-7-andriin@fb.com
---
 src/btf.c      | 30 ++++++++++++++++++++++++++++++
 src/btf.h      |  1 +
 src/libbpf.map |  1 +
 3 files changed, 32 insertions(+)

diff --git a/src/btf.c b/src/btf.c
index b22f4af..6506857 100644
--- a/src/btf.c
+++ b/src/btf.c
@@ -565,6 +565,36 @@ void btf__free(struct btf *btf)
 	free(btf);
 }
 
+struct btf *btf__new_empty(void)
+{
+	struct btf *btf;
+
+	btf = calloc(1, sizeof(*btf));
+	if (!btf)
+		return ERR_PTR(-ENOMEM);
+	btf->fd = -1;
+	btf->ptr_sz = sizeof(void *);
+
+	/* +1 for empty string at offset 0 */
+	btf->raw_size = sizeof(struct btf_header) + 1;
+	btf->raw_data = calloc(1, btf->raw_size);
+	if (!btf->raw_data) {
+		free(btf);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	btf->hdr = btf->raw_data;
+	btf->hdr->hdr_len = sizeof(struct btf_header);
+	btf->hdr->magic = BTF_MAGIC;
+	btf->hdr->version = BTF_VERSION;
+
+	btf->types_data = btf->raw_data + btf->hdr->hdr_len;
+	btf->strs_data = btf->raw_data + btf->hdr->hdr_len;
+	btf->hdr->str_len = 1; /* empty string at offset 0 */
+
+	return btf;
+}
+
 struct btf *btf__new(const void *data, __u32 size)
 {
 	struct btf *btf;
diff --git a/src/btf.h b/src/btf.h
index d88b644..b387d0b 100644
--- a/src/btf.h
+++ b/src/btf.h
@@ -27,6 +27,7 @@ struct bpf_object;
 
 LIBBPF_API void btf__free(struct btf *btf);
 LIBBPF_API struct btf *btf__new(const void *data, __u32 size);
+LIBBPF_API struct btf *btf__new_empty(void);
 LIBBPF_API struct btf *btf__parse(const char *path, struct btf_ext **btf_ext);
 LIBBPF_API struct btf *btf__parse_elf(const char *path, struct btf_ext **btf_ext);
 LIBBPF_API struct btf *btf__parse_raw(const char *path);
diff --git a/src/libbpf.map b/src/libbpf.map
index f1518c5..d02d8f5 100644
--- a/src/libbpf.map
+++ b/src/libbpf.map
@@ -307,6 +307,7 @@ LIBBPF_0.2.0 {
 		bpf_program__section_name;
 		btf__add_str;
 		btf__find_str;
+		btf__new_empty;
 		perf_buffer__buffer_cnt;
 		perf_buffer__buffer_fd;
 		perf_buffer__epoll_fd;