bpftool: Add support for custom BTF path in prog load/loadall

This patch exposes the btf_custom_path feature to bpftool, allowing users
to specify a custom BTF file when loading BPF programs using prog load or
prog loadall commands.

The argument 'btf_custom_path' in libbpf is used for those kernels that
don't have CONFIG_DEBUG_INFO_BTF enabled but still want to perform CO-RE
relocations.

Suggested-by: Quentin Monnet <[e-mail]>
Reviewed-by: Quentin Monnet <[e-mail]>
Signed-off-by: Jiayuan Chen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/bpf/bpftool/Documentation/bpftool-prog.rst

···
 | **bpftool** **prog dump xlated** *PROG* [{ **file** *FILE* | [**opcodes**] [**linum**] [**visual**] }]
 | **bpftool** **prog dump jited**  *PROG* [{ **file** *FILE* | [**opcodes**] [**linum**] }]
 | **bpftool** **prog pin** *PROG* *FILE*
-| **bpftool** **prog** { **load** | **loadall** } *OBJ* *PATH* [**type** *TYPE*] [**map** { **idx** *IDX* | **name** *NAME* } *MAP*] [{ **offload_dev** | **xdpmeta_dev** } *NAME*] [**pinmaps** *MAP_DIR*] [**autoattach**]
+| **bpftool** **prog** { **load** | **loadall** } *OBJ* *PATH* [**type** *TYPE*] [**map** { **idx** *IDX* | **name** *NAME* } *MAP*] [{ **offload_dev** | **xdpmeta_dev** } *NAME*] [**pinmaps** *MAP_DIR*] [**autoattach**] [**kernel_btf** *BTF_FILE*]
 | **bpftool** **prog attach** *PROG* *ATTACH_TYPE* [*MAP*]
 | **bpftool** **prog detach** *PROG* *ATTACH_TYPE* [*MAP*]
 | **bpftool** **prog tracelog**
···
     Note: *FILE* must be located in *bpffs* mount. It must not contain a dot
     character ('.'), which is reserved for future extensions of *bpffs*.
 
-bpftool prog { load | loadall } *OBJ* *PATH* [type *TYPE*] [map { idx *IDX* | name *NAME* } *MAP*] [{ offload_dev | xdpmeta_dev } *NAME*] [pinmaps *MAP_DIR*] [autoattach]
+bpftool prog { load | loadall } *OBJ* *PATH* [type *TYPE*] [map { idx *IDX* | name *NAME* } *MAP*] [{ offload_dev | xdpmeta_dev } *NAME*] [pinmaps *MAP_DIR*] [autoattach] [kernel_btf *BTF_FILE*]
     Load bpf program(s) from binary *OBJ* and pin as *PATH*. **bpftool prog
     load** pins only the first program from the *OBJ* as *PATH*. **bpftool prog
     loadall** pins all programs from the *OBJ* under *PATH* directory. **type**
···
     object file, in particular, it's not supported for all program types. If a
     program does not support autoattach, bpftool falls back to regular pinning
     for that program instead.
+
+    The **kernel_btf** option allows specifying an external BTF file to replace
+    the system's own vmlinux BTF file for CO-RE relocations. Note that any
+    other feature relying on BTF (such as fentry/fexit programs, struct_ops)
+    requires the BTF file for the actual kernel running on the host, often
+    exposed at /sys/kernel/btf/vmlinux.
 
     Note: *PATH* must be located in *bpffs* mount. It must not contain a dot
     character ('.'), which is reserved for future extensions of *bpffs*.

tools/bpf/bpftool/bash-completion/bpftool

···
                             _bpftool_get_map_names
                             return 0
                             ;;
-                        pinned|pinmaps)
+                        pinned|pinmaps|kernel_btf)
                             _filedir
                             return 0
                             ;;
                         *)
                             COMPREPLY=( $( compgen -W "map" -- "$cur" ) )
-                            _bpftool_once_attr 'type pinmaps autoattach'
+                            _bpftool_once_attr 'type pinmaps autoattach kernel_btf'
                             _bpftool_one_of_list 'offload_dev xdpmeta_dev'
                             return 0
                             ;;

tools/bpf/bpftool/prog.c

···
 		} else if (is_prefix(*argv, "autoattach")) {
 			auto_attach = true;
 			NEXT_ARG();
+		} else if (is_prefix(*argv, "kernel_btf")) {
+			NEXT_ARG();
+
+			if (!REQ_ARGS(1))
+				goto err_free_reuse_maps;
+
+			open_opts.btf_custom_path = GET_ARG();
 		} else {
-			p_err("expected no more arguments, 'type', 'map' or 'dev', got: '%s'?",
+			p_err("expected no more arguments, "
+			      "'type', 'map', 'offload_dev', 'xdpmeta_dev', 'pinmaps', "
+			      "'autoattach', or 'kernel_btf', got: '%s'?",
 			      *argv);
 			goto err_free_reuse_maps;
 		}
···
 		"                         [map { idx IDX | name NAME } MAP]\\\n"
 		"                         [pinmaps MAP_DIR]\n"
 		"                         [autoattach]\n"
+		"                         [kernel_btf BTF_FILE]\n"
 		"       %1$s %2$s attach PROG ATTACH_TYPE [MAP]\n"
 		"       %1$s %2$s detach PROG ATTACH_TYPE [MAP]\n"
 		"       %1$s %2$s run PROG \\\n"