btf: separate btf creation and loading

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, which will do DWARF to BTF
conversion and then use libbpf to do type deduplication, while then
handling ELF sections overwriting and other concerns on its own.

Fixes: 2d3feca8c44f ("bpf: btf: print map dump and lookup with btf info")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 30 insertions, 24 deletions

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 4324eb47d214..46db0a3b5cb7 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -367,8 +367,6 @@ 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;
 
@@ -378,15 +376,6 @@ 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;
@@ -396,17 +385,6 @@ 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;
@@ -418,8 +396,6 @@ 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);
@@ -428,6 +404,36 @@ done:
         return btf;
 }
 
+int btf__load(struct btf *btf)
+{
+        __u32 log_buf_size = BPF_LOG_BUF_SIZE;
+        char *log_buf = NULL;
+        int err = 0;
+
+        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) {
+                err = -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 err;
+}
+
 int btf__fd(const struct btf *btf)
 {
         return btf->fd;