bpftool: Support dumping a map with btf_vmlinux_value_type_id

This patch makes bpftool support dumping a map's value properly
when the map's value type is a type of the running kernel's btf.
(i.e. map_info.btf_vmlinux_value_type_id is set instead of
map_info.btf_value_type_id).  The first usecase is for the
BPF_MAP_TYPE_STRUCT_OPS.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200115230044.1103008-1-kafai@fb.com

diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
index 4c5b15d..86f8ab0 100644 (file)
--- a/tools/bpf/bpftool/map.c
+++ b/tools/bpf/bpftool/map.c
@@ -252,6 +252,7 @@ static int do_dump_btf(const struct btf_dumper *d,
                       struct bpf_map_info *map_info, void *key,
                       void *value)
 {
+       __u32 value_id;
        int ret;
 
        /* start of key-value pair */
@@ -265,9 +266,12 @@ static int do_dump_btf(const struct btf_dumper *d,
                        goto err_end_obj;
        }
 
+       value_id = map_info->btf_vmlinux_value_type_id ?
+               : map_info->btf_value_type_id;
+
        if (!map_is_per_cpu(map_info->type)) {
                jsonw_name(d->jw, "value");
-               ret = btf_dumper_type(d, map_info->btf_value_type_id, value);
+               ret = btf_dumper_type(d, value_id, value);
        } else {
                unsigned int i, n, step;
 
@@ -279,8 +283,7 @@ static int do_dump_btf(const struct btf_dumper *d,
                        jsonw_start_object(d->jw);
                        jsonw_int_field(d->jw, "cpu", i);
                        jsonw_name(d->jw, "value");
-                       ret = btf_dumper_type(d, map_info->btf_value_type_id,
-                                             value + i * step);
+                       ret = btf_dumper_type(d, value_id, value + i * step);
                        jsonw_end_object(d->jw);
                        if (ret)
                                break;
@@ -932,6 +935,44 @@ static int maps_have_btf(int *fds, int nb_fds)
        return 1;
 }
 
+static struct btf *btf_vmlinux;
+
+static struct btf *get_map_kv_btf(const struct bpf_map_info *info)
+{
+       struct btf *btf = NULL;
+
+       if (info->btf_vmlinux_value_type_id) {
+               if (!btf_vmlinux) {
+                       btf_vmlinux = libbpf_find_kernel_btf();
+                       if (IS_ERR(btf_vmlinux))
+                               p_err("failed to get kernel btf");
+               }
+               return btf_vmlinux;
+       } else if (info->btf_value_type_id) {
+               int err;
+
+               err = btf__get_from_id(info->btf_id, &btf);
+               if (err || !btf) {
+                       p_err("failed to get btf");
+                       btf = err ? ERR_PTR(err) : ERR_PTR(-ESRCH);
+               }
+       }
+
+       return btf;
+}
+
+static void free_map_kv_btf(struct btf *btf)
+{
+       if (!IS_ERR(btf) && btf != btf_vmlinux)
+               btf__free(btf);
+}
+
+static void free_btf_vmlinux(void)
+{
+       if (!IS_ERR(btf_vmlinux))
+               btf__free(btf_vmlinux);
+}
+
 static int
 map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
         bool show_header)
@@ -952,13 +993,10 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
        prev_key = NULL;
 
        if (wtr) {
-               if (info->btf_id) {
-                       err = btf__get_from_id(info->btf_id, &btf);
-                       if (err || !btf) {
-                               err = err ? : -ESRCH;
-                               p_err("failed to get btf");
-                               goto exit_free;
-                       }
+               btf = get_map_kv_btf(info);
+               if (IS_ERR(btf)) {
+                       err = PTR_ERR(btf);
+                       goto exit_free;
                }
 
                if (show_header) {
@@ -999,7 +1037,7 @@ exit_free:
        free(key);
        free(value);
        close(fd);
-       btf__free(btf);
+       free_map_kv_btf(btf);
 
        return err;
 }
@@ -1067,6 +1105,7 @@ exit_close:
                close(fds[i]);
 exit_free:
        free(fds);
+       free_btf_vmlinux();
        return err;
 }