[PATCH bpf-next v3] bpftool: Add support for custom BTF path in prog load/loadall

[Jiayuan Chen <jiayuan.chen@linux.dev>]

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 <qmo@kernel.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

---
V2 -> V3: Optimized document grammar and some prompts
https://lore.kernel.org/bpf/20250515065018.240188-1-jiayuan.chen@linux.dev/
V1 -> V2: Added bash completion and documentation
https://lore.kernel.org/bpf/20250513035853.75820-1-jiayuan.chen@linux.dev/
---
 tools/bpf/bpftool/Documentation/bpftool-prog.rst |  8 +++++++-
 tools/bpf/bpftool/bash-completion/bpftool        |  4 ++--
 tools/bpf/bpftool/prog.c                         | 12 +++++++++++-
 3 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/tools/bpf/bpftool/Documentation/bpftool-prog.rst b/tools/bpf/bpftool/Documentation/bpftool-prog.rst
index d6304e01afe0..4dce43e8e8a3 100644
--- a/tools/bpf/bpftool/Documentation/bpftool-prog.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-prog.rst
@@ -127,7 +127,7 @@ bpftool prog pin *PROG* *FILE*
     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**
@@ -153,6 +153,12 @@ bpftool prog { load | loadall } *OBJ* *PATH* [type *TYPE*] [map { idx *IDX* | na
     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*.
 
diff --git a/tools/bpf/bpftool/bash-completion/bpftool b/tools/bpf/bpftool/bash-completion/bpftool
index 1ce409a6cbd9..27512feb5c70 100644
--- a/tools/bpf/bpftool/bash-completion/bpftool
+++ b/tools/bpf/bpftool/bash-completion/bpftool
@@ -505,13 +505,13 @@ _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
                             ;;
diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
index f010295350be..96eea8a67225 100644
--- a/tools/bpf/bpftool/prog.c
+++ b/tools/bpf/bpftool/prog.c
@@ -1681,8 +1681,17 @@ static int load_with_options(int argc, char **argv, bool first_prog_only)
 		} 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;
 		}
@@ -2474,6 +2483,7 @@ static int do_help(int argc, char **argv)
 		"                         [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"
-- 
2.47.1