Re: [PATCH bpf-next v3 1/2] libbpf: Introduce 'btf_custom_path' to 'bpf_obj_open_opts'

btf_custom_path allows developers to load custom BTF, and subsequent
CO-RE will use custom BTF for relocation.

Learn from Andrii's comments in [0], add the btf_custom_path parameter
to bpf_obj_open_opts, you can directly use the skeleton's
<objname>_bpf__open_opts function to pass in the btf_custom_path
parameter.

Prior to this, there was also a developer who provided a patch with
similar functions. It is a pity that the follow-up did not continue to
advance. See [1].

        [0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t (local)
        [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/

Signed-off-by: Shuyi Cheng <redacted>
---
 tools/lib/bpf/libbpf.c | 46 ++++++++++++++++++++++++++++++++++++++++++++--
 tools/lib/bpf/libbpf.h |  6 +++++-
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 1e04ce7..6702b7f 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -498,6 +498,10 @@ struct bpf_object {
         * it at load time.
         */
        struct btf *btf_vmlinux;
+       /* custom BTF is in addition to vmlinux BTF (i.e., Use the CO-RE
+        * feature in the old kernel).
+        */

+       char *btf_custom_path;
        /* vmlinux BTF override for CO-RE relocations */
        struct btf *btf_vmlinux_override;
        /* Lazily initialized kernel module BTFs */

@@ -2668,6 +2672,27 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
        return false;
 }

+static int bpf_object__load_override_btf(struct bpf_object *obj)
+{
+       int err;
+
+       if (obj->btf_vmlinux_override)
+               return 0;
+
+       if (!obj->btf_custom_path)
+               return 0;
+
+       obj->btf_vmlinux_override = btf__parse(obj->btf_custom_path, NULL);
+       err = libbpf_get_error(obj->btf_vmlinux_override);
+       pr_debug("loading custom BTF '%s': %d\n", obj->btf_custom_path, err);
+       if (err) {
+               pr_warn("failed to parse custom BTF\n");
+               obj->btf_vmlinux_override = NULL;
+       }
+
+       return err;
+}

+
 static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
 {
        int err;

@@ -7554,7 +7579,7 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
 __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
                   const struct bpf_object_open_opts *opts)
 {
-       const char *obj_name, *kconfig;
+       const char *obj_name, *kconfig, *btf_tmp_path;
        struct bpf_program *prog;
        struct bpf_object *obj;
        char tmp_name[64];

@@ -7584,6 +7609,19 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
        obj = bpf_object__new(path, obj_buf, obj_buf_sz, obj_name);
        if (IS_ERR(obj))
                return obj;
+
+       btf_tmp_path = OPTS_GET(opts, btf_custom_path, NULL);
+       if (btf_tmp_path) {
+               if (strlen(btf_tmp_path) >= PATH_MAX) {
+                       err = -ENAMETOOLONG;
+                       goto out;
+               }
+               obj->btf_custom_path = strdup(btf_tmp_path);
+               if (!obj->btf_custom_path) {
+                       err = -ENOMEM;
+                       goto out;
+               }
+       }

        kconfig = OPTS_GET(opts, kconfig, NULL);
        if (kconfig) {

@@ -8049,6 +8087,7 @@ int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
                bpf_gen__init(obj->gen_loader, attr->log_level);

        err = bpf_object__probe_loading(obj);
+       err = err ? : bpf_object__load_override_btf(obj);

@@ -8075,9 +8114,11 @@ int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
        }
        free(obj->btf_modules);

-       /* clean up vmlinux BTF */
+       /* clean up vmlinux BTF and custom BTF*/
        btf__free(obj->btf_vmlinux);
        obj->btf_vmlinux = NULL;
+       btf__free(obj->btf_vmlinux_override);
+       obj->btf_vmlinux_override = NULL;

        obj->loaded = true; /* doesn't matter if successfully or not */

@@ -8702,6 +8743,7 @@ void bpf_object__close(struct bpf_object *obj)
        for (i = 0; i < obj->nr_maps; i++)
                bpf_map__destroy(&obj->maps[i]);

+       zfree(&obj->btf_custom_path);
        zfree(&obj->kconfig);
        zfree(&obj->externs);
        obj->nr_extern = 0;

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 6e61342..5002d1f 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h

@@ -94,8 +94,12 @@ struct bpf_object_open_opts {
         * system Kconfig for CONFIG_xxx externs.
         */
        const char *kconfig;
+       /* custom BTF is in addition to vmlinux BTF (i.e., Use the CO-RE
+        * feature in the old kernel).