[PATCH bpf-next] libbpf: prevent loading vmlinux BTF twice

[Andrii Nakryiko]

Prevent loading/parsing vmlinux BTF twice in some cases: for CO-RE relocations
and for BTF-aware hooks (tp_btf, fentry/fexit, etc).

Fixes: a6ed02cac690 ("libbpf: Load btf_vmlinux only once per object.")
Signed-off-by: Andrii Nakryiko <andr...@fb.com>
---
 tools/lib/bpf/libbpf.c | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 18461deb1b19..1e77bbfe6c63 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2504,22 +2504,31 @@ static inline bool libbpf_prog_needs_vmlinux_btf(struct 
bpf_program *prog)
 
 static int bpf_object__load_vmlinux_btf(struct bpf_object *obj)
 {
+       bool need_vmlinux_btf = false;
        struct bpf_program *prog;
        int err;
 
+       /* CO-RE relocations need kernel BTF */
+       if (obj->btf_ext && obj->btf_ext->field_reloc_info.len)
+               need_vmlinux_btf = true;
+
        bpf_object__for_each_program(prog, obj) {
                if (libbpf_prog_needs_vmlinux_btf(prog)) {
-                       obj->btf_vmlinux = libbpf_find_kernel_btf();
-                       if (IS_ERR(obj->btf_vmlinux)) {
-                               err = PTR_ERR(obj->btf_vmlinux);
-                               pr_warn("Error loading vmlinux BTF: %d\n", err);
-                               obj->btf_vmlinux = NULL;
-                               return err;
-                       }
-                       return 0;
+                       need_vmlinux_btf = true;
+                       break;
                }
        }
 
+       if (!need_vmlinux_btf)
+               return 0;
+
+       obj->btf_vmlinux = libbpf_find_kernel_btf();
+       if (IS_ERR(obj->btf_vmlinux)) {
+               err = PTR_ERR(obj->btf_vmlinux);
+               pr_warn("Error loading vmlinux BTF: %d\n", err);
+               obj->btf_vmlinux = NULL;
+               return err;
+       }
        return 0;
 }
 
@@ -4945,8 +4954,8 @@ bpf_core_reloc_fields(struct bpf_object *obj, const char 
*targ_btf_path)
        if (targ_btf_path)
                targ_btf = btf__parse_elf(targ_btf_path, NULL);
        else
-               targ_btf = libbpf_find_kernel_btf();
-       if (IS_ERR(targ_btf)) {
+               targ_btf = obj->btf_vmlinux;
+       if (IS_ERR_OR_NULL(targ_btf)) {
                pr_warn("failed to get target BTF: %ld\n", PTR_ERR(targ_btf));
                return PTR_ERR(targ_btf);
        }
@@ -4987,7 +4996,9 @@ bpf_core_reloc_fields(struct bpf_object *obj, const char 
*targ_btf_path)
        }
 
 out:
-       btf__free(targ_btf);
+       /* obj->btf_vmlinux is freed at the end of object load phase */
+       if (targ_btf != obj->btf_vmlinux)
+               btf__free(targ_btf);
        if (!IS_ERR_OR_NULL(cand_cache)) {
                hashmap__for_each_entry(cand_cache, entry, i) {
                        bpf_core_free_cands(entry->value);
-- 
2.24.1