Merge branch 'Add btf_custom_path in bpf_obj_open_opts'

Shuyi Cheng says:

====================

This patch set adds the ability to point to a custom BTF for the
purposes of BPF CO-RE relocations. This is useful for using BPF CO-RE
on old kernels that don't yet natively support kernel (vmlinux) BTF
and thus libbpf needs application's help in locating kernel BTF
generated separately from the kernel itself. This was already possible
to do through bpf_object__load's attribute struct, but that makes it
inconvenient to use with BPF skeleton, which only allows to specify
bpf_object_open_opts during the open step. Thus, add the ability to
override vmlinux BTF at open time.

Patch #1 adds libbpf changes.
Patch #2 fixes pre-existing memory leak detected during the code review.
Patch #3 switches existing selftests to using open_opts for custom BTF.

Changelog:
----------

v3: https://lore.kernel.org/bpf/CAEf4BzY2cdT44bfbMus=gei27ViqGE1BtGo6XrErSsOCnqtVJg@mail.gmail.com/T/#m877eed1d4cf0a1d3352d3f3d6c5ff158be45c542
v3->v4:
 - Follow Andrii's suggestion to modify cover letter description.
 - Delete function bpf_object__load_override_btf.
 - Follow Dan's suggestion to add fixes tag and modify commit msg to patch #2.
 - Add pathch #3 to switch existing selftests to using open_opts.

v2: https://lore.kernel.org/bpf/CAEf4Bza_ua+tjxdhyy4nZ8Boeo+scipWmr_1xM1pC6N5wyuhAA@mail.gmail.com/T/#mf9cf86ae0ffa96180ac29e4fd12697eb70eccd0f
v2->v3:
  - Load the BTF specified by btf_custom_path to btf_vmlinux_override
    instead of btf_bmlinux.
  - Fix the memory leak that may be introduced by the second version
    of the patch.
  - Add a new patch to fix the possible memory leak caused by
    obj->kconfig.

v1: https://lore.kernel.org/bpf/CAEf4BzaGjEC4t1OefDo11pj2-HfNy0BLhs_G2UREjRNTmb2u=A@mail.gmail.com/t/#m4d9f7c6761fbd2b436b5dfe491cd864b70225804
v1->v2:
  - Change custom_btf_path to btf_custom_path.
  - If the length of btf_custom_path of bpf_obj_open_opts is too long,
    return ERR_PTR(-ENAMETOOLONG).
  - Add `custom BTF is in addition to vmlinux BTF` with btf_custom_path field.
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

2 files changed, 36 insertions, 7 deletions

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 4ccfae30e681..17bc922f543f 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;
+	/* 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];
@@ -7627,11 +7633,26 @@ __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);
-		if (!obj->kconfig)
-			return ERR_PTR(-ENOMEM);
+		if (!obj->kconfig) {
+			err = -ENOMEM;
+			goto out;
+		}
 	}
 
 	err = bpf_object__elf_init(obj);
@@ -8097,7 +8118,7 @@ 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 +8765,7 @@ 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 6e61342ba56c..6b08c1023609 100644
--- 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 *