bpf, libbpf: add support for BTF Var and DataSec

This adds libbpf support for BTF Var and DataSec kinds. Main point
here is that libbpf needs to do some preparatory work before the
whole BTF object can be loaded into the kernel, that is, fixing up
of DataSec size taken from the ELF section size and non-static
variable offset which needs to be taken from the ELF's string section.

Upstream LLVM doesn't fix these up since at time of BTF emission
it is too early in the compilation process thus this information
isn't available yet, hence loader needs to take care of it.

Note, deduplication handling has not been in the scope of this work
and needs to be addressed in a future commit.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://reviews.llvm.org/D59441
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 96 insertions, 1 deletions

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 87e3020ac1bc..0f9079b9539e 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -24,6 +24,8 @@
                 ((k) == BTF_KIND_CONST) || \
                 ((k) == BTF_KIND_RESTRICT))
 
+#define IS_VAR(k) ((k) == BTF_KIND_VAR)
+
 static struct btf_type btf_void;
 
 struct btf {
@@ -212,6 +214,10 @@ static int btf_type_size(struct btf_type *t)
                 return base_size + vlen * sizeof(struct btf_member);
         case BTF_KIND_FUNC_PROTO:
                 return base_size + vlen * sizeof(struct btf_param);
+        case BTF_KIND_VAR:
+                return base_size + sizeof(struct btf_var);
+        case BTF_KIND_DATASEC:
+                return base_size + vlen * sizeof(struct btf_var_secinfo);
         default:
                 pr_debug("Unsupported BTF_KIND:%u\n", BTF_INFO_KIND(t->info));
                 return -EINVAL;
@@ -283,6 +289,7 @@ __s64 btf__resolve_size(const struct btf *btf, __u32 type_id)
                 case BTF_KIND_STRUCT:
                 case BTF_KIND_UNION:
                 case BTF_KIND_ENUM:
+                case BTF_KIND_DATASEC:
                         size = t->size;
                         goto done;
                 case BTF_KIND_PTR:
@@ -292,6 +299,7 @@ __s64 btf__resolve_size(const struct btf *btf, __u32 type_id)
                 case BTF_KIND_VOLATILE:
                 case BTF_KIND_CONST:
                 case BTF_KIND_RESTRICT:
+                case BTF_KIND_VAR:
                         type_id = t->type;
                         break;
                 case BTF_KIND_ARRAY:
@@ -326,7 +334,8 @@ int btf__resolve_type(const struct btf *btf, __u32 type_id)
         t = btf__type_by_id(btf, type_id);
         while (depth < MAX_RESOLVE_DEPTH &&
                !btf_type_is_void_or_null(t) &&
-               IS_MODIFIER(BTF_INFO_KIND(t->info))) {
+               (IS_MODIFIER(BTF_INFO_KIND(t->info)) ||
+                IS_VAR(BTF_INFO_KIND(t->info)))) {
                 type_id = t->type;
                 t = btf__type_by_id(btf, type_id);
                 depth++;
@@ -408,6 +417,92 @@ done:
         return btf;
 }
 
+static int compare_vsi_off(const void *_a, const void *_b)
+{
+        const struct btf_var_secinfo *a = _a;
+        const struct btf_var_secinfo *b = _b;
+
+        return a->offset - b->offset;
+}
+
+static int btf_fixup_datasec(struct bpf_object *obj, struct btf *btf,
+                             struct btf_type *t)
+{
+        __u32 size = 0, off = 0, i, vars = BTF_INFO_VLEN(t->info);
+        const char *name = btf__name_by_offset(btf, t->name_off);
+        const struct btf_type *t_var;
+        struct btf_var_secinfo *vsi;
+        struct btf_var *var;
+        int ret;
+
+        if (!name) {
+                pr_debug("No name found in string section for DATASEC kind.\n");
+                return -ENOENT;
+        }
+
+        ret = bpf_object__section_size(obj, name, &size);
+        if (ret || !size || (t->size && t->size != size)) {
+                pr_debug("Invalid size for section %s: %u bytes\n", name, size);
+                return -ENOENT;
+        }
+
+        t->size = size;
+
+        for (i = 0, vsi = (struct btf_var_secinfo *)(t + 1);
+             i < vars; i++, vsi++) {
+                t_var = btf__type_by_id(btf, vsi->type);
+                var = (struct btf_var *)(t_var + 1);
+
+                if (BTF_INFO_KIND(t_var->info) != BTF_KIND_VAR) {
+                        pr_debug("Non-VAR type seen in section %s\n", name);
+                        return -EINVAL;
+                }
+
+                if (var->linkage == BTF_VAR_STATIC)
+                        continue;
+
+                name = btf__name_by_offset(btf, t_var->name_off);
+                if (!name) {
+                        pr_debug("No name found in string section for VAR kind\n");
+                        return -ENOENT;
+                }
+
+                ret = bpf_object__variable_offset(obj, name, &off);
+                if (ret) {
+                        pr_debug("No offset found in symbol table for VAR %s\n", name);
+                        return -ENOENT;
+                }
+
+                vsi->offset = off;
+        }
+
+        qsort(t + 1, vars, sizeof(*vsi), compare_vsi_off);
+        return 0;
+}
+
+int btf__finalize_data(struct bpf_object *obj, struct btf *btf)
+{
+        int err = 0;
+        __u32 i;
+
+        for (i = 1; i <= btf->nr_types; i++) {
+                struct btf_type *t = btf->types[i];
+
+                /* Loader needs to fix up some of the things compiler
+                 * couldn't get its hands on while emitting BTF. This
+                 * is section size and global variable offset. We use
+                 * the info from the ELF itself for this purpose.
+                 */
+                if (BTF_INFO_KIND(t->info) == BTF_KIND_DATASEC) {
+                        err = btf_fixup_datasec(obj, btf, t);
+                        if (err)
+                                break;
+                }
+        }
+
+        return err;
+}
+
 int btf__load(struct btf *btf)
 {
         __u32 log_buf_size = BPF_LOG_BUF_SIZE;