[PATCH v6 bpf-next 03/14] libbpf: add internal helper to load BTF data by FD

[Andrii Nakryiko]

Add a btf_get_from_fd() helper, which constructs struct btf from in-kernel BTF
data by FD. This is used for loading module BTFs.

Signed-off-by: Andrii Nakryiko <and...@kernel.org>
---
 tools/lib/bpf/btf.c             | 61 +++++++++++++++++++--------------
 tools/lib/bpf/libbpf_internal.h |  1 +
 2 files changed, 36 insertions(+), 26 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 1935e83d309c..3c3f2bc6c652 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -1323,35 +1323,27 @@ 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 btf *btf_get_from_fd(int btf_fd, struct btf *base_btf)
 {
-       struct bpf_btf_info btf_info = { 0 };
+       struct bpf_btf_info btf_info;
        __u32 len = sizeof(btf_info);
        __u32 last_size;
-       int btf_fd;
+       struct btf *btf;
        void *ptr;
        int err;
 
-       err = 0;
-       *btf = NULL;
-       btf_fd = bpf_btf_get_fd_by_id(id);
-       if (btf_fd < 0)
-               return 0;
-
        /* we won't know btf_size until we call bpf_obj_get_info_by_fd(). so
         * let's start with a sane default - 4KiB here - and resize it only if
         * bpf_obj_get_info_by_fd() needs a bigger buffer.
         */
-       btf_info.btf_size = 4096;
-       last_size = btf_info.btf_size;
+       last_size = 4096;
        ptr = malloc(last_size);
-       if (!ptr) {
-               err = -ENOMEM;
-               goto exit_free;
-       }
+       if (!ptr)
+               return ERR_PTR(-ENOMEM);
 
-       memset(ptr, 0, last_size);
+       memset(&btf_info, 0, sizeof(btf_info));
        btf_info.btf = ptr_to_u64(ptr);
+       btf_info.btf_size = last_size;
        err = bpf_obj_get_info_by_fd(btf_fd, &btf_info, &len);
 
        if (!err && btf_info.btf_size > last_size) {
@@ -1360,31 +1352,48 @@ int btf__get_from_id(__u32 id, struct btf **btf)
                last_size = btf_info.btf_size;
                temp_ptr = realloc(ptr, last_size);
                if (!temp_ptr) {
-                       err = -ENOMEM;
+                       btf = ERR_PTR(-ENOMEM);
                        goto exit_free;
                }
                ptr = temp_ptr;
-               memset(ptr, 0, last_size);
+
+               len = sizeof(btf_info);
+               memset(&btf_info, 0, sizeof(btf_info));
                btf_info.btf = ptr_to_u64(ptr);
+               btf_info.btf_size = last_size;
+
                err = bpf_obj_get_info_by_fd(btf_fd, &btf_info, &len);
        }
 
        if (err || btf_info.btf_size > last_size) {
-               err = errno;
+               btf = err ? ERR_PTR(-errno) : ERR_PTR(-E2BIG);
                goto exit_free;
        }
 
-       *btf = btf__new((__u8 *)(long)btf_info.btf, btf_info.btf_size);
-       if (IS_ERR(*btf)) {
-               err = PTR_ERR(*btf);
-               *btf = NULL;
-       }
+       btf = btf_new(ptr, btf_info.btf_size, base_btf);
 
 exit_free:
-       close(btf_fd);
        free(ptr);
+       return btf;
+}
 
-       return err;
+int btf__get_from_id(__u32 id, struct btf **btf)
+{
+       struct btf *res;
+       int btf_fd;
+
+       *btf = NULL;
+       btf_fd = bpf_btf_get_fd_by_id(id);
+       if (btf_fd < 0)
+               return -errno;
+
+       res = btf_get_from_fd(btf_fd, NULL);
+       close(btf_fd);
+       if (IS_ERR(res))
+               return PTR_ERR(res);
+
+       *btf = res;
+       return 0;
 }
 
 int btf__get_map_kv_tids(const struct btf *btf, const char *map_name,
diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index d99bc847bf84..e569ae63808e 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -155,6 +155,7 @@ int bpf_object__section_size(const struct bpf_object *obj, 
const char *name,
                             __u32 *size);
 int bpf_object__variable_offset(const struct bpf_object *obj, const char *name,
                                __u32 *off);
+struct btf *btf_get_from_fd(int btf_fd, struct btf *base_btf);
 
 struct btf_ext_info {
        /*
-- 
2.24.1