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Date:   Wed, 23 Sep 2020 08:54:33 -0700
From:   Andrii Nakryiko <andriin@...com>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...com>,
        <daniel@...earbox.net>
CC:     <andrii.nakryiko@...il.com>, <kernel-team@...com>,
        Andrii Nakryiko <andriin@...com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: [PATCH bpf-next 6/9] 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@...com>
---
 tools/lib/bpf/btf.c      | 28 ++++++++++++++++++++++++++++
 tools/lib/bpf/btf.h      |  1 +
 tools/lib/bpf/libbpf.map |  1 +
 3 files changed, 30 insertions(+)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 8c8f15703608..4d0e532d7b3d 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -565,6 +565,34 @@ 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);
+
+	/* +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/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 84cb0502af95..830f798b9a77 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -26,6 +26,7 @@ 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/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index b4274c0fdd04..180c871b04b6 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -305,6 +305,7 @@ LIBBPF_0.2.0 {
 		bpf_prog_bind_map;
 		bpf_program__section_name;
 		btf__add_str;
+		btf__new_empty;
 		perf_buffer__buffer_cnt;
 		perf_buffer__buffer_fd;
 		perf_buffer__epoll_fd;
-- 
2.24.1

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