[PATCH v6] scripts/config: honor $KBUILD_OUTPUT by default

[Dmitry Voytik <voytikd@gmail.com>]

If $KBUILD_OUTPUT is set, use it by default if --file is not passed.

Fix the issue when the script silently updates a stale .config in
the root directory, while the user expects ${KBUILD_OUTPUT}/.config
to be updated.
Handle properly if the config file is a relative path.

The problem can be worked around with --file, but this is easy to miss,
and is inconsistent with how the Makefile and other tools work
(e.g., scripts/diffconfig).

How to reproduce the issue:
	make mrproper
	export KBUILD_OUTPUT=.out
	make defconfig
	./scripts/config \
	       -e CONFIG_UBSAN
Error reported:
	grep: .config: No such file or directory

How the patch was tested:
	make mrproper
	export KBUILD_OUTPUT=.out
	make defconfig
	grep "CONFIG_UBSAN=y" $KBUILD_OUTPUT/.config || echo "OK"

Test the basic usage:
	./scripts/config -e CONFIG_UBSAN
	grep "CONFIG_UBSAN=y" $KBUILD_OUTPUT/.config && echo "OK"

Test --refresh picks up correct config file by using $KBUILD_OUTPUT:
	./scripts/config -d CONFIG_UBSAN
	grep "CONFIG_UBSAN=y" $KBUILD_OUTPUT/.config && echo "FAIL"
	./scripts/config --refresh -e CONFIG_UBSAN
	grep "CONFIG_UBSAN=y" $KBUILD_OUTPUT/.config && echo "OK"

Test --file takes precedence over $KBUILD_OUTPUT and a relative
path is handled properly:
	./scripts/config -d CONFIG_UBSAN
	mkdir .out2
	cp .out/.config .out2/.my_config
	grep "CONFIG_UBSAN=y" .out2/.my_config  && echo "FAIL"
	./scripts/config --file .out2/.my_config --refresh \
	       -e CONFIG_UBSAN
	grep "CONFIG_UBSAN=y" .out2/.my_config && echo "OK"
	ls .out/.out2 && echo "FAIL"
	ls .out/.out && echo "FAIL"

Test --file can accept files with hyphen:
	./scripts/config -d CONFIG_UBSAN
	cp .out/.config -- -my_config
	grep "CONFIG_UBSAN=y" -- -my_config && echo "FAIL"
	./scripts/config  --refresh \
	       -e CONFIG_UBSAN --file -my_config
	grep "CONFIG_UBSAN=y" -- -my_config && echo "OK"

Signed-off-by: Dmitry Voytik <voytikd@gmail.com>
---
v6:
 * Per Nathan Chancellor's suggestions, simplified
 * https://lore.kernel.org/lkml/179121581235.2275809.12565811683284940522.b4-review@b4/
v5:
 * Per Nathan Chancellor's suggestion, improved handling $FN path by
   realpath
 * https://lore.kernel.org/lkml/179097210878.309835.10523307262544578337.b4-review@b4/
 * Documented the new test scenario in the git commit message
v4:
 * Addressed Nicolas Schier's comments - dropped the change how '--refresh'
   is handled
 * Implemented conversion of $FN to the absolute path which helps with
   '--refresh' and relative paths
 * Documented testing steps in the commit message
v3:
 * Added an example in the commit message how to reproduce the issue
v2:
 * Addressed comments by sashiko:
   * https://sashiko.dev/#/patchset/20260920064350.17999-1-voytikd%40gmail.com
   * use basename when --refresh is used
   * remove echo which can be misleading when --file is passed
v1:
 * https://lore.kernel.org/all/20260920064350.17999-1-voytikd@gmail.com/
---
 scripts/config | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/scripts/config b/scripts/config
index 32428ea909c2..d1d59d5fece6 100755
--- a/scripts/config
+++ b/scripts/config
@@ -130,13 +130,23 @@ on_exit() {
 trap on_exit EXIT
 
 FN=.config
+
+if [ -n "$KBUILD_OUTPUT" ]; then
+	if [ ! -d "$KBUILD_OUTPUT" ]; then
+		echo "KBUILD_OUTPUT is set but does not exist or is not a " >&2
+		echo "directory. Create it before running $myname." >&2
+		exit 1
+	fi
+	FN=$(realpath -- "$KBUILD_OUTPUT")/"$FN"
+fi
+
 CMDS=()
 while [[ $# -gt 0 ]]; do
 	if [ "$1" = "--file" ]; then
 		if [ "$2" = "" ]; then
 			usage
 		fi
-		FN="$2"
+		FN=$(realpath -q -- "$2" || printf -- "%s\n" "$2")
 		shift 2
 	else
 		CMDS+=("$1")
@@ -144,6 +154,7 @@ while [[ $# -gt 0 ]]; do
 	fi
 done
 
+
 set -- "${CMDS[@]}"
 if [ "$1" = "" ] ; then
 	usage
@@ -200,7 +211,7 @@ while [ "$1" != "" ] ; do
 		;;
 
 	--state|-s)
-		if grep -q "# ${CONFIG_}$ARG is not set" $FN ; then
+		if grep -q "# ${CONFIG_}$ARG is not set" "$FN" ; then
 			echo n
 		else
 			V="$(grep "^${CONFIG_}$ARG=" $FN)"
@@ -229,7 +240,7 @@ while [ "$1" != "" ] ; do
 		;;
 
 	--refresh)
-		yes "" | make oldconfig KCONFIG_CONFIG=$FN
+		yes "" | make oldconfig KCONFIG_CONFIG="$FN"
 		;;
 
 	*)
-- 
2.55.0