[PATCH bpf-next v1] scripts/gen-btf.sh: Use CONFIG_SHELL for execution

[Ihor Solodrai <ihor.solodrai@linux.dev>]

According to the docs [1], kernel build scripts should be executed via
CONFIG_SHELL, which is sh by default.

Fixup gen-btf.sh to be runnable with sh, and use CONFIG_SHELL at every
invocation site.

See relevant discussion for context [2].

[1] https://docs.kernel.org/kbuild/makefiles.html#script-invocation
[2] https://lore.kernel.org/bpf/CAADnVQ+dxmSNoJAGb6xV89ffUCKXe5CJXovXZt22nv5iYFV5mw@mail.gmail.com/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 scripts/Makefile.modfinal | 2 +-
 scripts/gen-btf.sh        | 8 ++++----
 scripts/link-vmlinux.sh   | 2 +-
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 422c56dc878e..adcbcde16a07 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -43,7 +43,7 @@ quiet_cmd_btf_ko = BTF [M] $@
 	if [ ! -f $(objtree)/vmlinux ]; then				\
 		printf "Skipping BTF generation for %s due to unavailability of vmlinux\n" $@ 1>&2; \
 	else	\
-		$(srctree)/scripts/gen-btf.sh --btf_base $(objtree)/vmlinux $@; \
+		$(CONFIG_SHELL) $(srctree)/scripts/gen-btf.sh --btf_base $(objtree)/vmlinux $@; \
 	fi;
 
 # Same as newer-prereqs, but allows to exclude specified extra dependencies
diff --git a/scripts/gen-btf.sh b/scripts/gen-btf.sh
index be21ccee3487..8ca96eb10a69 100755
--- a/scripts/gen-btf.sh
+++ b/scripts/gen-btf.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 # Copyright (c) 2025 Meta Platforms, Inc. and affiliates.
 #
@@ -81,7 +81,7 @@ gen_btf_data()
 
 gen_btf_o()
 {
-	local btf_data=${ELF_FILE}.btf.o
+	btf_data=${ELF_FILE}.btf.o
 
 	# Create ${btf_data} which contains just .BTF section but no symbols. Add
 	# SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all
@@ -107,11 +107,11 @@ embed_btf_data()
 	${OBJCOPY} --add-section .BTF=${ELF_FILE}.BTF ${ELF_FILE}
 
 	# a module might not have a .BTF_ids or .BTF.base section
-	local btf_base="${ELF_FILE}.BTF.base"
+	btf_base="${ELF_FILE}.BTF.base"
 	if [ -f "${btf_base}" ]; then
 		${OBJCOPY} --add-section .BTF.base=${btf_base} ${ELF_FILE}
 	fi
-	local btf_ids="${ELF_FILE}.BTF_ids"
+	btf_ids="${ELF_FILE}.BTF_ids"
 	if [ -f "${btf_ids}" ]; then
 		${RESOLVE_BTFIDS} --patch_btfids ${btf_ids} ${ELF_FILE}
 	fi
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 08cd8e25c65c..16d6a048e07c 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -206,7 +206,7 @@ fi
 
 if is_enabled CONFIG_DEBUG_INFO_BTF; then
 	info BTF .tmp_vmlinux1
-	if ! ${srctree}/scripts/gen-btf.sh .tmp_vmlinux1; then
+	if ! ${CONFIG_SHELL} ${srctree}/scripts/gen-btf.sh .tmp_vmlinux1; then
 		echo >&2 "Failed to generate BTF for vmlinux"
 		echo >&2 "Try to disable CONFIG_DEBUG_INFO_BTF"
 		exit 1
-- 
2.52.0