kbuild: rewrite check-local-export in sh/awk

Remove the bash build dependency for those who otherwise do not
have it installed. This also provides a significant speedup:

$ make defconfig
$ make yes2modconfig

...

$ find  .  -name "*.o" | grep -v vmlinux | wc
     3169      3169     89615
$ export NM=nm
$ time sh -c 'find . -name "*.o" | grep -v vmlinux | xargs -n1
./scripts/check-local-export'

Without patch:
    0m15.90s real     0m12.17s user     0m05.28s system

With patch:
dash + nawk
    0m02.16s real     0m02.92s user     0m00.34s system

dash + busybox awk
    0m02.36s real     0m03.36s user     0m00.34s system

dash + gawk
    0m02.07s real     0m03.26s user     0m00.32s system

bash + gawk
    0m03.55s real     0m05.00s user     0m00.54s system

Signed-off-by: Owen Rafferty <owen@owenrafferty.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/check-local-export

@@ -1,25 +1,14 @@
-#!/usr/bin/env bash
+#!/bin/sh
 # SPDX-License-Identifier: GPL-2.0-only
 #
 # Copyright (C) 2022 Masahiro Yamada <masahiroy@kernel.org>
+# Copyright (C) 2022 Owen Rafferty <owen@owenrafferty.com>
 #
 # Exit with error if a local exported symbol is found.
 # EXPORT_SYMBOL should be used for global symbols.
 
 set -e
-
-# catch errors from ${NM}
-set -o pipefail
-
-# Run the last element of a pipeline in the current shell.
-# Without this, the while-loop would be executed in a subshell, and
-# the changes made to 'symbol_types' and 'export_symbols' would be lost.
-shopt -s lastpipe
-
-declare -A symbol_types
-declare -a export_symbols
-
-exit_code=0
+pid=$$
 
 # If there is no symbol in the object, ${NM} (both GNU nm and llvm-nm) shows
 # 'no symbols' diagnostic (but exits with 0). It is harmless and hidden by
@@ -29,43 +18,53 @@ exit_code=0
 # TODO:
 # Use --quiet instead of 2>/dev/null when we upgrade the minimum version of
 # binutils to 2.37, llvm to 13.0.0.
-# Then, the following line will be really simple:
-#   ${NM} --quiet ${1} |
+# Then, the following line will be simpler:
+#   { ${NM} --quiet ${1} || kill 0; } |
+
+{ ${NM} ${1} 2>/dev/null || { echo "${0}: ${NM} failed" >&2; kill $pid; } } |
+${AWK} -v "file=${1}" '
+BEGIN {
+	i = 0
+}
+
+# Skip the line if the number of fields is less than 3.
+#
+# case 1)
+#   For undefined symbols, the first field (value) is empty.
+#   The outout looks like this:
+#     "                 U _printk"
+#   It is unneeded to record undefined symbols.
+#
+# case 2)
+#   For Clang LTO, llvm-nm outputs a line with type t but empty name:
+#     "---------------- t"
+!length($3) {
+	next
+}
 
-{ ${NM} ${1} 2>/dev/null || { echo "${0}: ${NM} failed" >&2; false; } } |
-while read value type name
-do
-	# Skip the line if the number of fields is less than 3.
-	#
-	# case 1)
-	#   For undefined symbols, the first field (value) is empty.
-	#   The outout looks like this:
-	#     "                 U _printk"
-	#   It is unneeded to record undefined symbols.
-	#
-	# case 2)
-	#   For Clang LTO, llvm-nm outputs a line with type 't' but empty name:
-	#     "---------------- t"
-	if [[ -z ${name} ]]; then
-		continue
-	fi
+# save (name, type) in the associative array
+{ symbol_types[$3]=$2 }
 
-	# save (name, type) in the associative array
-	symbol_types[${name}]=${type}
+# append the exported symbol to the array
+($3 ~ /^__ksymtab_/) {
+	export_symbols[i] = $3
+	sub(/^__ksymtab_/, "", export_symbols[i])
+	i++
+}
 
-	# append the exported symbol to the array
-	if [[ ${name} == __ksymtab_* ]]; then
-		export_symbols+=(${name#__ksymtab_})
-	fi
-done
+END {
+	exit_code = 0
+	for (j = 0; j < i; ++j) {
+		name = export_symbols[j]
+		# nm(3) says "If lowercase, the symbol is usually local"
+		if (symbol_types[name] ~ /[a-z]/) {
+			printf "%s: error: local symbol %s was exported\n",
+				file, name | "cat 1>&2"
+			exit_code = 1
+		}
+	}
 
-for name in "${export_symbols[@]}"
-do
-	# nm(3) says "If lowercase, the symbol is usually local"
-	if [[ ${symbol_types[$name]} =~ [a-z] ]]; then
-		echo "$@: error: local symbol '${name}' was exported" >&2
-		exit_code=1
-	fi
-done
+	exit exit_code
+}'
 
-exit ${exit_code}
+exit $?