libbpf: Introduce 'btf_custom_path' to 'bpf_obj_open_opts'

btf_custom_path allows developers to load custom BTF which libbpf will
subsequently use for CO-RE relocation instead of vmlinux BTF.

Having btf_custom_path in bpf_object_open_opts one can directly use the
skeleton's <objname>_bpf__open_opts() API to pass in the btf_custom_path
parameter, as opposed to using bpf_object__load_xattr() which is slated to be
deprecated ([0]).

This work continues previous work started by another developer ([1]).

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

Signed-off-by: Shuyi Cheng <chengshuyi@linux.alibaba.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1626180159-112996-2-git-send-email-chengshuyi@linux.alibaba.com

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 4ccfae3..8bff28f 100644 (file)
--- 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;
+       /* Path to the custom BTF to be used for BPF CO-RE relocations as an
+        * override for vmlinux BTF.
+        */
+       char *btf_custom_path;
        /* vmlinux BTF override for CO-RE relocations */
        struct btf *btf_vmlinux_override;
        /* Lazily initialized kernel module BTFs */
@@ -2645,8 +2649,10 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
        struct bpf_program *prog;
        int i;
 
-       /* CO-RE relocations need kernel BTF */
-       if (obj->btf_ext && obj->btf_ext->core_relo_info.len)
+       /* CO-RE relocations need kernel BTF, only when btf_custom_path
+        * is not specified
+        */
+       if (obj->btf_ext && obj->btf_ext->core_relo_info.len && !obj->btf_custom_path)
                return true;
 
        /* Support for typed ksyms needs kernel BTF */
@@ -7596,7 +7602,7 @@ static struct bpf_object *
 __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];
@@ -7627,6 +7633,19 @@ __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
        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) {
                obj->kconfig = strdup(kconfig);
@@ -8097,7 +8116,7 @@ int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
        err = err ? : bpf_object__sanitize_maps(obj);
        err = err ? : bpf_object__init_kern_struct_ops_maps(obj);
        err = err ? : bpf_object__create_maps(obj);
-       err = err ? : bpf_object__relocate(obj, attr->target_btf_path);
+       err = err ? : bpf_object__relocate(obj, obj->btf_custom_path ? : attr->target_btf_path);
        err = err ? : bpf_object__load_progs(obj, attr->log_level);
 
        if (obj->gen_loader) {
@@ -8744,6 +8763,7 @@ 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..6b08c10 100644 (file)
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -94,8 +94,15 @@ struct bpf_object_open_opts {
         * system Kconfig for CONFIG_xxx externs.
         */
        const char *kconfig;
+       /* Path to the custom BTF to be used for BPF CO-RE relocations.
+        * This custom BTF completely replaces the use of vmlinux BTF
+        * for the purpose of CO-RE relocations.
+        * NOTE: any other BPF feature (e.g., fentry/fexit programs,
+        * struct_ops, etc) will need actual kernel BTF at /sys/kernel/btf/vmlinux.
+        */
+       const char *btf_custom_path;
 };
-#define bpf_object_open_opts__last_field kconfig
+#define bpf_object_open_opts__last_field btf_custom_path
 
 LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
 LIBBPF_API struct bpf_object *