[PATCH bpf-next 1/2] btf: separate btf creation and loading

[Andrii Nakryiko <andriin@fb.com>]

This change splits out previous btf__new functionality of constructing
struct btf and loading it into kernel into two:
- btf__new() just creates and initializes struct btf
- btf__load() attempts to load existing struct btf into kernel

btf__free will still close BTF fd, if it was ever loaded successfully
into kernel.

This change allows users of libbpf to manipulate BTF using its API,
without the need to unnecessarily load it into kernel.

One of the intended use cases is pahole using libbpf to do DWARF to BTF
conversion and deduplication using libbpf, while handling ELF sections
overwrites and other concerns on its own.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
---
 tools/lib/bpf/btf.c      | 53 ++++++++++++++++++++++------------------
 tools/lib/bpf/btf.h      |  1 +
 tools/lib/bpf/libbpf.c   |  2 +-
 tools/lib/bpf/libbpf.map |  1 +
 4 files changed, 32 insertions(+), 25 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 4949f8840bda..065d51fa63e5 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -366,8 +366,6 @@ void btf__free(struct btf *btf)
 
 struct btf *btf__new(__u8 *data, __u32 size)
 {
-	__u32 log_buf_size = 0;
-	char *log_buf = NULL;
 	struct btf *btf;
 	int err;
 
@@ -377,15 +375,6 @@ struct btf *btf__new(__u8 *data, __u32 size)
 
 	btf->fd = -1;
 
-	log_buf = malloc(BPF_LOG_BUF_SIZE);
-	if (!log_buf) {
-		err = -ENOMEM;
-		goto done;
-	}
-
-	*log_buf = 0;
-	log_buf_size = BPF_LOG_BUF_SIZE;
-
 	btf->data = malloc(size);
 	if (!btf->data) {
 		err = -ENOMEM;
@@ -395,17 +384,6 @@ struct btf *btf__new(__u8 *data, __u32 size)
 	memcpy(btf->data, data, size);
 	btf->data_size = size;
 
-	btf->fd = bpf_load_btf(btf->data, btf->data_size,
-			       log_buf, log_buf_size, false);
-
-	if (btf->fd == -1) {
-		err = -errno;
-		pr_warning("Error loading BTF: %s(%d)\n", strerror(errno), errno);
-		if (log_buf && *log_buf)
-			pr_warning("%s\n", log_buf);
-		goto done;
-	}
-
 	err = btf_parse_hdr(btf);
 	if (err)
 		goto done;
@@ -417,8 +395,6 @@ struct btf *btf__new(__u8 *data, __u32 size)
 	err = btf_parse_type_sec(btf);
 
 done:
-	free(log_buf);
-
 	if (err) {
 		btf__free(btf);
 		return ERR_PTR(err);
@@ -427,6 +403,35 @@ struct btf *btf__new(__u8 *data, __u32 size)
 	return btf;
 }
 
+int btf__load(struct btf* btf) {
+	__u32 log_buf_size = BPF_LOG_BUF_SIZE;
+	char *log_buf = NULL;
+
+	if (btf->fd >= 0) {
+		return -EEXIST;
+	}
+
+	log_buf = malloc(log_buf_size);
+	if (!log_buf)
+		return -ENOMEM;
+
+	*log_buf = 0;
+
+	btf->fd = bpf_load_btf(btf->data, btf->data_size,
+			       log_buf, log_buf_size, false);
+	if (btf->fd < 0) {
+		btf->fd = -errno;
+		pr_warning("Error loading BTF: %s(%d)\n", strerror(errno), errno);
+		if (*log_buf)
+			pr_warning("%s\n", log_buf);
+		goto done;
+	}
+
+done:
+	free(log_buf);
+	return btf->fd;
+}
+
 int btf__fd(const struct btf *btf)
 {
 	return btf->fd;
diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 25a9d2db035d..e8410887f93a 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -57,6 +57,7 @@ struct btf_ext_header {
 
 LIBBPF_API void btf__free(struct btf *btf);
 LIBBPF_API struct btf *btf__new(__u8 *data, __u32 size);
+LIBBPF_API int btf__load(struct btf* btf);
 LIBBPF_API __s32 btf__find_by_name(const struct btf *btf,
 				   const char *type_name);
 LIBBPF_API __u32 btf__get_nr_types(const struct btf *btf);
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 47969aa0faf8..75b82c1cfc72 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -835,7 +835,7 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
 			obj->efile.maps_shndx = idx;
 		else if (strcmp(name, BTF_ELF_SEC) == 0) {
 			obj->btf = btf__new(data->d_buf, data->d_size);
-			if (IS_ERR(obj->btf)) {
+			if (IS_ERR(obj->btf) || btf__load(obj->btf) < 0) {
 				pr_warning("Error loading ELF section %s: %ld. Ignored and continue.\n",
 					   BTF_ELF_SEC, PTR_ERR(obj->btf));
 				obj->btf = NULL;
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 89c1149e32ee..ffa1fe044f6a 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -134,6 +134,7 @@ LIBBPF_0.0.2 {
 		bpf_object__find_map_fd_by_name;
 		bpf_get_link_xdp_id;
 		btf__dedup;
+		btf__load;
 		btf__get_map_kv_tids;
 		btf__get_nr_types;
 		btf__get_strings;
-- 
2.17.1